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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2018	Feb. 28, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	STIM
Entity Registrant Name	NEURONETICS, INC.
Entity Central Index Key	0001227636
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		18,054,628
Entity Shell Company	false
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 352.4

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 104,583	$ 29,147
Accounts receivable, net	5,620	4,267
Inventory	2,432	2,468
Prepaid expenses and other current assets	1,838	1,123
Total current assets	114,473	37,005
Property and equipment, net	1,378	1,359
Other assets	1,171	574
Total Assets	117,022	38,938
Current liabilities:
Accounts payable	3,756	2,513
Accrued expenses	7,548	7,511
Deferred revenue	2,255	1,970
Total current liabilities	13,559	11,994
Long-term debt, net	30,395	29,556
Deferred revenue	1,940	2,275
Deferred rent	86	151
Convertible preferred stock warrant liability		478
Total Liabilities	45,980	44,454
Commitments and contingencies (Note 15)
Convertible preferred stock, $0.01 par value: 308,593 shares previously authorized prior to initial public offering, issuable in series; no and 304,958 shares issued and outstanding at December 31, 2018 and 2017, respectively; no liquidation value at December 31, 2018		187,136
Stockholders’ Equity (Deficit):
Preferred stock, $0.01 par value: 10,000 shares authorized; no shares issued or outstanding at December 31, 2018 and 2017, respectively
Common stock, $0.01 par value: 200,000 shares authorized; 17,744 and 231 shares issued and outstanding at December 31, 2018 and 2017, respectively	177	2
Additional paid-in capital	291,908	4,292
Accumulated deficit	(221,043)	(196,946)
Total Stockholders' Equity (Deficit)	71,042	(192,652)
Total Liabilities, Convertible Preferred Stock and Stockholders’ Equity (Deficit)	$ 117,022	$ 38,938

Balance Sheets (Parenthetical) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Convertible preferred stock, par value	$ 0.01	$ 0.01
Convertible preferred stock, authorized	308,593,000	308,593,000
Convertible preferred stock, issued	0	304,958,000
Convertible preferred stock, outstanding	0	304,958,000
Liquidation preference	$ 0	$ 108,324,000
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	17,744,000	231,000
Common stock, shares outstanding	17,744,000	231,000

Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenues	$ 15,635	$ 13,737	$ 13,252	$ 10,152	$ 12,108	$ 10,491	$ 10,308	$ 7,526	$ 52,776	$ 40,433	$ 34,228
Cost of revenues	3,711	3,034	3,245	2,457	2,957	2,636	2,501	1,538	12,447	9,632	6,622
Gross Profit	11,924	10,703	10,007	7,695	9,151	7,855	7,807	5,988	40,329	30,801	27,606
Operating expenses:
Sales and marketing	10,648	9,672	9,835	8,109	8,628	6,566	6,400	6,306	38,264	27,900	21,794
General and administrative	4,715	3,238	3,078	2,636	2,837	2,256	1,837	1,642	13,667	8,572	6,926
Research and development	2,222	2,125	2,330	1,555	1,919	1,843	2,147	2,028	8,232	7,937	8,223
Total operating expenses	17,585	15,035	15,243	12,300	13,384	10,665	10,384	9,976	60,163	44,409	36,943
Loss from Operations	(5,661)	(4,332)	(5,236)	(4,605)	(4,233)	(2,810)	(2,577)	(3,988)	(19,834)	(13,608)	(9,337)
Other (income) expense:
Interest expense	939	928	900	921	740	807	711	550	3,688	2,808	1,835
Other (income) expense, net	(457)	(299)	1,360	(29)	(49)	136	(420)	(24)	575	(357)	62
Net Loss	$ (6,143)	$ (4,961)	$ (7,496)	$ (5,497)	$ (4,924)	$ (3,753)	$ (2,868)	$ (4,514)	$ (24,097)	$ (16,059)	$ (11,234)
Net loss per share of common stock outstanding, basic and diluted	$ (0.35)	$ (0.29)	$ (30.60)	$ (24.43)	$ (23.34)	$ (19.35)	$ (16.58)	$ (27.03)	$ (2.69)	$ (86.34)	$ (76.95)
Weighted-average common shares outstanding, basic and diluted	17,655	17,382	245	226	211	194	173	167	8,948	186	146

Statement of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Beginning balance at Dec. 31, 2015	$ (166,118)		$ 1	$ 3,534	$ (169,653)
Beginning balance, shares at Dec. 31, 2015			126,000
Beginning balance, shares at Dec. 31, 2015		264,374,000
Beginning balance at Dec. 31, 2015		$ 172,311
Issuance of restricted stock awards			$ 1	(1)
Issuance of restricted stock awards, shares			42,000
Exercises of stock options	67			67
Exercises of stock options, shares			19,000
Share-based compensation expense	161			161
Net loss	(11,234)				(11,234)
Ending balance at Dec. 31, 2016	(177,124)		$ 2	3,761	(180,887)
Ending balance, shares at Dec. 31, 2016			187,000
Ending balance, shares at Dec. 31, 2016		264,374,000
Ending balance at Dec. 31, 2016		$ 172,311
Issuance of Series G convertible preferred stock, net of issuance costs		$ 14,825
Issuance of restricted stock awards, shares			11,000
Exercises of stock options	35			35
Exercises of stock options, shares			33,000
Share-based compensation expense	496			496
Net loss	(16,059)				(16,059)
Ending balance at Dec. 31, 2017	$ (192,652)		$ 2	4,292	(196,946)
Ending balance, shares at Dec. 31, 2017			231,000
Ending balance, shares at Dec. 31, 2017	304,958,000	304,958,000
Ending balance at Dec. 31, 2017	$ 187,136	$ 187,136
Issuance of Series G convertible stock, net of issuance costs of $175, shares		40,584,000
Conversion of convertible preferred stock into common stock	187,136	$ (187,136)	$ 109	187,027
Conversion of convertible preferred stock into common stock, shares		(304,958,000)	10,994,000
Conversion of convertible preferred stock warrants into common stock warrants	1,874			1,874
Issuance of common stock initial public offering, net of issuance costs of $3,463	96,535		$ 64	96,471
Issuance of common stock initial public offering, net of issuance costs of $3,463, shares			6,325,000
Exercises of stock options	503		$ 2	501
Exercises of stock options, shares			194,000
Share-based compensation expense	1,743			1,743
Net loss	(24,097)				(24,097)
Ending balance at Dec. 31, 2018	$ 71,042		$ 177	$ 291,908	$ (221,043)
Ending balance, shares at Dec. 31, 2018			17,744,000
Ending balance, shares at Dec. 31, 2018	0

Statement of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Statement Of Stockholders Equity [Abstract]
Issuance of series G convertible preferred stock issuance cost		$ 175
Issuance of common stock in initial public offering, issuance costs	$ 3,463

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Flows from Operating Activities:
Net loss	$ (24,097)	$ (16,059)	$ (11,234)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	882	596	673
Share-based compensation	1,743	496	161
Non-cash interest expense	839	722	391
Change in fair value of convertible preferred stock warrant liability	1,396	(271)	108
Cost of rental units purchased by customers	229	216	15
Changes in certain assets and liabilities:
Accounts receivable, net	(1,353)	(690)	(123)
Inventory	(435)	(1,068)	(646)
Prepaid expenses and other assets	(237)	(175)	(156)
Accounts payable	606	788	734
Accrued expenses	(52)	1,391	1,201
Deferred revenue	(49)	2,915	363
Deferred rent	(63)	(5)	(28)
Net Cash Used in Operating Activities	(20,591)	(11,144)	(8,541)
Cash Flows from Investing Activities:
Purchases of property and equipment and capitalized software	(1,011)	(594)	(324)
Net Cash Used in Investing Activities	(1,011)	(594)	(324)
Cash Flows from Financing Activities:
Proceeds from issuance of common stock in initial public offering	99,998
Payments of public offering costs	(3,463)
Proceeds from exercises of stock options	503	35	67
Proceeds from issuance of Series G convertible preferred stock, net		14,825
Borrowings under credit facilities		10,000	5,000
Payments of debt issuance costs		(1,015)	(171)
Net Cash Provided by Financing Activities	97,038	23,845	4,896
Net Increase (Decrease) in Cash and Cash Equivalents	75,436	12,107	(3,969)
Cash and Cash Equivalents, Beginning of Year	29,147	17,040	21,009
Cash and Cash Equivalents, End of Year	104,583	29,147	17,040
Supplemental disclosure of cash flow information:
Cash paid for interest	2,786	2,043	1,406
Transfer of inventory to property and equipment	365	296	531
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment and capitalized software in accounts payable and accrued expenses	263	45	16
Conversion of convertible preferred stock into common stock	187,136
Conversion of convertible preferred stock warrants into common stock warrants	$ 1,874
Deferred public offering costs included in accounts payable and accrued expenses		55
Allocation of proceeds from debt financing to convertible preferred stock warrant liability		$ 290	$ 135

Description of Business

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Description of Business

1.	DESCRIPTION OF BUSINESS

Neuronetics, Inc., or the Company, is a commercial stage medical technology company focused on designing, developing and marketing products that improve the quality of life for patients who suffer from psychiatric disorders. The Company’s first commercial product, the NeuroStar Advanced Therapy System, is a non-invasive and non-systemic office-based treatment that uses transcranial magnetic stimulation, or TMS, to create a pulsed, MRI-strength magnetic field that induces electrical currents designed to stimulate specific areas of the brain associated with mood. The system was cleared in 2008 by the United States Food and Drug Administration, or the FDA, to treat adult patients with major depressive disorder, or MDD, who have failed to achieve satisfactory improvement from prior antidepressant medication in the current episode. The Company intends to continue to pursue development of its NeuroStar Advanced Therapy System for additional indications.

Initial Public Offering

On July 2, 2018, the Company closed its initial public offering, or IPO, in which the Company issued and sold 6.325 million shares of its common stock, which included shares sold pursuant to an option granted to the underwriters to purchase additional shares, at a public offering price of $17.00 per share. The Company received net proceeds of $96.5 million after deducting underwriting discounts, commissions and other offering expenses paid by the Company. The Company’s common stock is listed on the Nasdaq Global Market under the trading symbol “STIM.” In addition, immediately prior to the closing of the IPO on July 2, 2018, (i) all of the Company’s outstanding shares of convertible preferred stock converted into an aggregate 11.0 million shares of common stock; (ii) all of the Company’s outstanding warrants to purchase convertible preferred stock converted into warrants to purchase common stock; and (iii) the Company filed an amended and restated certificate of incorporation to, among other things, decrease the number of shares of common stock, $0.01 par value per share, authorized for issuance to 200.0 million and to authorize the board of directors to issue up to 10.0 million shares of “blank check” preferred stock, $0.01 par value per share.

Liquidity

As of December 31, 2018, the Company had cash and cash equivalents of $104.6 million and an accumulated deficit of $221.0 million. The Company incurred negative cash flows from operating activities of $20.6 million, $11.1 million and $8.5 million for the years ended December 31, 2018, 2017 and 2016, respectively. The Company has incurred operating losses since its inception, and management anticipates that its operating losses will continue in the near term as the Company seeks to expand its sales and marketing initiatives to support its growth into existing and new markets and invest in additional research and development activities. The Company’s primary sources of capital to date have been from its IPO, private placements of its convertible preferred securities, borrowings under its credit facilities and revenues from sales of its products. As of December 31, 2018, the Company had $30.0 million of borrowings outstanding under its credit facility, which matures in March 2022. Management believes that the Company’s cash and cash equivalents as of December 31, 2018 and anticipated revenues from sales of its products are sufficient to fund the Company’s operations for at least the next 24 months after December 31, 2018.

Basis of Presentation

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

2.	BASIS OF PRESENTATION

The accompanying financial statements have been prepared in accordance with United States generally accepted accounting principles, or GAAP. Any reference in these notes to applicable guidance is meant to refer to GAAP as found in the Accounting Standards Codification, or ASC, and Accounting Standards Updates, or ASUs, promulgated by the Financial Accounting Standards Board, or FASB.

Use of Estimates

The preparation of financial statements in accordance with GAAP and the rules and regulations of the United States Securities and Exchange Commission, or SEC, requires the use of estimates and assumptions, based on judgments considered reasonable, which affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. The Company bases its estimates and assumptions on historical experience, known trends and events and various other factors that management believes to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying value of assets and liabilities that are not readily apparent from other sources. Although management believes its estimates and assumptions are reasonable when made, they are based upon information available at the time they are made. Management evaluates the estimates and assumptions on an ongoing basis and, if necessary, makes adjustments. Due to the risks and uncertainties involved in the Company’s business and evolving market conditions and given the subjective element of the estimates and assumptions made, actual results may differ from estimated results. The most significant estimates and judgments impact share-based compensation and the valuation of common stock prior to the IPO.

Recapitalization

The Company effected a 0.0345-for-1 reverse split of its common stock on June 14, 2018. The reverse split combined each approximately 29 shares of the Company’s issued and outstanding common stock into one share of common stock and correspondingly adjusted the conversion price of its outstanding convertible preferred stock. No fractional shares were issued in connection with the reverse split. Any fractional share resulting from the reverse split was rounded down to the nearest whole share, and in lieu of any fractional share, the Company paid in cash to the holders of such fractional shares an amount equal to the fair market value, as determined by the board of directors, of such fractional shares. All share, per share and related information presented in these financial statements and the related notes thereto have been retroactively adjusted, where applicable, to reflect the reverse stock split.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

3.	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Cash and Cash Equivalents

The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents. As of December 31, 2018 and 2017, cash equivalents consisted of money market funds.

Concentrations of Credit Risk

The Company’s cash is held on deposit in demand accounts at a large financial institution in amounts in excess of the Federal Deposit Insurance Corporation, or FDIC, insurance coverage limit of $250,000 per depositor, per FDIC-insured bank, per ownership category. Management has reviewed the financial statements of this institution and believes it has sufficient assets and liquidity to conduct its operations in the ordinary course of business with little or no credit risk to the Company.

Financial instruments that potentially subject the Company to concentrations of credit risk principally consist of cash equivalents and accounts receivable. The Company limits its credit risk associated with cash equivalents by placing investments in highly-rated money market funds. The Company limits its credit risk with respect to accounts receivable by performing credit evaluations when deemed necessary, but it does not require collateral to secure amounts owed by its customers.

Allowance for Doubtful Accounts

The Company maintains allowances for doubtful accounts for estimated losses resulting from amounts deemed to be uncollectible from its customers. These allowances are for specific amounts on certain customer accounts based on facts and circumstances determined on a case-by-case basis.

Inventory

Inventory is stated at the lower of cost and net realizable value, with cost being determined on a first in, first out basis. The Company’s inventory is primarily comprised of finished goods.

Property and Equipment and Capitalized Software

Property and equipment are recorded at cost. Maintenance and repairs are charged to expense as incurred and costs of improvements and renewals are capitalized. Depreciation and amortization are recognized using the straight-line method based on the estimated useful lives of the related assets. The Company uses an estimated useful life of three years for computers and software, five years for laboratory and office equipment, six years for devices in the rental agreement program and the lesser of five years or the remaining life of the underlying facility lease for leasehold improvements.

Software development costs relating to assets to be sold in the normal course of business are included in research and development and are expensed as incurred until technological feasibility is established. After technological feasibility is established, material software development costs are capitalized. The Company uses an estimated useful life of two years for capitalized software and amortizes these costs beginning at the product release.

Impairment of Long-Lived Assets

Long-lived assets, such as property and equipment, are tested for impairment when events or changes in circumstances indicate that the carrying value of the asset may not be recoverable. Impairment testing requires management to estimate the future net undiscounted cash flows of an asset using assumptions believed to be reasonable. Actual cash flows may differ from the estimates used in the impairment testing. If such assets are considered to be impaired, the Company recognizes an impairment loss when and to the extent that the estimated fair value of an asset is less than its carrying value. The Company has not recorded any impairment of its long-lived assets for the years ended December 31, 2018, 2017, and 2016.

Warrant Liability

The Company’s current and previous credit facilities required the Company to issue to the lender of warrants to purchase the Company’s convertible preferred stock at the date of borrowing. Because the convertible preferred stock warrants were a form of a contingently redeemable instrument, they were classified as liabilities on the Company’s balance sheet. At the date of borrowing, the Company bifurcated the estimated fair value of the convertible preferred stock warrants from the proceeds from borrowing, resulting in the recognition of a debt discount, and recorded a warrant liability on its balance sheet. This warrant liability was revalued at each reporting period, with changes in fair value recorded in the Company’s statement of operations as a component of other income or expense. The valuation of the warrant liability was based upon estimates of the fair value of the underlying convertible preferred stock and the related volatility and expected term for an illiquid instrument, which could vary significantly from period to period.

Immediately prior to the closing of the IPO on July 2, 2018, all of the Company’s outstanding convertible preferred stock warrants converted into common stock warrants. The warrant liability was remeasured at its estimated fair value and reclassified to additional paid-in capital on the Company’s balance sheet.

Deferred Debt Issuance Costs

The Company capitalizes direct costs incurred to obtain debt financing and amortizes these costs to interest expense over the term of the debt using the effective interest method. These costs are recorded as a debt discount are netted against the related debt on the Company’s balance sheet.

Revenue Recognition

Revenue is recognized when title and risk of ownership has been transferred, provided that persuasive evidence of an arrangement exists, the price is fixed and determinable and collectability is reasonably assured. Transfer of title and risk of ownership occurs when the product is shipped or transferred to the customer. The Company sells to end users in the United States and to third-party distributors outside the United States and does not provide return rights. Sales to distributors outside the United States are in U.S. dollars.

The Company generates revenues from sales of NeuroStar Advanced Therapy Systems and treatment sessions. NeuroStar Advanced Therapy System revenue consists primarily of a capital component, including updates to the equipment, attributable to the initial sale of the NeuroStar system unit. NeuroStar Advanced Therapy Systems can be purchased outright or on a rent-to-own basis by certain customers. Treatment session revenue primarily includes sales of NeuroStar treatment sessions and SenStar treatment links. The NeuroStar treatment sessions are access codes delivered electronically in the United States. The SenStar treatment links are disposable units containing single-use access codes that are sold and used outside the United States. Access codes are purchased separately by customers, primarily on an as-needed basis, and are required by the NeuroStar Advanced Therapy System in order to deliver the treatment sessions.

The Company’s NeuroStar Advanced Therapy System sales in the United States typically have a post-sale training obligation. This obligation is fulfilled after product shipment, and the Company defers recognizing revenue until training occurs. In accordance with the accounting guidance related to multiple element arrangements, the Company defers the fair value attributable to the post-shipment training and recognizes such revenue when the obligation is fulfilled. The Company bases the fair value of the training using stand-alone service rates. The Company’s sales to its third-party distributors outside the United States do not have these post-sale obligations. The Company’s treatment sessions have no post-sale obligations and no return rights. Revenues on the sales of treatment sessions are recognized upon delivery. Revenue related to operating leases for the Company’s NeuroStar Advanced Therapy System is recognized over the term of the lease. Revenue attributable to the NeuroStar Advanced Therapy Systems purchased on a rent-to-own basis are accounted for as operating leases and revenue is recognized on a straight-line basis over the term of the lease.

The Company provides a one to two-year warranty for systems sold in the United States. Terms of product warranty differ amongst its third-party distributors outside the United States but are generally three years or less. The Company provides for the estimated cost to repair or replace products under any warranty at the time of sale. The Company also offers its customers in the United States annual service contracts. Revenue from the sale of annual service contracts is recognized on a straight-line basis over the period of the applicable contract. The Company also earns revenue from customers from services outside of their warranty term or annual service contracts. Such service revenue is recognized as the services are provided.

Research and Development Expenses

Research and development activities are expensed as incurred. Costs incurred in obtaining technology licenses are charged immediately to research and development expense if the technology licensed has not reached technological feasibility and has no alternative future uses.

Share-based Compensation

The Company recognizes the grant-date fair value of share-based awards issued as compensation as expense on a straight-line basis over the requisite service period, which is generally the vesting period of the award. To date, the Company has not issued awards where vesting is subject to market conditions. There was one performance grant in 2018 to the board of directors subject to completion of the IPO, which was achieved in July 2018. The fair value of restricted stock awards and restricted stock units is estimated at the time of grant, based on the grant date fair value of the Company’s common stock. The fair value of stock options is estimated at the time of grant using the Black-Scholes option pricing model, which requires the use of inputs and assumptions such as the fair value of the underlying common stock, exercise price of the option, expected term, risk-free interest rate, expected volatility and dividend yield, the most critical of which is the fair value of the Company’s common stock prior to the IPO.

The fair value of each grant of stock options awarded during the years ended December 31, 2018, 2017 and 2016 was determined using the following methods and assumptions:

•

Fair Value of Common Stock. Prior to the IPO, its board of directors periodically estimated the fair value of the Company’s common stock considering, among other things, contemporaneous valuations of its preferred and common stock prepared by an unrelated third-party valuation firm in accordance with the guidance provided by the American Institute of Certified Public Accountants 2013 Practice Aid, Valuation of Privately-Held-Company Equity Securities Issued as Compensation. Since the closing of the IPO, its board of directors has determined the per share fair value of the Company’s common stock based on the closing price as reported by the Nasdaq Global Market on the date of grant.

•

Expected Term. Due to the historical lack of a public market for the trading of the Company’s common stock and the lack of sufficient company-specific historical data, the expected term of employee stock options is determined using the “simplified” method, as prescribed in SEC Staff Accounting Bulletin (SAB) No. 107 (SAB 107), whereby the expected life equals the arithmetic average of the vesting term and the original contractual term of the option. The expected term of nonemployee stock options is equal to the contractual term.

•

Risk-free Interest Rate. The risk-free interest rate is based on the interest rate payable on United States Treasury securities in effect at the time of grant for a period that is commensurate with the assumed expected term.

•

Expected Volatility. The expected volatility is based on historical volatilities of peer companies within the Company’s industry which were commensurate with the expected term assumption, as described in SAB 107.

•

Dividend Yield. The dividend yield is 0% because the Company has never paid, and for the foreseeable future does not expect to pay, a dividend on its common stock.

The inputs and assumptions used to estimate the fair value of share-based payment awards represent management’s best estimates and involve inherent uncertainties and the application of management’s judgment. As a result, if factors change and management uses different inputs and assumptions, the Company’s share-based compensation expense could be materially different for future awards.

Income Taxes

Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in operations in the period that includes the enactment date. A valuation allowance is recorded to the extent it is more likely than not that some portion or all of the deferred tax assets will not be realized.

Unrecognized income tax benefits represent income tax positions taken on income tax returns that have not been recognized in the financial statements. The Company recognizes the benefit of an income tax position only if it is more likely than not (greater than 50%) that the tax position will be sustained upon tax examination, based solely on the technical merits of the tax position. Otherwise, no benefit is recognized. The tax benefits recognized are measured based on the largest benefit that has a greater than 50% likelihood of being realized upon ultimate settlement. The Company accrues interest and related penalties are classified as income tax expense in the statements of operations. The Company does not anticipate significant changes in the amount of unrecognized income tax benefits over the next year.

As of December 31, 2018 and 2017, the Company had deferred tax assets of $53.8 million and $47.3 million, respectively; these deferred tax assets are primarily attributable to federal and state net operating loss carryforwards. Although the loss carryforwards are available to offset future taxable income, they will begin to expire in 2020 for state and 2023 for federal. In addition, prior ownership changes may create a limitation in the Company’s ability to use the net operating loss carryforwards for federal and state income tax purposes. These loss carryforwards have been fully offset by a valuation allowance because management does not consider realization of these deferred tax assets to be more likely than not.

Corporate tax reform was enacted on December 22, 2017 and is effective for the Company for year ended December 31, 2017. The provisions of the corporate tax reform did not have any impact to the Company due to the full valuation allowance position. As a result of the reduced corporate rate, the Company’s deferred tax assets were revalued from 34% to 21%, which was fully offset by a reduction in the valuation allowance. In connection with the corporate tax reform, the Medical Device Tax was suspended for another two years.

Recent Accounting Pronouncements

12 Months Ended

Dec. 31, 2018

New Accounting Pronouncements And Changes In Accounting Principles [Abstract]

Recent Accounting Pronouncements

4.	RECENT ACCOUNTING PRONOUNCEMENTS

JOBS Act Accounting Election

The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012, or the JOBS Act. Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it (i) is no longer an emerging growth company or (ii) affirmatively and irrevocably opts out of the extended transition period provided in the JOBS Act. As a result, these financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates.

In May 2014, the FASB issued ASU 2014-09, “Revenue from Contracts with Customers” (Topic 606), regarding the accounting for and disclosures of revenue recognition, with an effective date for public companies of annual and interim periods beginning after December 15, 2016. In July 2015, the FASB issued ASU 2015-14, “Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date,” which deferred the effective date of the previously issued revenue recognition guidance by one year. This guidance is effective for public companies for annual and interim periods beginning after December 15, 2017. For all other entities, including emerging growth companies, this standard is effective for annual reporting periods beginning after December 15, 2018, and interim periods with annual periods beginning after December 15, 2019. Early adoption is permitted.

In April 2016 and May 2016, the FASB issued ASU 2016-10, “Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing” and ASU 2016-12, “Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients,” respectively. Both updates provide improvements and clarification to the previously issued revenue recognition guidance.

The new revenue standard provides a single comprehensive model for accounting for revenue from contracts with customers. The model requires that revenue recognized reflects the actual consideration to which the entity expects to be entitled in exchange for the goods or services defined in the contract, including in situations with multiple performance obligations. In addition, the standard requires quantitative and qualitative disclosures of the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers, as well as any judgements related to the standard.

The new revenue standard is principles-based and interpretation of those principles may vary from company to company based on their unique circumstances. It is possible that interpretation, industry practice, and guidance may evolve as companies and the accounting profession work to implement this new standard. The Company will continue to monitor industry activities and any additional guidance provided by regulators, standards setters, or the accounting profession. The Company has also implemented internal controls and processes to enable the preparation of financial information and has reached conclusions on key accounting assessments related to the standard.

The new standard can be adopted using one of two methods: the full retrospective method, which requires the standard to be applied to each prior period presented, or the modified retrospective method, which requires the cumulative effect of adoption to be recognized as an adjustment to opening retained earnings in the period of adoption. The Company adopted the standard on January 1, 2019, using the modified retrospective approach and applying the completed contracts practical expedient, with the cumulative effect of adoption recorded within retained earnings on January 1, 2019. However, the Company concluded that all sales prior to January 1, 2019 are completed contracts, as all or substantially all the revenue related to these sales has been recognized as of that date. Therefore, there was no cumulative effect of adoption recorded within retained earnings on January 1, 2019.

The Company determined that the new guidance will not have a material impact on its revenue recognition practices for NeuroStar Advance Therapy Systems and Treatment Sessions. However, the standard changes the Company’s accounting treatment for incremental costs to obtain a contract, including sales commissions. The incremental costs of obtaining a contract are those that an entity incurs to obtain a contract with a customer that it would not have incurred if the contract had not been obtained.

Historically, the Company expensed incremental costs to obtain a contract as incurred, including the sales commissions paid for NeuroStar Advance Therapy Systems and Treatment Sessions. Under Topic 606, the Company will capitalize a portion of the sales commissions paid on NeuroStar Advanced Therapy Systems sold after January 1, 2019 and amortize the expense on a straight-line basis over a seven-year period, which is consistent with the transfer of specific anticipated Treatment Session contracts to customers.

Additionally, the standard changes the Company’s accounting treatment for milestone payments, specifically related to a future contingent milestone payment in the Company’s distribution agreement with Teijin Pharma Limited, or Teijin. Teijin is required to pay the Company a milestone payment tied to Japanese Ministry of Health, Labour and Welfare, or JMHLW, issuing reimbursement for use of its products for the treatment of MDD in Japan. The initial reimbursement amount received from JMHLW is subject to a revision period based the completion of post marketing surveillance studies, which in turn would cause a revision in the milestone payment. The Company estimated the revenue likely to be earned, subject to the constraint that the amount recorded is not probable of a significant revenue reversal, by probability weighting likely reimbursement outcomes and the related milestone payment amounts. As a result of this analysis, the Company concluded that the new guidance will not have an impact on the Company’s revenue as of the adoption date.

In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), with guidance regarding the accounting for and disclosure of leases. The update requires lessees to recognize all leases, including operating leases, with a term greater than 12 months on the balance sheet. This update also requires lessees and lessors to disclose key information about their leasing transactions. This guidance will be effective for public companies for annual and interim periods beginning after December 15, 2018. For all other entities, including emerging growth companies, this standard will be effective for annual reporting periods beginning after December 15, 2019, and interim periods within annual periods beginning after December 15, 2020. Early adoption is permitted.

As a lessee, leases will be classified as either finance or operating, with classification affecting the pattern of expense recognition in the income statement. As a lessor, leases will be classified as either direct financing, sales-type, or operating, with classification affecting the pattern of revenue recognition and related expense, in the income statement. Additionally, the new standard requires quantitative and qualitative disclosures of amount, timing, and judgments related to the accounting for leases and the related cash flows.

The leases standard is required to be applied to leases in existence as of the date of initial application using a modified retrospective transition approach; a full retrospective transition approach is not permitted. The Company adopted the standard on January 1, 2019, using the effective date modified retrospective method transition. Under this adoption method, comparative periods are presented in accordance with Topic 840 and do not include any retrospective adjustments to reflect the adoption of Topic 842.

The Company elected the package of practical expedients permitted under the transition guidance within the new standard, which among other things, allows the Company to carryforward the historical lease classification. The Company made an accounting policy election to keep leases with an initial term of 12 months or less off the balance sheet. The Company will recognize those lease payments in the Statements of Operations on a straight-line basis over the lease term. The Company made an accounting policy election to present all funds collected from lessees for sales and other similar taxes net of the related sales tax expense.

As a lessee, the Company will recognize additional right of use assets and lease liabilities of approximately $1.0 million as of January 1, 2019. The right of use asset and lease liabilities primarily relate to the Company’s current non-cancelable office lease with an expiration date in 2021.

Certain costumers have purchased NeuroStar Advanced Therapy Systems on a rent-to-own basis. Prior to adoption of Topic 842, revenue attributable to the NeuroStar Advanced Therapy Systems purchased on a rent-to-own basis were accounted for as operating leases and revenue is recognized on a straight-line basis over the term of the lease. Given the package of practical expedients elected, the Company will continue this revenue recognition for NeuroStar Advanced Therapy Systems sold on a rent-to-own basis as of December 31, 2018.

By granting a customer the right to use a NeuroStar Advanced Therapy System, the Company is performing a revenue generating activity and the revenue recognition should be consistent with the framework in Topic 606. Due to developing customer purchase history, the Company has recently determined customers who purchase NeuroStar Advanced Therapy Systems on a rent-to-own basis are now reasonably certain to exercise the purchase option at the end of the lease. Therefore, NeuroStar Advanced Therapy Systems purchased on a rent-to-own basis will be accounted for as sales-type leases.

As a sales-type lease classification under Topic 842, selling profit or loss will be recognized at lease commencement and the underlying NeuroStar Advance Therapy System will be derecognized. The Company will record a corresponding lease receivable equal to the present value of the future lease payments, adjusted prospectively by interest income and payments. As of January 1, 2019, there was no impact on the Company’s financial statements related to the NeuroStar Advanced Therapy Systems purchased on a rent-to-own basis.

In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718): “Improvements to Nonemployee Share-Based Payment Accounting,” which largely aligns the accounting for share-based payment awards issued to nonemployees with the accounting for share-based payment awards issued to employees. Under previous GAAP, the accounting for nonemployee share-based payments differed from that applied to employee awards, particularly with regard to the measurement date and the impact of performance conditions. Under the new guidance, (i) equity-classified share-based payment awards issued to nonemployees will be measured at the grant date, instead of the previous requirement to remeasure the awards through the performance completion date, (ii) for performance conditions, compensation cost associated with the award will be recognized when the achievement of the performance condition is probable, rather than upon achievement of the performance condition, and (iii) the current requirement to reassess the classification (equity or liability) for nonemployee awards upon vesting will be eliminated, except for awards in the form of convertible instruments. This new guidance will be effective for public companies for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. The Company currently has no outstanding nonemployee share-based payment awards for which there is unrecognized compensation expense and therefore does not currently expect the adoption of the new guidance to have an effect on its financial statements.

Fair Value Measurement and Financial Instruments

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurement and Financial Instruments

5.	FAIR VALUE MEASUREMENT AND FINANCIAL INSTRUMENTS

The carrying values of cash equivalents, accounts receivable, prepaids and other current assets, and accounts payable on the Company’s balance sheets approximated their fair values as of December 31, 2018 and 2017 due to their short-term nature. The carrying values of the Company’s current credit facility approximated its fair value as of December 31, 2018 and 2017 due to its variable interest rate.

Certain of the Company’s financial instruments are measured at fair value using a three-level hierarchy that prioritizes the inputs used to measure fair value. This hierarchy maximizes the use of observable inputs and minimizes the use of unobservable inputs. The three levels of inputs used to measure fair value are as follows:

Level 1:	Inputs are quoted prices for identical instruments in active markets.

Level 2:	Inputs are quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; or model-derived valuations whose inputs are observable or whose significant value drivers are observable.

Level 3:	Inputs are unobservable and reflect the Company’s own assumptions, based on the best information available, including the Company’s own data.

The following tables set forth the carrying amounts and fair values of the Company’s financial instruments as of December 31, 2018 and 2017 (in thousands):

	December 31, 2018
					Fair Value Measurement Based on
	Carrying Amount		Fair Value		Quoted Prices In Active Markets (Level 1)		Significant other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets
Money market funds (cash equivalents)	$	87,062	$	87,062	$	87,062	$	-	$	-

	December 31, 2017
					Fair Value Measurement Based on
	Carrying Amount		Fair Value		Quoted Prices In Active Markets (Level 1)		Significant other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets
Money market funds (cash equivalents)	$	11,149	$	11,149	$	11,149	$	-	$	-
Liabilities
Convertible preferred stock warrant liability	$	478	$	478	$	-	$	-	$	478

The fair value of the convertible preferred stock warrant liability was estimated using the Black-Scholes option pricing model and the following inputs and assumptions as of December 31, 2017:

	December 31, 2017
	Series E			Series F
Estimated fair value of convertible preferred stock	$	9.57		$	11.01
Exercise price	$	19.55		$	9.73
Remaining term (in years)		5.0			3.1 - 7.0
Risk-free interest rate		2	%	2.0% - 2.3%
Expected volatility		43	%	43% - 44%
Dividend yield		0	%		0	%

The following table presents the changes in Level 3 instruments measured on a recurring basis for the years ended December 31, 2018 and 2017 (in thousands):

Balance at December 31, 2016	$	459
Issuance of warrants		290
Change in fair value		(271	)
Balance at December 31, 2017		478
Change in fair value		1,396
Reclassification to additional paid in capital		(1,874	)
Balance at December 31, 2018	$	-

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Accounts Receivable

12 Months Ended

Dec. 31, 2018

Receivables [Abstract]

Accounts Receivable

6.	ACCOUNTS RECEIVABLE

The following table presents the composition of accounts receivable, net as of December 31, 2018 and 2017 (in thousands):

	December 31,
	2018			2017
Gross accounts receivable - trade	$	6,120		$	4,684
Less: Allowances for doubtful accounts		(500	)		(417	)
Accounts receivable, net	$	5,620		$	4,267

Bad debt expense was $0.2 million, $0.1 million and $0.1 million for the years ended December 31, 2018, 2017 and 2016, respectively.

The following table presents a rollforward of the allowance for doubtful accounts (in thousands):

	Balance at Beginning of Period			Bad Debt Expense Recognized			Write-offs of Uncollectible Balances		Balance at End of Period
Year ended December 31, 2016	$	(274	)		(75	)		6	$	(343	)
Year ended December 31, 2017	$	(343	)		(116	)		42	$	(417	)
Year ended December 31, 2018	$	(417	)		(153	)		70	$	(500	)

- Definition

The entire disclosure for claims held for amounts due a entity, excluding financing receivables. Examples include, but are not limited to, trade accounts receivables, notes receivables, loans receivables. Includes disclosure for allowance for credit losses.

+ References + Details

Property and Equipment and Capitalized Software

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Property and Equipment and Capitalized Software

7.	PROPERTY AND EQUIPMENT AND CAPITALIZED SOFTWARE

The following table presents the composition of property and equipment, net as of December 31, 2018 and 2017 (in thousands):

	December 31,
	2018			2017
Laboratory equipment	$	150		$	150
Office equipment		487			487
Computer equipment and software		1,050			680
Manufacturing equipment		273			273
Leasehold improvements		172			153
Rental equipment		1,262			1,447
Property and equipment, gross		3,394			3,190
Less: Accumulated depreciation		(2,016	)		(1,831	)
Property and equipment, net	$	1,378		$	1,359

As of December 31, 2018 and 2017, the Company had capitalized software costs, net of $1.0 million and $0.4 million, respectively, which are included in “Other assets” on the balance sheet.

Depreciation and amortization expense was $0.9 million, $0.6 million and $0.7 million for the years ended December 31, 2018, 2017 and 2016, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2018

Payables And Accruals [Abstract]

Accrued expenses

8.	ACCRUED EXPENSES

The following table presents the composition of accrued expenses as of December 31, 2018 and 2017 (in thousands):

	December 31,
	2018		2017
Compensation and related benefits	$	4,909	$	4,465
Consulting and professional fees		342		461
Research and development expenses		191		497
Sales and marketing expenses		127		620
Warranty		629		570
Sales tax payable		606		322
Interest payable		251		188
Other		493		388
Accrued expenses	$	7,548	$	7,511

Debt

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Debt

DEBT

The following table presents the composition of debt as of December 31, 2018 and 2017 (in thousands):

	December 31,
	2018			2017
Outstanding principal	$	30,000		$	30,000
Accrued final payment fees		1,468			940
Less debt discounts		(1,073	)		(1,384	)
Total long-term debt, net		30,395			29,556
Less current portion of long-term debt		-			-
Long-term debt, net	$	30,395		$	29,556

For the year ended December 31, 2018, the Company recognized interest expense of $3.7 million, of which $2.9 million was cash and $0.8 million was non-cash interest expense related to the amortization of deferred debt issuance costs and accrual of final payment fees. For the year ended December 31, 2017, the Company recognized interest expense of $2.8 million, of which $2.1 million was cash and $0.7 million was non-cash interest expense related to the amortization of deferred debt issuance costs and accrual of final payment fees. For the year ended December 31, 2016, the Company recognized interest expense of $1.8 million, of which $1.4 million was cash and $0.4 million was non-cash interest expense related to the amortization of deferred debt issuance costs and accrual of final payment fees.

Current Credit Facility

In March 2017, the Company entered into a new loan and security agreement with Oxford Finance LLC, or Oxford, for a credit facility that replaced its previous $25.0 million credit facility with Oxford and which allowed it to borrow up to $35.0 million in three tranches of term loans: a Term A Loan in the amount of $25.0 million, which was drawn down immediately upon closing in March 2017, a Term B Loan in the amount of $5.0 million, which was drawn down in December 2017, and a Term C Loan in the amount of $5.0 million, which became available to the Company upon the achievement of $45.0 million of trailing twelve month revenues during the second quarter of 2018. Upon achieving the required revenue milestone, the Company had 60 days to notify Oxford if it elected to borrow the Term C Loan. As a result of completing its IPO on July 2, 2018 and receiving the net proceeds therefrom, the Company elected not to borrow the additional $5.0 million, and it is no longer available to the Company. Each term loan accrues interest from the date of borrowing through the date of repayment at a floating per annum rate of interest, which resets monthly and is equal to the greater of (a) 8.15% or (b) the 30-day U.S. LIBOR on the last business day of the month plus 7.38%. The Company was also required to issue to Oxford at the date of each borrowing warrants to purchase its Series F or later series of convertible preferred stock with a seven-year term and in an amount equal to 3.95% of the first $5.0 million of each tranche borrowed. The credit facility matures and all amounts borrowed thereunder are due on March 1, 2022. As of December 31, 2018, the Company had borrowed and had outstanding an aggregate of $30.0 million of principal under the credit facility.

The Term A Loan featured an interest-only period through March 2019, during which time the Company was required to make monthly interest payments, after which time the Company was required to make monthly payments of principal and interest based on a 36-month amortization schedule. However, due to the achievement of $45.0 million of revenues during the fourth quarter of 2018, the interest-only period was extended for an additional 12 months through March 2020, after which time the Company will be required to make monthly payments of principal and interest based on a 24-month amortization schedule. In connection with the drawdown of the Term A Loan, the Company issued to Oxford a warrant to purchase shares of its Series F convertible preferred stock. On July 2, 2018, this convertible preferred stock warrant converted into a warrant to purchase 20,303 shares of the Company’s common stock at an exercise price of $9.73 per share. The warrant will expire in March 2024.

The Term B Loan featured an interest-only period through March 2019, during which time the Company was required to make monthly interest payments, after which time the Company was required to make monthly payments of principal and interest based on a 36-month amortization schedule. However, due to the achievement of $45.0 million of revenues during the fourth quarter of 2018, the interest-only period was extended for an additional 12 months through March 2020, after which time the Company will be required to make monthly payments of principal and interest based on a 24-month amortization schedule. In connection with the drawdown of the Term B Loan, the Company issued to Oxford a warrant to purchase shares of its Series F convertible preferred stock. On July 2, 2018, this convertible preferred stock warrant converted into a warrant to purchase 20,303 shares of the Company’s common stock at an exercise price of $9.73 per share. The warrant will expire in December 2024.

In addition to principal and interest payments due under the credit facility, the Company is required to make final payment fees to Oxford due upon the earlier of prepayment or maturity of each tranche, which increased as a result the extension of the interest-only period and are now equal to 8.5% and 7.5% of the principal amounts of the Term A and Term B Loans, respectively. The Company accrues the estimated final payment fees using the effective interest rate, with a charge to non-cash interest expense, over the term of borrowing for each tranche. As of December 31, 2018, the effective interest rates for the Term A and Term B Loans were 11.87% and 12.14%, respectively. As of December 31, 2017, the effective interest rates for the Term A and Term B Loans were 10.7% and 11.6%, respectively. If the Company prepays its term loans prior to their respective scheduled maturities, it will also be required to make prepayment fees to Oxford equal to 3% if prepaid on or before the first anniversary of funding, 2% if prepaid after the first and on or before the second anniversary of funding, or 1% if prepaid after the second anniversary of funding of the principal amounts borrowed.

The Company’s obligations under the credit facility are secured by a first priority security interest in substantially all of its assets, other than its intellectual property. The Company has agreed not to pledge or otherwise encumber any of its intellectual property. The loan and security agreement related to the credit facility includes a financial maintenance covenant that requires the Company to achieve at least 75% of its trailing 12-month forecasted revenues, as measured each month in accordance with a forecast that the Company provided to Oxford upon signing the agreement and future forecasts that the Company is required to deliver to Oxford each year for the life of the credit facility, as well as customary affirmative and negative covenants. The Company was in compliance with all of the covenants under its credit facility as of December 31, 2018.

The loan and security agreement related to the credit facility contains events of default, including, without limitation, events of default upon: (i) failure to make payment pursuant to the terms of the agreement; (ii) violation of covenants; (iii) material adverse changes to the Company’s business; (iv) attachment or levy on the Company’s assets or judicial restraint on its business; (v) insolvency; (vi) significant judgments, orders or decrees for payments by the Company not covered by insurance; (vii) incorrectness of representations and warranties; (viii) incurrence of subordinated debt; (ix) revocation of governmental approvals necessary for the Company to conduct its business; and (x) failure by the Company to maintain a valid and perfected lien on the collateral securing the borrowing.

Based on a 24-month amortization of the outstanding principal amounts of the Term A and Term B Loans beginning on April 1, 2020 as discussed above, the following table sets forth by year the Company’s required future principal payments (in thousands):

Year:	Principal Payments
2019	$	-
2020		11,250
2021		15,000
2022		3,750
Total principal payments	$	30,000

Previous $25.0 Million Credit Facility

Prior to March 2017, the Company had a $25.0 million credit facility in place with Oxford, which it entered into in February 2014 and which allowed it to borrow up to $25.0 million in three tranches of term loans: a Term A Loan in the amount of $15.0 million, a Term B Loan in the amount of $5.0 million and a Term C Loan in the amount of $5.0 million, which was never drawn down. Each term loan accrued interest at per annum rates ranging from 8.5% to 8.9%. This facility featured an interest-only period on all tranches through March 2017, and the Company was also required to issue convertible preferred stock warrants to Oxford at the time of borrowing of each tranche. These convertible preferred stock warrants converted into common stock warrants immediately prior to the closing of the Company’s IPO on July 2, 2018.

In addition to principal and interest payments due under the previous $25.0 million credit facility, the Company was required to make final payment fees to Oxford upon the earlier of prepayment or maturity and equal to 8.5% and 4.7% of the principal amounts of the Term A and Term B Loans, respectively. The Company accrued final payment fees using the effective interest rate, with a charge to non-cash interest expense, over the term of borrowing and until its entry into the current credit facility in March 2017, at which time the Company paid Oxford $1.0 million in satisfaction of all final payment fee liabilities due under the prior credit facility.

Management evaluated whether the current credit facility entered into in March 2017 represented a debt modification or extinguishment in accordance with ASC 470-50, Debt—Modifications and Extinguishments. Upon determining that the change in cash flows between the previous and current credit facilities was not greater than 10%, management accounted for the transaction as a debt modification. As of March 2017, the unamortized balance of deferred debt issuance costs incurred in connection with the $25.0 million credit facility, and certain additional deferred debt issuance costs incurred in connection with entry into the current credit facility, are being amortized to interest expense through March 2022 utilizing the effective interest method.

Convertible Preferred Stock and Stockholders' Deficit

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Convertible Preferred Stock and Stockholders' Deficit

10.	CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ DEFICIT

Initial Public Offering

On July 2, 2018, the Company closed its IPO, in which the Company issued and sold 6.325 million shares of its common stock, which included shares sold pursuant to an option granted to the underwriters to purchase additional shares, at a public offering price of $17.00 per share. The Company received net proceeds of $96.5 million after deducting underwriting discounts, commissions and other offering expenses paid by the Company. In addition, immediately prior to the closing of the IPO on July 2, 2018, (i) all of the Company’s outstanding shares of convertible preferred stock converted into an aggregate of 11.0 million shares of common stock; (ii) all of the Company’s outstanding warrants to purchase convertible preferred stock converted into warrants to purchase common stock; and (iii) the Company filed an amended and restated certificate of incorporation to, among other things, decrease the number of shares of common stock, $0.01 par value per share, authorized for issuance to 200.0 million and to authorize the board of directors to issue up to 10.0 million shares of “blank check” preferred stock, $0.01 par value per share.

Common Stock

The Company’s amended and restated certificate of incorporation as of December 31, 2018 authorized the issuance of 200.0 million shares of common stock, $0.01 par value per share, of which 17.744 million were issued and outstanding as of December 31, 2018.

Prior to the IPO, the Company was required to reserve and keep available out of its authorized but unissued shares of common stock a number of shares sufficient to effect the conversion into common stock of all outstanding shares of convertible preferred stock and convertible preferred stock warrants, convertible preferred stock or common stock warrants issuable upon borrowing the Term C Loan under the current credit facility, stock options granted and shares available for grant under its stock incentive plan.

The following table summarizes the total number of shares of the Company’s common stock issued and reserved for issuance as of December 31, 2018 and 2017 (in thousands):

	December 31,
	2018		2017
Shares of common stock issued		17,744		231
Shares of common stock reserved for issuance for:
Convertible preferred stock outstanding:
Series A-1		-		166
Series A-2		-		898
Series B		-		697
Series C		-		1,063
Series D		-		1,705
Series E		-		1,534
Series F		-		3,531
Series G		-		1,400
Convertible preferred stock warrants outstanding:
Series E		-		14
Series F		-		91
Warrants issuable upon Term C Loan borrowing		-		20
Common stock warrants outstanding		105		-
Stock options outstanding		2,711		2,444
Restricted stock units outstanding		43		-
Shares available for grant under stock incentive plan		1,312		246
Shares available for sale under employee stock purchase plan		244		-
Total shares of common stock issued and reserved for issuance		22,159		14,040

Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Holders of common stock are entitled to receive any dividends that the Company’s board of directors may declare out of funds legally available for that purpose on a non-cumulative basis. The Company has never paid, and for the foreseeable future does not expect to pay, a dividend on its common stock.

Convertible Preferred Stock

Prior to the filing of the July 2, 2018 amended certificate of incorporation, the Company’s amended certificate of incorporation authorized the issuance of 308.6 million shares of convertible preferred stock, $0.01 par value per share, of which the Company had designated and issued Series A-1, Series A-2, Series B, Series C, Series D, Series E, Series F and Series G shares. Series A-1 through Series E shares of convertible preferred stock are referred to collectively as Junior Securities and are subordinate to shares of Series G and Series F convertible preferred stock. All of the Company’s convertible preferred stock was classified outside of stockholders’ equity (deficit) because the shares contained deemed liquidation rights that were a contingent redemption feature not solely within the control of the Company.

The following table summarizes the Company’s outstanding convertible preferred stock as of December 31, 2017:

	Shares Authorized and Designated (in thousands)		Shares Issued and Outstanding (in thousands)		Carrying Value (in thousands)		Liquidation Value per Share		Liquidation Value (in thousands)
Series A-1		4,800		4,800	$	900	$	0.0617	$	296
Series A-2		25,385		25,385		16,428	$	0.2052		5,209
Series B		17,000		17,000		16,859	$	0.3168		5,386
Series C		20,958		20,958		34,841	$	0.5253		11,009
Series D		49,426		49,426		29,970	$	0.2874		14,205
Series E		44,873		44,471		29,800	$	0.5144		22,876
Series F		105,567		102,334		43,513	$	0.3356		34,343
Series G		40,584		40,584		14,825	$	0.3696		15,000
Balance at December 31, 2017		308,593		304,958	$	187,136			$	108,324

Conversion

Each share and series of convertible preferred stock was convertible into common stock at any time at the option of the holder thereof at the conversion price then in effect (each subject to adjustments upon the occurrence of certain dilutive events). As of July 2, 2018, the conversion price for Series A-1, Series D, Series E, Series F and Series G shares was equal to the original issue price, resulting in a common stock conversion ratio of 1:0.0345. As of July 2, 2018, as a result of past anti-dilution adjustments, the conversion price for Series A-2, Series B and Series C shares was below the original issue price, resulting in common stock conversion ratios of 1:0.03539, 1:0.04103 and 1:0.05071, respectively. Immediately prior to the closing of the IPO on July 2, 2018, all of the Company’s outstanding shares of convertible preferred stock converted into 11.0 million shares of common stock.

Liquidation Preferences

As of December 31, 2017

In the event of a liquidation, dissolution or winding up of the Company, either voluntary or involuntary, or in the event of a deemed liquidation event, which includes a sale of the Company as defined in the Company’s certificate of incorporation, holders of Series G convertible preferred stock were entitled to receive, in preference to all other stockholders, an amount equal to their original investment amount plus any declared and unpaid dividends. If upon the occurrence of such event the assets and funds available for distribution are insufficient to pay such holders the full amount to which they are entitled, then the entire assets and funds legally available for distribution would be distributed ratably among the holders of the Series G convertible preferred stock in proportion to the full amounts to which they would otherwise be entitled.

After payment in full of the liquidation preference of the Series G convertible preferred stock, holders of Series F convertible preferred stock were entitled to receive, in preference to all holders of Junior Securities and common stock, an amount equal to their original investment amount plus any declared and unpaid dividends. If upon the occurrence of such event the assets and funds available for distribution are insufficient to pay such holders the full amount to which they are entitled, then the entire remaining assets and funds legally available for distribution would be distributed ratably among the holders of the Series F convertible preferred stock in proportion to the full amounts to which they would otherwise be entitled.

After payment in full of the liquidation preferences of the Series G and Series F convertible preferred stock, holders of Junior Securities were entitled to receive an amount equal to $59.2 million in the aggregate. If upon the occurrence of such event the assets and funds available for distribution are insufficient to pay such holders the full amount to which they are entitled, then the entire assets and funds legally available for distribution would be distributed ratably among the holders of the Junior Securities in proportion to the full amounts to which they would otherwise be entitled.

After payment in full of the liquidation preferences of the Series G, Series F and Junior Securities convertible preferred stock, holders of Series F convertible preferred stock were entitled to receive an additional liquidation preference at an amount equal to $0.1678 per share. If upon the occurrence of such event the assets and funds available for distribution are insufficient to pay such holders the full amount to which they would be entitled, then the entire assets and funds legally available for distribution would be distributed ratably among the holders of the Series F convertible preferred stock in proportion to the full amounts to which they would otherwise be entitled.

After payment in full of the liquidation preferences of the Series G, Series F and Junior Securities convertible preferred stock and the additional liquidation preference for holders of Series F convertible preferred stock, holders of common stock and holders of Junior Securities, Series F and Series G convertible preferred stock would be entitled to receive a liquidation preference until the amount distributed to holders of the Series F convertible preferred stock equals $1.0068 plus declared but unpaid dividends on each share and then to the holders of common stock and holders of Junior Securities and Series G convertible preferred stock until the aggregate amount distributed to such holders equals the amount distributed to holders of Series F convertible preferred stock divided by the Series F ownership percentage.

After payments of the above liquidation preferences have been made, any remaining assets would be distributed ratably to holders of common stock and holders of Series G, Series F and Junior Securities convertible preferred stock on an “as-converted” basis.

As of July 2, 2018

Immediately prior to the closing of the IPO on July 2, 2018, all of the Company’s outstanding shares of convertible preferred stock converted into 11.0 million shares of common stock, resulting in the elimination of the Company’s outstanding liquidation preferences.

Dividends

Each class of convertible preferred stock was entitled to receive non-cumulative annual dividends at a rate of 9.0%, if and when declared by the Company’s board of directors. The holders of Series G convertible preferred stock were entitled to dividends in preference to holders of any other class or series of the Company’s stock. The holders of Series F convertible preferred stock were entitled to dividends in preference to all holders of Junior Securities and holders of common stock. The holders of Junior Securities were entitled to dividends in preference to holders of common stock.

In the event a dividend was declared to common stockholders, holders of each class of convertible preferred stock would also receive an equivalent dividend on an “as-converted” basis.

Voting

The holders of each class of convertible preferred stock were entitled to one vote for each share of common stock into which their shares of convertible preferred stock may be converted and, subject to certain convertible preferred stock class votes specified in the Company’s certificate of incorporation or as required by law, the holders of convertible preferred stock and common stock vote together on an “as-converted” basis.

Common Stock Warrants

The following table summarizes the Company’s outstanding common stock warrants as of December 31, 2018:

Warrants Outstanding (in thousands)		Exercise Price		Expiration Date
	14	$	19.55	Dec-2022
	30	$	9.73	Feb-2021
	20	$	9.73	Aug-2023
	20	$	9.73	Mar-2024
	21	$	9.73	Dec-2024
	105

Convertible Preferred Stock Warrants

The following table summarizes the Company’s outstanding convertible preferred stock warrants as of December 31, 2017:

	Warrants Outstanding (in thousands)		Exercise Price		Expiration Date
Series E		402	$	0.6746	Dec-2022
Series F		878	$	0.3356	Feb-2021
Series F		589	$	0.3356	Aug-2023
Series F		589	$	0.3356	Mar-2024
Series F		588	$	0.3356	Dec-2024
		3,046

Loss Per Share

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Loss Per Share

11.	LOSS PER SHARE

The Company’s basic loss per common share is computed by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period. The Company’s restricted stock awards (non-vested shares) are issued and outstanding at the time of grant but are excluded from the Company’s computation of weighted-average shares outstanding in the determination of basic loss per share until vesting occurs.

A net loss cannot be diluted, so when the Company is in a net loss position, basic and diluted loss per common share are the same. If in the future the Company achieves profitability, the denominator of a diluted earnings per common share calculation will include both the weighted-average number of shares outstanding and the number of common stock equivalents, if the inclusion of such common stock equivalents would be dilutive. Dilutive common stock equivalents potentially include warrants, stock options and non-vested restricted stock awards and units using the treasury stock method, along with the effect, if any, from the potential conversion of outstanding securities, such as convertible preferred stock.

The following potentially dilutive securities outstanding as of December 31, 2018, 2017 and 2016 have been excluded from the denominator of the diluted loss per share of common stock outstanding calculation (in thousands):

	December 31,
	2018		2017		2016
Stock options		2,711		2,444		1,826
Non-vested restricted stock awards		4		16		22
Non-vested restricted stock units		43		-		-
Convertible preferred stock warrants		-		105		64
Common stock warrants		105		-		-
Shares of convertible preferred stock “as-converted”		-		10,994		9,594

Share-based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Share-based Compensation

12.	SHARE-BASED COMPENSATION

The amount of share-based compensation expense recognized by the Company by location in its statements of operations for the years ended December 31, 2018, 2017 and 2016 is as follows (in thousands):

	Year ended December 31,
	2018		2017		2016
Cost of revenues	$	21	$	18	$	5
Sales and marketing		926		141		49
General and administrative		646		226		70
Research and development		150		111		37
Total	$	1,743	$	496	$	161

2018 Equity Incentive Plan

In June 2018, the Company adopted the 2018 Equity Incentive Plan, or 2018 Plan, which authorized the issuance of up to 1.4 million shares in the form of restricted stock, stock appreciation rights and stock options to the Company’s directors, employees and consultants. The amount and terms of grants are determined by the Company’s board of directors. All stock options granted to date have had exercise prices equal to the fair value, as determined by the closing price as reported by the Nasdaq Global Market on the date of grant, of the underlying common stock on the date of grant. The contractual term of stock options is up to 10 years, and stock options are exercisable in cash or as otherwise determined by the board of directors. Generally, stock options vest 25% upon the first anniversary of the date of grant and the remainder ratably monthly thereafter for 36 months. As of December 31, 2018, there were 1.3 million shares available for future issuance under the 2018 Plan.

2003 Stock Incentive Plan

In April 2003 (and as subsequently amended), the Company adopted the 2003 Stock Incentive Plan, or 2003 Plan, which authorized the issuance of up to 3.1 million shares in the form of restricted stock, stock appreciation rights and stock options to the Company’s directors, employees and consultants. The amount and terms of grants are determined by the Company’s board of directors. All stock options granted to date have had exercise prices equal to the estimated fair value, as determined by the board of directors, of the underlying common stock on the date of the grant. The contractual term of stock options is up to 10 years, and stock options are exercisable in cash or as otherwise determined by the board of directors. Generally, stock options vest 25% upon the first anniversary of the date of grant and the remainder ratably monthly thereafter for 36 months. As of December 31, 2018, there were no shares available for future issuance under the 2003 Plan. As of the closing of the IPO, all shares available for issuance under the 2003 Plan were carried over to the newly adopted 2018 Plan.

Stock Options

The following table summarizes the Company’s stock option activity for the years ended December 31, 2018, 2017 and 2016:

	Number of Shares under Option (in thousands)			Weighted- average Exercise Price per Option		Weighted- average Remaining Contractual Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2015		1,767		$	2.52
Granted		255		$	2.45
Exercised		(19	)	$	3.58
Forfeited		(161	)	$	5.68
Expired		(16	)	$	19.63
Outstanding at December 31, 2016		1,826		$	2.07
Granted		967		$	3.03
Exercised		(33	)	$	1.05
Forfeited		(316	)	$	2.66
Outstanding at December 31, 2017		2,444		$	2.39
Granted		584		$	8.33
Exercised		(194	)	$	2.60
Forfeited		(123	)	$	3.73
Outstanding at December 31, 2018		2,711		$	3.59		7.3	$	42,773
Exercisable at December 31, 2018		1,613		$	2.22		6.2	$	27,921
Vested and expected to vest at December 31, 2018		2,711		$	3.59		7.3	$	42,773

The Company recognized share-based compensation expense related to stock options of $1.6 million, $0.4 million and $0.2 million for the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018, there was $3.1 million of total unrecognized compensation cost related to non-vested stock options which the Company expects to recognize over a weighted-average period of 3.1 years. The weighted-average grant-date fair value of stock options granted during the years ended December 31, 2018, 2017 and 2016 was estimated at $5.03, $1.45 and $1.16 per option. The total intrinsic value of stock options exercised during the years ended December 31, 2018, 2017 and 2016 was $3.3 million, $0.1 million and de minimis, respectively.

For the years ended December 31, 2018, 2017 and 2016, the grant-date fair value of stock options was estimated at the time of grant using the following weighted-average inputs and assumptions in the Black-Scholes option pricing model:

	2018			2017			2016
Estimated fair value of common stock	$	8.41		$	2.90		$	2.32
Exercise price	$	8.41		$	2.90		$	2.32
Expected term (in years)		6.0			6.0			6.0
Risk-free interest rate		2.7	%		2.0	%		1.4	%
Expected volatility		65.3	%		48.0	%		44.2	%
Dividend yield		0	%		0	%		0	%

In April 2018, the Company’s board of directors granted options to purchase 54,794 shares of common stock to the members of the board. These options have an exercise price of $5.22 and vest in 12 equal monthly installments beginning in March 2018; however, the entire grant was subject to forfeiture if an initial public offering of the Company’s common stock did not occur by December 31, 2018. The estimated grant-date fair value of the options awards was $0.2 million. Upon the closing of the IPO on July 2, 2018, the Company recognized the fair value of the vested portion of the awards as share-based compensation expense, with the unvested portion to be recognized as share-based compensation expense ratably over the remaining service period.

Restricted Stock Awards and Restricted Stock Units

The following table summarizes the Company’s restricted stock award and restricted stock unit activity for the years ended December 31, 2018, 2017 and 2016:

	Non-vested Restricted Stock Awards (in thousands)			Weighted- average Grant-date Fair Value		Non-vested Restricted Stock Units (in thousands)		Weighted- average Grant-date Fair Value
Non-vested at December 31, 2015		-		$	-		-	$	-
Granted		42		$	2.03		-	$	-
Vested		(20	)	$	2.03		-	$	-
Non-vested at December 31, 2016		22		$	2.03		-	$	-
Granted		11		$	4.06		-	$	-
Vested		(17	)	$	2.90		-	$	-
Non-vested at December 31, 2017		16		$	2.32		-	$	-
Granted		-		$	-		43	$	25.21
Vested		(12	)	$	2.32		-	$	25.92
Non-vested at December 31, 2018		4		$	2.32		43	$	25.21

The Company recognized share-based compensation expense related to restricted stock awards and restricted stock units of $0.2 million, $0.1 million and de minimis during the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018, there was $0.9 million of unrecognized compensation cost related to non-vested restricted stock awards and restricted stock units which the Company expects to recognize over a weighted-average period of 3.4 years. The total fair value at the vesting date of restricted stock awards and restricted stock units vested during the years ended December 31, 2018, 2017 and 2016 was $0.2 million, $0.1 million and de minimis, respectively.

Employee Benefit Plans

12 Months Ended

Dec. 31, 2018

Compensation And Retirement Disclosure [Abstract]

Employee Benefit Plans

13.	EMPLOYEE BENEFIT PLANS

401(k) Defined Contribution Plan

The Company maintains a 401(k) defined contribution retirement plan which covers all of its employees. Employees are eligible to participate on the first of the month following their date of hire. Under the 401(k) plan, participating employees may defer up to 100% of their pre-tax salary but not more than statutory limits. There is currently no employer matching of employee contributions and employee contributions vest immediately.

2018 Employee Stock Purchase Plan

In July 2018, the Company adopted the 2018 Employee Stock Purchase Plan, or 2018 ESPP, with an initial 0.2 million share reserve, subject to automatic annual increases on January 1st of each year for a period of up to ten years, as defined in the plan document. The purpose of the 2018 ESPP is to enhance employee interest in the success and progress of the Company by encouraging employee ownership of common stock of the Company. The 2018 ESPP provides the opportunity to purchase the Company’s common stock at a 15% discount to the market price through payroll deductions or lump sum cash investments. As of December 31, 2018, the Company had not yet approved any offering under the plan.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

14.	INCOME TAXES

The Company’s loss before income taxes was $24.1 million, $16.1 million and $11.2 million for the years ended December 31, 2018, 2017 and 2016, respectively, and was generated entirely in the United States. The Company did not record current or deferred income tax expense or benefit during the years ended December 31, 2018, 2017 and 2016.

A reconciliation of the statutory United States federal income tax rate to the Company’s effective tax rate is as follows:

	Tax Year ended December 31,
	2018			2017			2016
U.S. federal statutory income tax rate		21.0	%		34.0	%		34.0	%
State and local taxes, net of federal benefit		3.8	%		2.7	%		2.6	%
Nondeductible expenses		0.7	%		-0.3	%		-1.2	%
Research and development credits		0.6	%		1.3	%		2.0	%
Tax rate change and true-up		0.6	%		-138.0	%		-1.2	%
Change in valuation allowance		-26.7	%		100.3	%		-36.2	%
Effective income tax rate		0.0	%		0.0	%		0.0	%

The tax effects of temporary differences that gave rise to significant portions of the deferred tax assets were as follows (in thousands):

	December 31,
	2018			2017
Deferred tax assets:
Net operating loss carryforwards	$	46,352		$	41,430
Research and development credits		3,208			3,059
Share-based compensation		523			316
Accruals		853			753
Interest expense - 163(j)		592			-
Capitalized start-up costs		1,295			1,530
Other temporary differences		951			253
Gross deferred tax assets		53,774			47,341
Less: Valuation allowance		(53,774	)		(47,341	)
Net deferred taxes	$	-		$	-

In assessing the realizability of the net deferred tax asset, the Company considers all relevant positive and negative evidence in determining whether it is more likely than not that some portion or all of the deferred income tax assets will not be realized. The realization of the gross deferred tax assets is dependent on several factors, including the generation of sufficient taxable income prior to the expiration of the net operating loss carryforwards. The Company believes that it is more likely than not that the Company’s deferred income tax asset associated with its net operating losses will not be realized in the immediate future. As such, there is a full valuation allowance against the net deferred tax assets as of December 31, 2018 and 2017. The valuation allowance increased / (decreased) by $6.4 and $(16.1) million during the years ended December 31, 2018 and 2017, respectively, due primarily to the generation of net operating losses and the federal tax rate reduction during the periods.

	December 31,
	2018		2017
Balance at the beginning of the period	$	47,341	$	63,449
Amounts charges to expense	$	6,433	$	6,099
Tax rate change (34% to 21%)		-		(22,207	)
Balance at the end of the period	$	53,774	$	47,341

The following table summarizes carryforwards of federal net operating losses and tax credits as of December 31, 2018 (in thousands):

	Amount		Expiration Beginning in
Federal net operating losses	$	187	2023
State net operating losses	$	112	2020
Research and development credits	$	3	2023

Under the Tax Reform Act of 1986 (the Act), the net operating loss and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. Net operating loss and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant shareholders over a three-year period in excess of 50 percent, as defined under Sections 382 and 383 of the Internal Revenue Code, respectively, as well as similar state provisions. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years. The Company has not done an analysis to determine whether or not ownership changes, as defined by the Act, have occurred since inception.

The Company will recognize interest and penalties related to uncertain tax positions in income tax expense. As of December 31, 2018, the Company had no accrued interest or penalties related to uncertain tax positions and no amounts have been recognized in the Company’s statements of operations. Due to net operating loss and tax credit carry forwards that remain unutilized, income tax returns for tax years from inception through 2017 remain subject to examination by the taxing jurisdictions.

In December 2017, the Tax Cuts and Jobs Act (the “2017 Tax Act”) was enacted. The 2017 Tax Act includes a number of changes to existing U.S. tax laws that impact the company, most notably a reduction of the U.S. corporate income tax rate from 35 percent to 21 percent for tax years beginning after December 31, 2017. The 2017 Tax Act also provides for a one-time transition tax on certain foreign earnings and the acceleration of depreciation for certain assets placed into service after September 27, 2017 as well as prospective changes beginning in 2018, including repeal of the domestic manufacturing deduction, acceleration of tax revenue recognition, capitalization of research and development expenditures, additional limitations on executive compensation and limitations on the deductibility of interest.

On December 22, 2017, the SEC staff issued Staff Accounting Bulletin No. 118 (“SAB 118”), which provided guidance on accounting for the federal tax rate change and other tax effects of the Tax Act. SAB 118 provided a measurement period that should not extend beyond one year from the Tax Act enactment date for companies to complete the accounting under ASC 740, Income Taxes. In connection with our adoption of the Tax Act and in consideration of SAB 118, there were no changes made to the provisional amounts recognized in 2017 in connection with the enactment of the Tax Reform Act. The accounting for the income tax effects of the Tax Reform Act is complete as of December 31, 2018.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

15.	COMMITMENTS AND CONTINGENCIES

Executive Employment Agreements

The Company has entered into an employment agreement and offer letters with certain key executives, providing for compensation and severance in certain circumstances, as defined in the agreements.

Leases

In January 2013, the Company entered into a 93-month lease for its headquarters office and warehouse. The Company also rents certain office equipment. The Company recognizes rent expense on a straight-line basis over the lease period and has accrued for rent expense incurred but not yet paid. Landlord allowances for tenant improvements are deferred and recognized as a reduction to rent expense on a straight-line basis and over the remaining lease term.

Rent expense under operating leases was $0.5 million, $0.5 million and $0.6 for the years ended December 31, 2018, 2017 and 2016, respectively.

The following is a schedule of future minimum annual payments at December 31, 2018 under non-cancelable operating lease agreements (in thousands):

For the years ending December 31,
2019	$	547
2020		560
2021		88
Total future minimum lease payments	$	1,195

Legal Matters

The Company is subject from time to time to various claims and legal actions arising during the ordinary course of its business. Management believes that there are currently no claims or legal actions that would reasonably be expected to have a material adverse effect on the Company’s results of operations, financial condition or cash flows.

Distribution Agreement with Teijin Pharma Limited

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Distribution Agreement with Teijin Pharma Limited

16.	DISTRIBUTION AGREEMENT WITH TEIJIN PHARMA LIMITED

In October 2017, the Company entered into a seven-and-a-half-year distribution agreement with Teijin Pharma Limited, or Teijin, for the exclusive distribution of its NeuroStar Advanced Therapy System to customers who will treat patients with MDD in Japan. Under the distribution agreement, Teijin is generally restricted from selling competing products in Japan. The distribution agreement provides that the Company will have primary responsibility for obtaining reimbursement approval for use of NeuroStar Advanced Therapy System for the treatment of MDD in Japan, and Teijin will promote the sales of NeuroStar Advanced Therapy System for treatment of MDD in Japan. The Company has agreed to provide sales and technical support training to Teijin for its NeuroStar Advanced Therapy Systems.

Teijin is required to purchase minimum dollar values of NeuroStar Advanced Therapy Systems and treatment sessions from the Company following reimbursement approval by the Japanese Ministry of Health, Labour and Welfare, or JMHLW, for TMS treatment for MDD (or, if such approval requires certified training on the NeuroStar Advanced Therapy System by a third party, then upon the first psychiatrist being issued his or her training certification).

In 2017, under the distribution agreement with Teijin, the Company received an upfront payment of $0.75 million and a milestone payment of $2.0 million following JMHLW’s approval of marketing the NeuroStar Advanced Therapy System for the treatment of MDD in Japan. These upfront and milestone payments have been deferred and are being recognized as revenue over the seven- and one-half year term of the agreement. Teijin is required to pay the Company a milestone payment tied to JMHLW issuing reimbursement for use of its products for the treatment of MDD in Japan.

The distribution agreement is scheduled to expire on March 31, 2025, subject to earlier termination if we or Teijin breach the agreement, Teijin fails to maintain distributor-level permits and approvals, Teijin fails to purchase from us specified dollar values of its sales forecasts, reimbursement for treatment of MDD using the NeuroStar Advanced Therapy System is not obtained from JMHLW by specified dates or such reimbursement is below specified minimums, Teijin reasonably believes that it is not commercially reasonable to continue distributing the NeuroStar Advanced Therapy System in Japan or bankruptcy related events occur. The term of the distribution agreement will be automatically extended for two years unless either party gives the other party at least two years’ prior written of notice of non-renewal, except that the Company cannot decline to renew the agreement if Teijin has purchased 100% of its sales forecasts over the term of the agreement.

Geographical Segment Information

12 Months Ended

Dec. 31, 2018

Segments Geographical Areas [Abstract]

Geographical Segment Information

17.	GEOGRAPHICAL SEGMENT INFORMATION

Operating segments are defined as components of an enterprise about which separate discrete information is available for evaluation by the chief operating decision maker, or decision making group, in deciding how to allocate resources and in assessing performance. The Company currently operates in one business segment as it is managed and operated as one business. A single management team that reports to the chief operating decision maker comprehensively manages the entire business. The Company does not operate any material separate lines of business or separate business entities with respect to its products or product development.

The Company’s revenue was generated in the following geographic regions for the years indicated (in thousands):

	Revenue by Geography Year ended December 31,
	2018					2017
	Amount		% of Revenues			Amount		% of Revenues
United States	$	51,477		98	%	$	39,853		99	%
International		1,299		2	%		580		1	%
Total revenues	$	52,776		100	%	$	40,433		100	%

Supplementary Financial Information (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Supplementary Financial Information (Unaudited)

18.	SUPPLEMENTARY FINANCIAL INFORMATION (Unaudited)

Selected financial information for the quarterly periods noted is as follows:

	Three Months ended
	March 31, 2018			June 30, 2018			September 30, 2018			December 31, 2018
	(in thousands except per share data)
Revenues	$	10,152		$	13,252		$	13,737		$	15,635
Cost of revenues		2,457			3,245			3,034			3,711
Gross Profit		7,695			10,007			10,703			11,924
Operating expenses:
Sales and marketing		8,109			9,835			9,672			10,648
General and administrative		2,636			3,078			3,238			4,715
Research and development		1,555			2,330			2,125			2,222
Total operating expenses		12,300			15,243			15,035			17,585
Loss from Operations		(4,605	)		(5,236	)		(4,332	)		(5,661	)
Other (income) expense:
Interest expense		921			900			928			939
Other (income) expense, net		(29	)		1,360			(299	)		(457	)
Net Loss	$	(5,497	)	$	(7,496	)	$	(4,961	)	$	(6,143	)
Net loss per share of common stock outstanding, basic and diluted	$	(24.43	)	$	(30.60	)	$	(0.29	)	$	(0.35	)
Weighted-average common shares outstanding, basic and diluted		226			245			17,382			17,655

	Three Months ended
	March 31, 2017			June 30, 2017			September 30, 2017			December 31, 2017
	(in thousands except per share data)
Revenues	$	7,526		$	10,308		$	10,491		$	12,108
Cost of revenues		1,538			2,501			2,636			2,957
Gross Profit		5,988			7,807			7,855			9,151
Operating expenses:
Sales and marketing		6,306			6,400			6,566			8,628
General and administrative		1,642			1,837			2,256			2,837
Research and development		2,028			2,147			1,843			1,919
Total operating expenses		9,976			10,384			10,665			13,384
Loss from Operations		(3,988	)		(2,577	)		(2,810	)		(4,233	)
Other (income) expense:
Interest expense		550			711			807			740
Other (income) expense, net		(24	)		(420	)		136			(49	)
Net Loss	$	(4,514	)	$	(2,868	)	$	(3,753	)	$	(4,924	)
Net loss per share of common stock outstanding, basic and diluted	$	(27.03	)	$	(16.58	)	$	(19.35	)	$	(23.34	)
Weighted-average common shares outstanding, basic and diluted		167			173			194			211

Quarterly computations of net loss per share amounts are made independently for each quarterly reporting period, and the sum of the per share amounts for the quarterly reporting periods may not equal the per share amounts for the year-to-date reporting period. Net loss per share for the three months ended September 30, 2018 and December 31, 2018 include, on a weighted-average basis, the 11.0 million shares of common stock issued upon the conversion of all outstanding shares of convertible preferred stock immediately prior to the closing of the IPO and 6.325 million shares of common stock issued upon the closing of the IPO on July 2, 2018.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Use of Estimates

Recapitalization

Cash and Cash Equivalents

Concentration of Credit Risk

Concentrations of Credit Risk

Allowance for Doubtful Accounts

Inventory

Inventory is stated at the lower of cost and net realizable value, with cost being determined on a first in, first out basis. The Company’s inventory is primarily comprised of finished goods.

Property and Equipment and Capitalized Software

Impairment of Long-Lived Assets

Warrant Liability

Deferred Debt Issuance Costs

Revenue Recognition

Research and Development Expenses

Share-based Compensation

The fair value of each grant of stock options awarded during the years ended December 31, 2018, 2017 and 2016 was determined using the following methods and assumptions:

•

Dividend Yield. The dividend yield is 0% because the Company has never paid, and for the foreseeable future does not expect to pay, a dividend on its common stock.

Income Taxes

Fair Value Measurement and Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Schedule of Carrying Amounts and Fair Values of Financial Instruments

The following tables set forth the carrying amounts and fair values of the Company’s financial instruments as of December 31, 2018 and 2017 (in thousands):

	December 31, 2018
					Fair Value Measurement Based on
	Carrying Amount		Fair Value		Quoted Prices In Active Markets (Level 1)		Significant other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets
Money market funds (cash equivalents)	$	87,062	$	87,062	$	87,062	$	-	$	-

	December 31, 2017
					Fair Value Measurement Based on
	Carrying Amount		Fair Value		Quoted Prices In Active Markets (Level 1)		Significant other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Assets
Money market funds (cash equivalents)	$	11,149	$	11,149	$	11,149	$	-	$	-
Liabilities
Convertible preferred stock warrant liability	$	478	$	478	$	-	$	-	$	478

Summary of Fair Value Inputs and Assumptions

The fair value of the convertible preferred stock warrant liability was estimated using the Black-Scholes option pricing model and the following inputs and assumptions as of December 31, 2017:

	December 31, 2017
	Series E			Series F
Estimated fair value of convertible preferred stock	$	9.57		$	11.01
Exercise price	$	19.55		$	9.73
Remaining term (in years)		5.0			3.1 - 7.0
Risk-free interest rate		2	%	2.0% - 2.3%
Expected volatility		43	%	43% - 44%
Dividend yield		0	%		0	%

Summary of Changes in Instruments Measured on Recurring Basis

The following table presents the changes in Level 3 instruments measured on a recurring basis for the years ended December 31, 2018 and 2017 (in thousands):

Balance at December 31, 2016	$	459
Issuance of warrants		290
Change in fair value		(271	)
Balance at December 31, 2017		478
Change in fair value		1,396
Reclassification to additional paid in capital		(1,874	)
Balance at December 31, 2018	$	-

Accounts Receivable (Tables)

12 Months Ended

Dec. 31, 2018

Receivables [Abstract]

Composition of Accounts Receivable, Net

The following table presents the composition of accounts receivable, net as of December 31, 2018 and 2017 (in thousands):

	December 31,
	2018			2017
Gross accounts receivable - trade	$	6,120		$	4,684
Less: Allowances for doubtful accounts		(500	)		(417	)
Accounts receivable, net	$	5,620		$	4,267

Allowance for Doubtful Accounts

The following table presents a rollforward of the allowance for doubtful accounts (in thousands):

	Balance at Beginning of Period			Bad Debt Expense Recognized			Write-offs of Uncollectible Balances		Balance at End of Period
Year ended December 31, 2016	$	(274	)		(75	)		6	$	(343	)
Year ended December 31, 2017	$	(343	)		(116	)		42	$	(417	)
Year ended December 31, 2018	$	(417	)		(153	)		70	$	(500	)

Property and Equipment and Capitalized Software (Tables)

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Summary of Composition of Property and Equipment, Net

The following table presents the composition of property and equipment, net as of December 31, 2018 and 2017 (in thousands):

	December 31,
	2018			2017
Laboratory equipment	$	150		$	150
Office equipment		487			487
Computer equipment and software		1,050			680
Manufacturing equipment		273			273
Leasehold improvements		172			153
Rental equipment		1,262			1,447
Property and equipment, gross		3,394			3,190
Less: Accumulated depreciation		(2,016	)		(1,831	)
Property and equipment, net	$	1,378		$	1,359

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2018

Payables And Accruals [Abstract]

Summary of Composition of Accrued Expenses

The following table presents the composition of accrued expenses as of December 31, 2018 and 2017 (in thousands):

	December 31,
	2018		2017
Compensation and related benefits	$	4,909	$	4,465
Consulting and professional fees		342		461
Research and development expenses		191		497
Sales and marketing expenses		127		620
Warranty		629		570
Sales tax payable		606		322
Interest payable		251		188
Other		493		388
Accrued expenses	$	7,548	$	7,511

Debt (Tables)

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Summary of Composition of Debt

The following table presents the composition of debt as of December 31, 2018 and 2017 (in thousands):

	December 31,
	2018			2017
Outstanding principal	$	30,000		$	30,000
Accrued final payment fees		1,468			940
Less debt discounts		(1,073	)		(1,384	)
Total long-term debt, net		30,395			29,556
Less current portion of long-term debt		-			-
Long-term debt, net	$	30,395		$	29,556

Summary of Future Principal Payments

Year:	Principal Payments
2019	$	-
2020		11,250
2021		15,000
2022		3,750
Total principal payments	$	30,000

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

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+ Details

Convertible Preferred Stock and Stockholders' Deficit (Tables)

12 Months Ended

Dec. 31, 2018

Summary of Common Stock Issued and Reserved for Issuance

The following table summarizes the total number of shares of the Company’s common stock issued and reserved for issuance as of December 31, 2018 and 2017 (in thousands):

	December 31,
	2018		2017
Shares of common stock issued		17,744		231
Shares of common stock reserved for issuance for:
Convertible preferred stock outstanding:
Series A-1		-		166
Series A-2		-		898
Series B		-		697
Series C		-		1,063
Series D		-		1,705
Series E		-		1,534
Series F		-		3,531
Series G		-		1,400
Convertible preferred stock warrants outstanding:
Series E		-		14
Series F		-		91
Warrants issuable upon Term C Loan borrowing		-		20
Common stock warrants outstanding		105		-
Stock options outstanding		2,711		2,444
Restricted stock units outstanding		43		-
Shares available for grant under stock incentive plan		1,312		246
Shares available for sale under employee stock purchase plan		244		-
Total shares of common stock issued and reserved for issuance		22,159		14,040

Summary of Outstanding Convertible Preferred Stock

The following table summarizes the Company’s outstanding convertible preferred stock as of December 31, 2017:

	Shares Authorized and Designated (in thousands)		Shares Issued and Outstanding (in thousands)		Carrying Value (in thousands)		Liquidation Value per Share		Liquidation Value (in thousands)
Series A-1		4,800		4,800	$	900	$	0.0617	$	296
Series A-2		25,385		25,385		16,428	$	0.2052		5,209
Series B		17,000		17,000		16,859	$	0.3168		5,386
Series C		20,958		20,958		34,841	$	0.5253		11,009
Series D		49,426		49,426		29,970	$	0.2874		14,205
Series E		44,873		44,471		29,800	$	0.5144		22,876
Series F		105,567		102,334		43,513	$	0.3356		34,343
Series G		40,584		40,584		14,825	$	0.3696		15,000
Balance at December 31, 2017		308,593		304,958	$	187,136			$	108,324

Convertible Preferred Stock

Summary of Outstanding Warrants

The following table summarizes the Company’s outstanding convertible preferred stock warrants as of December 31, 2017:

	Warrants Outstanding (in thousands)		Exercise Price		Expiration Date
Series E		402	$	0.6746	Dec-2022
Series F		878	$	0.3356	Feb-2021
Series F		589	$	0.3356	Aug-2023
Series F		589	$	0.3356	Mar-2024
Series F		588	$	0.3356	Dec-2024
		3,046

Common Stock

Summary of Outstanding Warrants

The following table summarizes the Company’s outstanding common stock warrants as of December 31, 2018:

Warrants Outstanding (in thousands)		Exercise Price		Expiration Date
	14	$	19.55	Dec-2022
	30	$	9.73	Feb-2021
	20	$	9.73	Aug-2023
	20	$	9.73	Mar-2024
	21	$	9.73	Dec-2024
	105

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

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- Definition

Tabular disclosure of warrants or rights issued. Warrants and rights outstanding are derivative securities that give the holder the right to purchase securities (usually equity) from the issuer at a specific price within a certain time frame. Warrants are often included in a new debt issue to entice investors by a higher return potential. The main difference between warrants and call options is that warrants are issued and guaranteed by the company, whereas options are exchange instruments and are not issued by the company. Also, the lifetime of a warrant is often measured in years, while the lifetime of a typical option is measured in months. Disclose the title of issue of securities called for by warrants and rights outstanding, the aggregate amount of securities called for by warrants and rights outstanding, the date from which the warrants or rights are exercisable, and the price at which the warrant or right is exercisable.

+ References + Details

Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Schedule of Potentially Dilutive Securities Outstanding Excluded from Diluted Loss Per Share Calculation

	December 31,
	2018		2017		2016
Stock options		2,711		2,444		1,826
Non-vested restricted stock awards		4		16		22
Non-vested restricted stock units		43		-		-
Convertible preferred stock warrants		-		105		64
Common stock warrants		105		-		-
Shares of convertible preferred stock “as-converted”		-		10,994		9,594

Share-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Share-Based Compensation Expense

The amount of share-based compensation expense recognized by the Company by location in its statements of operations for the years ended December 31, 2018, 2017 and 2016 is as follows (in thousands):

	Year ended December 31,
	2018		2017		2016
Cost of revenues	$	21	$	18	$	5
Sales and marketing		926		141		49
General and administrative		646		226		70
Research and development		150		111		37
Total	$	1,743	$	496	$	161

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the years ended December 31, 2018, 2017 and 2016:

	Number of Shares under Option (in thousands)			Weighted- average Exercise Price per Option		Weighted- average Remaining Contractual Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2015		1,767		$	2.52
Granted		255		$	2.45
Exercised		(19	)	$	3.58
Forfeited		(161	)	$	5.68
Expired		(16	)	$	19.63
Outstanding at December 31, 2016		1,826		$	2.07
Granted		967		$	3.03
Exercised		(33	)	$	1.05
Forfeited		(316	)	$	2.66
Outstanding at December 31, 2017		2,444		$	2.39
Granted		584		$	8.33
Exercised		(194	)	$	2.60
Forfeited		(123	)	$	3.73
Outstanding at December 31, 2018		2,711		$	3.59		7.3	$	42,773
Exercisable at December 31, 2018		1,613		$	2.22		6.2	$	27,921
Vested and expected to vest at December 31, 2018		2,711		$	3.59		7.3	$	42,773

Summary of Weighted-average Inputs and Assumptions used to Estimate Grant-date Fair Value of Stock Options

	2018			2017			2016
Estimated fair value of common stock	$	8.41		$	2.90		$	2.32
Exercise price	$	8.41		$	2.90		$	2.32
Expected term (in years)		6.0			6.0			6.0
Risk-free interest rate		2.7	%		2.0	%		1.4	%
Expected volatility		65.3	%		48.0	%		44.2	%
Dividend yield		0	%		0	%		0	%

Summary of Restricted Stock Award and Restricted Stock Unit Activity

The following table summarizes the Company’s restricted stock award and restricted stock unit activity for the years ended December 31, 2018, 2017 and 2016:

	Non-vested Restricted Stock Awards (in thousands)			Weighted- average Grant-date Fair Value		Non-vested Restricted Stock Units (in thousands)		Weighted- average Grant-date Fair Value
Non-vested at December 31, 2015		-		$	-		-	$	-
Granted		42		$	2.03		-	$	-
Vested		(20	)	$	2.03		-	$	-
Non-vested at December 31, 2016		22		$	2.03		-	$	-
Granted		11		$	4.06		-	$	-
Vested		(17	)	$	2.90		-	$	-
Non-vested at December 31, 2017		16		$	2.32		-	$	-
Granted		-		$	-		43	$	25.21
Vested		(12	)	$	2.32		-	$	25.92
Non-vested at December 31, 2018		4		$	2.32		43	$	25.21

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Reconciliation of Statutory Federal Income Tax Rate

A reconciliation of the statutory United States federal income tax rate to the Company’s effective tax rate is as follows:

	Tax Year ended December 31,
	2018			2017			2016
U.S. federal statutory income tax rate		21.0	%		34.0	%		34.0	%
State and local taxes, net of federal benefit		3.8	%		2.7	%		2.6	%
Nondeductible expenses		0.7	%		-0.3	%		-1.2	%
Research and development credits		0.6	%		1.3	%		2.0	%
Tax rate change and true-up		0.6	%		-138.0	%		-1.2	%
Change in valuation allowance		-26.7	%		100.3	%		-36.2	%
Effective income tax rate		0.0	%		0.0	%		0.0	%

Schedule of Deferred Tax Assets

The tax effects of temporary differences that gave rise to significant portions of the deferred tax assets were as follows (in thousands):

	December 31,
	2018			2017
Deferred tax assets:
Net operating loss carryforwards	$	46,352		$	41,430
Research and development credits		3,208			3,059
Share-based compensation		523			316
Accruals		853			753
Interest expense - 163(j)		592			-
Capitalized start-up costs		1,295			1,530
Other temporary differences		951			253
Gross deferred tax assets		53,774			47,341
Less: Valuation allowance		(53,774	)		(47,341	)
Net deferred taxes	$	-		$	-

Summary of Valuation Allowance

	December 31,
	2018		2017
Balance at the beginning of the period	$	47,341	$	63,449
Amounts charges to expense	$	6,433	$	6,099
Tax rate change (34% to 21%)		-		(22,207	)
Balance at the end of the period	$	53,774	$	47,341

Summary of Net Operating Loss Carryforwards and Tax Credits

The following table summarizes carryforwards of federal net operating losses and tax credits as of December 31, 2018 (in thousands):

	Amount		Expiration Beginning in
Federal net operating losses	$	187	2023
State net operating losses	$	112	2020
Research and development credits	$	3	2023

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Schedule of Future Minimum Annual Payments Under Non-Cancelable Operating Lease Agreements

The following is a schedule of future minimum annual payments at December 31, 2018 under non-cancelable operating lease agreements (in thousands):

For the years ending December 31,
2019	$	547
2020		560
2021		88
Total future minimum lease payments	$	1,195

Geographical Segment Information (Tables)

12 Months Ended

Dec. 31, 2018

Segments Geographical Areas [Abstract]

Summary of Revenue Generated in Geographic Regions for Years Indicated

The Company’s revenue was generated in the following geographic regions for the years indicated (in thousands):

	Revenue by Geography Year ended December 31,
	2018					2017
	Amount		% of Revenues			Amount		% of Revenues
United States	$	51,477		98	%	$	39,853		99	%
International		1,299		2	%		580		1	%
Total revenues	$	52,776		100	%	$	40,433		100	%

Supplementary Financial Information (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Summary of Selected Financial Information for Quarterly Periods

Selected financial information for the quarterly periods noted is as follows:

	Three Months ended
	March 31, 2018			June 30, 2018			September 30, 2018			December 31, 2018
	(in thousands except per share data)
Revenues	$	10,152		$	13,252		$	13,737		$	15,635
Cost of revenues		2,457			3,245			3,034			3,711
Gross Profit		7,695			10,007			10,703			11,924
Operating expenses:
Sales and marketing		8,109			9,835			9,672			10,648
General and administrative		2,636			3,078			3,238			4,715
Research and development		1,555			2,330			2,125			2,222
Total operating expenses		12,300			15,243			15,035			17,585
Loss from Operations		(4,605	)		(5,236	)		(4,332	)		(5,661	)
Other (income) expense:
Interest expense		921			900			928			939
Other (income) expense, net		(29	)		1,360			(299	)		(457	)
Net Loss	$	(5,497	)	$	(7,496	)	$	(4,961	)	$	(6,143	)
Net loss per share of common stock outstanding, basic and diluted	$	(24.43	)	$	(30.60	)	$	(0.29	)	$	(0.35	)
Weighted-average common shares outstanding, basic and diluted		226			245			17,382			17,655

	Three Months ended
	March 31, 2017			June 30, 2017			September 30, 2017			December 31, 2017
	(in thousands except per share data)
Revenues	$	7,526		$	10,308		$	10,491		$	12,108
Cost of revenues		1,538			2,501			2,636			2,957
Gross Profit		5,988			7,807			7,855			9,151
Operating expenses:
Sales and marketing		6,306			6,400			6,566			8,628
General and administrative		1,642			1,837			2,256			2,837
Research and development		2,028			2,147			1,843			1,919
Total operating expenses		9,976			10,384			10,665			13,384
Loss from Operations		(3,988	)		(2,577	)		(2,810	)		(4,233	)
Other (income) expense:
Interest expense		550			711			807			740
Other (income) expense, net		(24	)		(420	)		136			(49	)
Net Loss	$	(4,514	)	$	(2,868	)	$	(3,753	)	$	(4,924	)
Net loss per share of common stock outstanding, basic and diluted	$	(27.03	)	$	(16.58	)	$	(19.35	)	$	(23.34	)
Weighted-average common shares outstanding, basic and diluted		167			173			194			211

Description of Business - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jul. 02, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Description Of Business [Line Items]
IPO closing date	Jul. 02, 2018
Net proceeds from initial public offering		$ 99,998
Common stock, par value per share		$ 0.01	$ 0.01
Common stock, shares authorized for issuance		200,000,000	200,000,000
Preferred stock, shares authorized for issuance		10,000,000	10,000,000
Preferred stock, par value per share		$ 0.01	$ 0.01
Cash and cash equivalents		$ 104,583	$ 29,147
Accumulated deficit		221,043	196,946
Cash flows from operating activities		20,591	$ 11,144	$ 8,541
$35.0 Million Credit Facility \| Oxford Finance LLC
Description Of Business [Line Items]
Borrowings outstanding under credit facility		$ 30,000
Credit facility maturity date		Mar. 01, 2022
Common Stock
Description Of Business [Line Items]
Common stock shares issued and sold	6,325,000	6,325,000
Convertible preferred stock converted into common stock	11,000,000	10,994,000
Initial Public Offering
Description Of Business [Line Items]
Net proceeds from initial public offering	$ 96,500
Common stock, par value per share	$ 0.01
Common stock, shares authorized for issuance	200,000,000
Preferred stock, shares authorized for issuance	10,000,000
Preferred stock, par value per share	$ 0.01
Initial Public Offering \| Common Stock
Description Of Business [Line Items]
Common stock shares issued and sold	6,325,000
Issuance price per share	$ 17.00
Convertible preferred stock converted into common stock	11,000,000

Basis of Presentation - Additional Information (Details)		12 Months Ended
	Jun. 14, 2018	Dec. 31, 2018
Accounting Policies [Abstract]
Reverse stock split description		0.0345-for-1 reverse split
Reverse stock split, Conversion ratio	0.0345

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Cash held on deposits at financial institution in excess of FDIC Insured limit	$ 250,000
Impairment of long-lived assets	0	$ 0	$ 0
Gross deferred tax assets	$ 53,774,000	$ 47,341,000
Deferred tax asset revaluation percentage	21.00%	34.00%	34.00%
Medical device tax suspension period	2 years
State
Summary Of Significant Accounting Policies [Line Items]
Operating loss carryforwards begin to expire year	2020
Federal
Summary Of Significant Accounting Policies [Line Items]
Operating loss carryforwards begin to expire year	2023
Measurement Input, Expected Dividend Rate
Summary Of Significant Accounting Policies [Line Items]
Dividend yield percentage	0
Minimum \| US
Summary Of Significant Accounting Policies [Line Items]
Product warranty period	1 year
Maximum
Summary Of Significant Accounting Policies [Line Items]
Deferred tax asset revaluation percentage		35.00%
Maximum \| US
Summary Of Significant Accounting Policies [Line Items]
Product warranty period	2 years
Maximum \| Non US
Summary Of Significant Accounting Policies [Line Items]
Product warranty period	3 years
Computers and Software
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful life	3 years
Laboratory and Office Equipment
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful life	5 years
Devices in Rental Agreement Program
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful life	6 years
Leasehold Improvements
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful life	5 years
Capitalized Software
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful life	2 years

Recent Accounting Pronouncements - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Jan. 01, 2019
Lessee Lease Description [Line Items]
Current non-cancelable office lease, expiration date	2021
ASU 2016-12 \| Subsequent Event
Lessee Lease Description [Line Items]
Cumulative effect on retained earnings		$ 0
ASU 2016-02, Leases \| Scenario Plan
Lessee Lease Description [Line Items]
Operating ease, right of use assets		1,000,000
Operating lease liabilities		$ 1,000,000

Fair Value Measurement and Financial Instruments - Schedule of Carrying Amounts and Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Carrying Amount \| Convertible preferred sock warrant liability
Liabilities
Total liabilities		$ 478
Fair Value \| Convertible preferred sock warrant liability
Liabilities
Total liabilities		478
Significant Unobservable Inputs (Level 3) \| Convertible preferred sock warrant liability
Liabilities
Total liabilities		478
Money market funds \| Carrying Amount
Assets
Total assets	$ 87,062	11,149
Money market funds \| Fair Value
Assets
Total assets	87,062	11,149
Money market funds \| Quoted Prices In Active Markets (Level 1)
Assets
Total assets	$ 87,062	$ 11,149

Fair Value Measurement and Financial Instruments - Summary of Fair Value Inputs and Assumptions (Details)	12 Months Ended
	Dec. 31, 2017 $ / shares
Estimated Fair Value of Convertible Preferred Stock \| Series E
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value assumption	$ 9.57
Estimated Fair Value of Convertible Preferred Stock \| Series F
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value assumption	11.01
Exercise Price \| Series E
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Exercise price	19.55
Exercise Price \| Series F
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Exercise price	$ 9.73
Remaining Term (in years) \| Series E
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Remaining term (in years)	5 years
Remaining Term (in years) \| Series F \| Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Remaining term (in years)	3 years 1 month 6 days
Remaining Term (in years) \| Series F \| Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Remaining term (in years)	7 years
Risk-free Interest Rate \| Series E
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Risk-free interest rate	2
Risk-free Interest Rate \| Series F \| Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Risk-free interest rate	2.0
Risk-free Interest Rate \| Series F \| Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Risk-free interest rate	2.3
Expected Volatility \| Series E
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Risk-free interest rate	43
Expected Volatility \| Series F \| Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Risk-free interest rate	43
Expected Volatility \| Series F \| Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Risk-free interest rate	44
Measurement Input, Expected Dividend Rate \| Series E
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Risk-free interest rate	0
Measurement Input, Expected Dividend Rate \| Series F
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Risk-free interest rate	0

Fair Value Measurement and Financial Instruments - Summary of Changes in Instruments Measured on Recurring Basis (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Abstract]
Balance at December 31, 2016	$ 478	$ 459
Issuance of warrants		290
Change in fair value	1,396	(271)
Reclassification to additional paid in capital	(1,874)
Balance at December 31, 2017	$ 0	$ 478

Accounts Receivable - Composition of Accounts Receivable, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Receivables [Abstract]
Gross accounts receivable - trade	$ 6,120	$ 4,684
Less: Allowances for doubtful accounts	(500)	(417)	$ (343)	$ (274)
Accounts receivable, net	$ 5,620	$ 4,267

Accounts Receivable - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Receivables [Abstract]
Bad debt expense	$ 153	$ 116	$ 75

Accounts Receivable - Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Receivables [Abstract]
Balance at Beginning of Period	$ (417)	$ (343)	$ (274)
Bad Debt Expense Recognized	(153)	(116)	(75)
Write-offs of Uncollectible Balances	70	42	6
Balance at End of Period	$ (500)	$ (417)	$ (343)

Property and Equipment and Capitalized Software - Summary of Composition of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 3,394	$ 3,190
Less: Accumulated depreciation	(2,016)	(1,831)
Property and equipment, net	1,378	1,359
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	150	150
Office Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	487	487
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,050	680
Manufacturing Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	273	273
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	172	153
Rental Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 1,262	$ 1,447

Property and Equipment and Capitalized Software - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Depreciation and amortization expense	$ 0.9	$ 0.6	$ 0.7
Other Assets [Member]
Property Plant And Equipment [Line Items]
Capitalized software cost, net	$ 1.0	$ 0.4

Accrued Expenses - Summary of Composition of Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Compensation and related benefits	$ 4,909	$ 4,465
Consulting and professional fees	342	461
Research and development expenses	191	497
Sales and marketing expenses	127	620
Warranty	629	570
Sales tax payable	606	322
Interest payable	251	188
Other	493	388
Accrued expenses	$ 7,548	$ 7,511

Debt - Summary of Composition of Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
Outstanding principal	$ 30,000	$ 30,000
Accrued final payment fees	1,468	940
Less debt discounts	(1,073)	(1,384)
Total long-term debt, net	30,395	29,556
Long-term debt, net	$ 30,395	$ 29,556

Debt - Additional Information (Details) - USD ($) $ / shares in Units, shares in Thousands			1 Months Ended	12 Months Ended
	Jul. 02, 2018	Feb. 28, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Interest expense				$ 3,700,000	$ 2,800,000	$ 1,800,000
Cash interest expense				2,900,000	2,100,000	1,400,000
Non-cash interest expense				$ 800,000	$ 700,000	$ 400,000
Debt instrument, final payment fee			$ 1,000,000
$35.0 Million Credit Facility
Debt Instrument [Line Items]
Percentage of forecasted revenue to be achieved				75.00%
$35.0 Million Credit Facility \| Prepaid on or Before First Anniversary of Funding
Debt Instrument [Line Items]
Prepayment fees percentage				3.00%
$35.0 Million Credit Facility \| Prepaid After First and on or Before Second Anniversary of Funding
Debt Instrument [Line Items]
Prepayment fees percentage				2.00%
$35.0 Million Credit Facility \| Prepaid After Second Anniversary of Funding
Debt Instrument [Line Items]
Prepayment fees percentage				1.00%
$35.0 Million Credit Facility \| Oxford Finance LLC
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 25,000,000	35,000,000
Borrowings outstanding under credit facility				$ 30,000,000
Available borrowing achievement of trailing twelve month revenues			45,000,000
Debt instrument, interest rate				8.15%
Warrants term				7 years
Percentage of borrowing capacity of tranche				3.95%
Borrowing capacity of tranche				$ 5,000,000
Credit facility maturity date				Mar. 01, 2022
$35.0 Million Credit Facility \| Oxford Finance LLC \| LIBOR
Debt Instrument [Line Items]
Debt instrument, description of variable rate basis				30-day U.S. LIBOR
Basis spread on variable rate				7.38%
$35.0 Million Credit Facility \| Term A Loan
Debt Instrument [Line Items]
Interest-only period				2019-03
Amortization period without interest-only period extension				36 months
Extension of interest-only period upon achievement of revenues				12 months
Interest-only period upon achievement of revenues				2020-03
Amortization period upon extension of interest-only period				24 months
Increase in final payment fees percentage upon extension interest only periods				8.50%
Effective interest rates				11.87%	10.70%
$35.0 Million Credit Facility \| Term A Loan \| Common Stock
Debt Instrument [Line Items]
Warrants to purchase shares	20,303
Warrants, exercise price	$ 9.73
Warrants expiration period	2024-03
$35.0 Million Credit Facility \| Term A Loan \| Oxford Finance LLC
Debt Instrument [Line Items]
Borrowings outstanding under credit facility			25,000,000
$35.0 Million Credit Facility \| Term B Loan
Debt Instrument [Line Items]
Interest-only period				2019-03
Amortization period without interest-only period extension				36 months
Extension of interest-only period upon achievement of revenues				12 months
Interest-only period upon achievement of revenues				2020-03
Amortization period upon extension of interest-only period				24 months
Increase in final payment fees percentage upon extension interest only periods				7.50%
Effective interest rates				12.14%	11.60%
$35.0 Million Credit Facility \| Term B Loan \| Common Stock
Debt Instrument [Line Items]
Warrants to purchase shares	20,303
Warrants, exercise price	$ 9.73
Warrants expiration period	2024-12
$35.0 Million Credit Facility \| Term B Loan \| Oxford Finance LLC
Debt Instrument [Line Items]
Borrowings outstanding under credit facility			5,000,000
$35.0 Million Credit Facility \| Term C Loan \| Oxford Finance LLC
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity			$ 5,000,000
Notification period for lenders to borrow loan upon achieving required revenue milestone				60 days
Line of credit facility additional borrowing capacity				$ 5,000,000
$25.0 Million Credit Facility \| Oxford Finance LLC
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 25,000,000
$25.0 Million Credit Facility \| Term A Loan
Debt Instrument [Line Items]
Final payment fees percentage		8.50%
$25.0 Million Credit Facility \| Term A Loan \| Minimum
Debt Instrument [Line Items]
Debt instrument, accrued interest rate		8.50%
$25.0 Million Credit Facility \| Term A Loan \| Maximum
Debt Instrument [Line Items]
Debt instrument, accrued interest rate		8.90%
$25.0 Million Credit Facility \| Term A Loan \| Oxford Finance LLC
Debt Instrument [Line Items]
Borrowings outstanding under credit facility		$ 15,000,000
$25.0 Million Credit Facility \| Term B Loan
Debt Instrument [Line Items]
Final payment fees percentage		4.70%
$25.0 Million Credit Facility \| Term B Loan \| Minimum
Debt Instrument [Line Items]
Debt instrument, accrued interest rate		8.50%
$25.0 Million Credit Facility \| Term B Loan \| Maximum
Debt Instrument [Line Items]
Debt instrument, accrued interest rate		8.90%
$25.0 Million Credit Facility \| Term B Loan \| Oxford Finance LLC
Debt Instrument [Line Items]
Borrowings outstanding under credit facility		$ 5,000,000
$25.0 Million Credit Facility \| Term C Loan \| Oxford Finance LLC
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity		$ 5,000,000

Debt - Schedule of Future Principal Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total principal payments	$ 30,000	$ 30,000
Term A and Term B Loans
Debt Instrument [Line Items]
2020	11,250
2021	15,000
2022	3,750
Total principal payments	$ 30,000

Convertible Preferred Stock and Stockholders' Deficit - Additional Information (Details) - USD ($)		12 Months Ended
	Jul. 02, 2018	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Stockholders Deficit [Line Items]
IPO closing date	Jul. 02, 2018
Net proceeds from initial public offering		$ 99,998,000
Common stock, par value per share		$ 0.01		$ 0.01
Common stock, shares authorized for issuance		200,000,000		200,000,000
Preferred stock, shares authorized for issuance		10,000,000		10,000,000
Preferred stock, par value per share		$ 0.01		$ 0.01
Common stock, shares issued		17,744,000		231,000
Common stock, shares outstanding		17,744,000		231,000
Convertible preferred stock, shares authorized		308,593,000		308,593,000
Convertible preferred stock, par value		$ 0.01		$ 0.01
Liquidation preference		$ 0		$ 108,324,000
Convertible preferred stock non-cumulative dividends rate		9.00%
Common stock voting rights		one
Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock converted into common stock		(304,958,000)
Convertible preferred stock, shares authorized			308,600,000
Convertible preferred stock, par value			$ 0.01
Oxford Finance LLC
Stockholders Deficit [Line Items]
Convertible preferred stock converted to common stock, ratio	2898.55%
Series D Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock, shares authorized				49,426,000
Convertible preferred stock converted to common stock, ratio	2898.55%
Liquidation preference				$ 14,205,000
Series E Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock, shares authorized				44,873,000
Convertible preferred stock converted to common stock, ratio	2898.55%
Liquidation preference				$ 22,876,000
Series F Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock, shares authorized				105,567,000
Convertible preferred stock converted to common stock, ratio	2898.55%
Liquidation preference				$ 34,343,000
Additional liquidation preference				$ 0.1678
Additional liquidation preference per share due				$ 1.0068
Series G Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock, shares authorized				40,584,000
Convertible preferred stock converted to common stock, ratio	2898.55%
Liquidation preference				$ 15,000,000
Series A-2 Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock, shares authorized				25,385,000
Convertible preferred stock converted to common stock, ratio	2825.65%
Liquidation preference				$ 5,209,000
Series B Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock, shares authorized				17,000,000
Convertible preferred stock converted to common stock, ratio	2437.24%
Liquidation preference				$ 5,386,000
Series C Convertible Preferred Stock
Stockholders Deficit [Line Items]
Convertible preferred stock, shares authorized				20,958,000
Convertible preferred stock converted to common stock, ratio	1971.99%
Liquidation preference				$ 11,009,000
Junior Securities
Stockholders Deficit [Line Items]
Liquidation preference				$ 59,200,000
Common Stock
Stockholders Deficit [Line Items]
Common stock shares issued and sold	6,325,000	6,325,000
Convertible preferred stock converted into common stock	11,000,000	10,994,000
Initial Public Offering
Stockholders Deficit [Line Items]
IPO closing date	Jul. 02, 2018
Net proceeds from initial public offering	$ 96,500,000
Common stock, par value per share	$ 0.01
Common stock, shares authorized for issuance	200,000,000
Preferred stock, shares authorized for issuance	10,000,000
Preferred stock, par value per share	$ 0.01
Initial Public Offering \| Common Stock
Stockholders Deficit [Line Items]
Common stock shares issued and sold	6,325,000
Issuance price per share	$ 17.00
Convertible preferred stock converted into common stock	11,000,000

Convertible Preferred Stock and Stockholders' Deficit - Summary of Common Stock Issued and Reserved for Issuance (Details) - shares	Dec. 31, 2018	Jul. 31, 2018	Dec. 31, 2017
Stockholders Deficit [Line Items]
Shares of common stock issued	17,744,000		231,000
Shares of common stock reserved for issuance for:
Total shares of common stock issued and reserved for issuance	22,159,000		14,040,000
Series A-1 Convertible Preferred Stock
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance			166,000
Series A-2 Convertible Preferred Stock
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance			898,000
Series B Convertible Preferred Stock
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance			697,000
Series C Convertible Preferred Stock
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance			1,063,000
Series D Convertible Preferred Stock
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance			1,705,000
Series E Convertible Preferred Stock
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance			1,534,000
Series F Convertible Preferred Stock
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance			3,531,000
Series G Convertible Preferred Stock
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance			1,400,000
Series E Convertible Preferred Stock Warrants
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance			14,000
Series F Convertible Preferred Stock Warrants
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance			91,000
Common Stock Warrants Outstanding
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance	105,000
Warrants Issuable Upon Term C Loan Borrowing
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance			20,000
Shares Available for Sales Under Employee Stock Purchase Plan
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance	244,000	200,000
Stock Options Outstanding
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance	2,711,000		2,444,000
Shares Available for Grant Under Stock Incentive Plan
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance	1,312,000		246,000
Restricted Stock Units Outstanding
Shares of common stock reserved for issuance for:
Shares of common stock reserved for issuance	43,000

Convertible Preferred Stock and Stockholders' Deficit - Summary of Outstanding Convertible Preferred Stock (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Shares Authorized and Designated	308,593,000	308,593,000
Shares Issued	0	304,958,000
Shares Outstanding	0	304,958,000
Carrying Value		$ 187,136,000
Liquidation Value	$ 0	$ 108,324,000
Series A-1 Convertible Preferred Stock
Class Of Stock [Line Items]
Shares Authorized and Designated		4,800,000
Shares Issued		4,800,000
Shares Outstanding		4,800,000
Carrying Value		$ 900,000
Liquidation Value per Share		$ 0.0617
Liquidation Value		$ 296,000
Series A-2 Convertible Preferred Stock
Class Of Stock [Line Items]
Shares Authorized and Designated		25,385,000
Shares Issued		25,385,000
Shares Outstanding		25,385,000
Carrying Value		$ 16,428,000
Liquidation Value per Share		$ 0.2052
Liquidation Value		$ 5,209,000
Series B Convertible Preferred Stock
Class Of Stock [Line Items]
Shares Authorized and Designated		17,000,000
Shares Issued		17,000,000
Shares Outstanding		17,000,000
Carrying Value		$ 16,859,000
Liquidation Value per Share		$ 0.3168
Liquidation Value		$ 5,386,000
Series C Convertible Preferred Stock
Class Of Stock [Line Items]
Shares Authorized and Designated		20,958,000
Shares Issued		20,958,000
Shares Outstanding		20,958,000
Carrying Value		$ 34,841,000
Liquidation Value per Share		$ 0.5253
Liquidation Value		$ 11,009,000
Series D Convertible Preferred Stock
Class Of Stock [Line Items]
Shares Authorized and Designated		49,426,000
Shares Issued		49,426,000
Shares Outstanding		49,426,000
Carrying Value		$ 29,970,000
Liquidation Value per Share		$ 0.2874
Liquidation Value		$ 14,205,000
Series E Convertible Preferred Stock
Class Of Stock [Line Items]
Shares Authorized and Designated		44,873,000
Shares Issued		44,471,000
Shares Outstanding		44,471,000
Carrying Value		$ 29,800,000
Liquidation Value per Share		$ 0.5144
Liquidation Value		$ 22,876,000
Series F Convertible Preferred Stock
Class Of Stock [Line Items]
Shares Authorized and Designated		105,567,000
Shares Issued		102,334,000
Shares Outstanding		102,334,000
Carrying Value		$ 43,513,000
Liquidation Value per Share		$ 0.3356
Liquidation Value		$ 34,343,000
Series G Convertible Preferred Stock
Class Of Stock [Line Items]
Shares Authorized and Designated		40,584,000
Shares Issued		40,584,000
Shares Outstanding		40,584,000
Carrying Value		$ 14,825,000
Liquidation Value per Share		$ 0.3696
Liquidation Value		$ 15,000,000

Convertible Preferred Stock and Stockholders' Deficit - Summary of Outstanding Common Stock Warrants (Details) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Class Of Warrant Or Right [Line Items]
Warrants Outstanding	105	3,046
Exercise Price $19.55
Class Of Warrant Or Right [Line Items]
Warrants Outstanding	14
Exercise price	$ 19.55
Expiration Date	2022-12
Exercise Price $9.73
Class Of Warrant Or Right [Line Items]
Warrants Outstanding	30
Exercise price	$ 9.73
Expiration Date	2021-02
Exercise Price $9.73
Class Of Warrant Or Right [Line Items]
Warrants Outstanding	20
Exercise price	$ 9.73
Expiration Date	2023-08
Exercise Price $9.73
Class Of Warrant Or Right [Line Items]
Warrants Outstanding	20
Exercise price	$ 9.73
Expiration Date	2024-03
Exercise Price $9.73
Class Of Warrant Or Right [Line Items]
Warrants Outstanding	21
Exercise price	$ 9.73
Expiration Date	2024-12

Convertible Preferred Stock and Stockholders' Deficit - Summary of Outstanding Convertible Preferred Stock Warrants (Details) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Class Of Warrant Or Right [Line Items]
Warrants Outstanding	3,046	105
Series E Convertible Preferred Stock Warrants
Class Of Warrant Or Right [Line Items]
Warrants Outstanding	402
Exercise price	$ 0.6746
Expiration Date	2022-12
Series F Convertible Preferred Stock Warrants
Class Of Warrant Or Right [Line Items]
Warrants Outstanding	878
Exercise price	$ 0.3356
Expiration Date	2021-02
Series F Convertible Preferred Stock Warrants
Class Of Warrant Or Right [Line Items]
Warrants Outstanding	589
Exercise price	$ 0.3356
Expiration Date	2023-08
Series F Convertible Preferred Stock Warrants
Class Of Warrant Or Right [Line Items]
Warrants Outstanding	589
Exercise price	$ 0.3356
Expiration Date	2024-03
Series F Convertible Preferred Stock Warrants
Class Of Warrant Or Right [Line Items]
Warrants Outstanding	588
Exercise price	$ 0.3356
Expiration Date	2024-12

Loss Per Share - Schedule of Potentially Dilutive Securities Outstanding Excluded from Diluted Loss Per Share Calculation (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock Options Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding excluded from diluted loss per share	2,711	2,444	1,826
Non-vested Restricted Stock Awards
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding excluded from diluted loss per share	4	16	22
Non-vested Restricted Stock Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding excluded from diluted loss per share	43
Common Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding excluded from diluted loss per share	105
Convertible Preferred Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding excluded from diluted loss per share		105	64
Shares of Convertible Preferred Stock "as-converted"
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding excluded from diluted loss per share		10,994	9,594

Share-Based Compensation - Summary of Share-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	$ 1,743	$ 496	$ 161
Cost of Revenues
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	21	18	5
Sales and Marketing
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	926	141	49
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	646	226	70
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total	$ 150	$ 111	$ 37

Share-Based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Apr. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expenses		$ 1,743	$ 496	$ 161
Weighted-average grant-date fair value of stock options granted		$ 5.03	$ 1.45	$ 1.16
Intrinsic value of stock options exercised		$ 3,300	$ 100
Options to purchase shares	54,794
Option exercise price	$ 5.22
Stock option vesting period, monthly installment	12 months
Estimated grant-date fair value of option awards	$ 200
Stock Options Outstanding
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expenses		1,600	$ 400	$ 200
Unrecognized compensation cost related to non-vested stock options		$ 3,100
Non-vested awards not yet recognized weighted-average period for recognition		3 years 1 month 6 days
Options to purchase shares		584,000	967,000	255,000
Restricted Stock Awards and Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expenses		$ 200	$ 100
Non-vested awards not yet recognized weighted-average period for recognition		3 years 4 months 24 days
Unrecognized compensation cost related to non-vested restricted stock		$ 900
Fair value of restricted stock vested		$ 200	$ 100
2018 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting terms of stock options		stock options vest 25% upon the first anniversary of the date of grant and the remainder ratably monthly thereafter for 36 months
Shares available for future issuance		1,300,000
2018 Equity Incentive Plan \| First Anniversary of Date of Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting percentage of stock options		25.00%
2018 Equity Incentive Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized for issuance		1,400,000
Maximum contractual term of stock options		10 years
2003 Stock Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting terms of stock options		stock options vest 25% upon the first anniversary of the date of grant and the remainder ratably monthly thereafter for 36 months
Shares available for future issuance		0
2003 Stock Incentive Plan \| First Anniversary of Date of Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting percentage of stock options		25.00%
2003 Stock Incentive Plan \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized for issuance		3,100,000
Maximum contractual term of stock options		10 years

Share-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Apr. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares under Option, Granted	54,794
Stock Options Outstanding
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares under Option, Outstanding, Beginning balance		2,444,000	1,826,000	1,767,000
Number of Shares under Option, Granted		584,000	967,000	255,000
Number of Shares under Option, Exercised		(194,000)	(33,000)	(19,000)
Number of Shares under Option, Forfeited		(123,000)	(316,000)	(161,000)
Number of Shares under Option, Expired				(16,000)
Number of Shares under Option, Outstanding, Ending balance		2,711,000	2,444,000	1,826,000
Number of Shares under Option, Exercisable		1,613,000
Number of Shares under Option, Vested and expected to vest		2,711,000
Weighted-average Exercise Price per Option, Outstanding, Beginning balance		$ 2.39	$ 2.07	$ 2.52
Weighted-average Exercise Price per Option, Granted		8.33	3.03	2.45
Weighted-average Exercise Price per Option, Exercised		2.60	1.05	3.58
Weighted-average Exercise Price per Option, Forfeited		3.73	2.66	5.68
Weighted-average Exercise Price per Option Expired				19.63
Weighted-average Exercise Price per Option, Outstanding, Ending balance		3.59	$ 2.39	$ 2.07
Weighted-average Exercise Price per Option, Exercisable		2.22
Weighted-average Exercise Price per Option, Vested and expected to vest		$ 3.59
Weighted-average Remaining Contractual Life, Outstanding		7 years 3 months 18 days
Weighted-average Remaining Contractual Life, Exercisable		6 years 2 months 12 days
Weighted-average Remaining Contractual Life, Vested and expected to vest		7 years 3 months 18 days
Aggregate Intrinsic Value, Outstanding		$ 42,773
Aggregate Intrinsic Value, Exercisable		27,921
Aggregate Intrinsic Value, Vested and expected to vest		$ 42,773

Share-Based Compensation - Summary of Weighted-average Inputs and Assumptions used to Estimate Grant-date Fair Value of Stock Options (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Estimated fair value of common stock	$ 8.41	$ 2.90	$ 2.32
Exercise price	$ 8.41	$ 2.90	$ 2.32
Expected term (in years)	6 years	6 years	6 years
Risk-free interest rate	2.70%	2.00%	1.40%
Expected volatility	65.30%	48.00%	44.20%
Dividend yield	0.00%	0.00%	0.00%

Share-Based Compensation - Summary of Restricted Stock Award and Restricted Stock Unit Activity (Details) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted Stock Awards [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Non-vested, Beginning balance	16	22
Granted		11	42
Vested	(12)	(17)	(20)
Non-vested, Ending balance	4	16	22
Weighted-average Grant-date Fair Value, Non-vested, Beginning balance	$ 2.32	$ 2.03
Weighted-average Grant-date Fair Value, Granted		4.06	$ 2.03
Weighted-average Grant-date Fair Value, Vested	2.32	2.90	2.03
Weighted-average Grant-date Fair Value, Non-vested, Ending balance	$ 2.32	$ 2.32	$ 2.03
Restricted Stock Units Outstanding
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted	43
Non-vested, Ending balance	43
Weighted-average Grant-date Fair Value, Granted	$ 25.21
Weighted-average Grant-date Fair Value, Vested	25.92
Weighted-average Grant-date Fair Value, Non-vested, Ending balance	$ 25.21

Employee Benefit Plans - Additional Information (Details) - shares shares in Thousands	1 Months Ended	12 Months Ended
	Jul. 31, 2018	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined contribution retirement plan, plan name		401(k) plan
2018 ESPP
Defined Benefit Plan Disclosure [Line Items]
Shares of common stock reserved for issuance	200	244
Common stock purchase price, discount rate	15.00%
Maximum \| 2018 ESPP
Defined Benefit Plan Disclosure [Line Items]
Period of automatic annual increase in the share reserve	10 years
401(k) Plan
Defined Benefit Plan Disclosure [Line Items]
Defined contribution retirement plan, maximum employee contribution deferred		100.00%

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Loss before income taxes	$ 24,100,000	$ 16,100,000	$ 11,200,000
Deferred income tax expense or benefit	0	0	0
Current Income Tax Expense (Benefit)	0	0	$ 0
Increase/ (decrease) in valuation allowance	6,400,000	$ (16,100,000)
Accrued interest or penalties related to uncertain tax positions	0
Interest or penalties related to uncertain tax positions recognized in statements of operations and comprehensive loss	$ 0
Deferred tax asset revaluation percentage	21.00%	34.00%	34.00%
Maximum
Income Taxes [Line Items]
Deferred tax asset revaluation percentage		35.00%

Income Taxes - Schedule of Reconciliation of Statutory Federal Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
U.S. federal statutory income tax rate	21.00%	34.00%	34.00%
State and local taxes, net of federal benefit	3.80%	2.70%	2.60%
Nondeductible expenses	0.70%	(0.30%)	(1.20%)
Research and development credits	0.60%	1.30%	2.00%
Tax rate change and true-up	0.60%	(138.00%)	(1.20%)
Change in valuation allowance	(26.70%)	100.30%	(36.20%)
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Schedule of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 46,352	$ 41,430
Research and development credits	3,208	3,059
Share-based compensation	523	316
Accruals	853	753
Interest expense - 163(j)	592
Capitalized start-up costs	1,295	1,530
Other temporary differences	951	253
Gross deferred tax assets	53,774	47,341
Less: Valuation allowance	$ (53,774)	$ (47,341)

Income Taxes - Summary of Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Balance at the beginning of the period	$ 47,341	$ 63,449
Amounts charges to expense	6,433	6,099
Tax rate change (34% to 21%)		(22,207)
Balance at the end of the period	$ 53,774	$ 47,341

Income Taxes - Summary of Valuation Allowance (Parenthetical) (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Tax rate change	21.00%	34.00%	34.00%

Income Taxes - Summary of Net Operating Loss Carryforwards and Tax Credits (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Income Taxes [Line Items]
Net operating losses, Amount	$ 187
Net operating losses, Amount	112
Net operating losses, Amount	$ 3
Federal
Income Taxes [Line Items]
Net operating loss carryforwards, Expiration beginning	2023
State
Income Taxes [Line Items]
Net operating loss carryforwards, Expiration beginning	2020
Research and Development Credits
Income Taxes [Line Items]
Net operating loss carryforwards, Expiration beginning	2023

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 31, 2013
Commitments And Contingencies Disclosure [Abstract]
Lease term				93 months
Operating leases, rent expense, net	$ 0.5	$ 0.5	$ 0.6

Commitments and Contingencies - Schedule of Future Minimum Annual Payments Under Non-Cancelable Operating Lease Agreements (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019	$ 547
2020	560
2021	88
Total future minimum lease payments	$ 1,195

Distribution Agreement with Teijin Pharma Limited - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Term of distribution agreement		7 years 6 months
Distribution agreement, renewal period		2 years
Distribution term agreement	The term of the distribution agreement will be automatically extended for two years unless either party gives the other party at least two years’ prior written of notice of non-renewal, except that the Company cannot decline to renew the agreement if Teijin has purchased 100% of its sales forecasts over the term of the agreement.
Teijin \| Distribution Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement expiry date		Mar. 31, 2025
Extended term of agreement		2 years
Related party purchased percentage of sales forecasts over agreement term.		100.00%
Upfront Payment
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payment received		$ 750
Milestone Payment
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payment received		$ 2,000

Geographical Segment Information - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018 Segment
Segments Geographical Areas [Abstract]
Number of operating business segment	1

Geographical Segment Information - Summary of Revenue Generated in Geographic Regions for Years Indicated (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 15,635	$ 13,737	$ 13,252	$ 10,152	$ 12,108	$ 10,491	$ 10,308	$ 7,526	$ 52,776	$ 40,433	$ 34,228
Revenues \| Geographic Concentration
Segment Reporting Information [Line Items]
Revenues									$ 52,776	$ 40,433
Percentage of Revenues									100.00%	100.00%
Revenues \| United States \| Geographic Concentration
Segment Reporting Information [Line Items]
Revenues									$ 51,477	$ 39,853
Percentage of Revenues									98.00%	99.00%
Revenues \| International \| Geographic Concentration
Segment Reporting Information [Line Items]
Revenues									$ 1,299	$ 580
Percentage of Revenues									2.00%	1.00%

Supplementary Financial Information (Unaudited) - Summary of Selected Financial Information for Quarterly Periods (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 15,635	$ 13,737	$ 13,252	$ 10,152	$ 12,108	$ 10,491	$ 10,308	$ 7,526	$ 52,776	$ 40,433	$ 34,228
Cost of revenues	3,711	3,034	3,245	2,457	2,957	2,636	2,501	1,538	12,447	9,632	6,622
Gross Profit	11,924	10,703	10,007	7,695	9,151	7,855	7,807	5,988	40,329	30,801	27,606
Operating expenses:
Sales and marketing	10,648	9,672	9,835	8,109	8,628	6,566	6,400	6,306	38,264	27,900	21,794
General and administrative	4,715	3,238	3,078	2,636	2,837	2,256	1,837	1,642	13,667	8,572	6,926
Research and development	2,222	2,125	2,330	1,555	1,919	1,843	2,147	2,028	8,232	7,937	8,223
Total operating expenses	17,585	15,035	15,243	12,300	13,384	10,665	10,384	9,976	60,163	44,409	36,943
Loss from Operations	(5,661)	(4,332)	(5,236)	(4,605)	(4,233)	(2,810)	(2,577)	(3,988)	(19,834)	(13,608)	(9,337)
Other (income) expense:
Interest expense	939	928	900	921	740	807	711	550	3,688	2,808	1,835
Other (income) expense, net	(457)	(299)	1,360	(29)	(49)	136	(420)	(24)	575	(357)	62
Net Loss	$ (6,143)	$ (4,961)	$ (7,496)	$ (5,497)	$ (4,924)	$ (3,753)	$ (2,868)	$ (4,514)	$ (24,097)	$ (16,059)	$ (11,234)
Net loss per share of common stock outstanding, basic and diluted	$ (0.35)	$ (0.29)	$ (30.60)	$ (24.43)	$ (23.34)	$ (19.35)	$ (16.58)	$ (27.03)	$ (2.69)	$ (86.34)	$ (76.95)
Weighted-average common shares outstanding, basic and diluted	17,655	17,382	245	226	211	194	173	167	8,948	186	146

Supplementary Financial Information (Unaudited) - Additional Information (Details) - shares shares in Thousands		12 Months Ended
	Jul. 02, 2018	Dec. 31, 2018
Supplementary Financial Information [Line Items]
IPO closing date	Jul. 02, 2018
Common Stock
Supplementary Financial Information [Line Items]
Conversion of convertible preferred stock into common stock, shares	11,000	10,994
Common stock shares issued and sold	6,325	6,325