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Reconciliation Between Income Taxes at Statutory Tax Rate (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule Of Effective Income Tax Rate Reconciliation [Line Items]      
Loss before income taxes $ (7,023) $ (6,750) $ (5,467)
Tax (benefit) @ statutory tax rate (2,388) (2,295) (1,859)
Effects of:      
Exclusion of incentive stock option expense 86 117 89
R&D tax credits (283) (252) (132)
Increase (decrease) in valuation allowance (1,447) 3,001 1,899
FMV of warrants (658) (582) 0
Prior year adjustments 5 6 0
Carryforward adjustment (252) 5 3
Rate change on net deferred tax asset 4,937 0 0
Provision for income taxes $ 0 $ 0 $ 0