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Reconciliation Between Income Taxes at Statutory Tax Rate (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Schedule Of Effective Income Tax Rate Reconciliation [Line Items]      
Loss before income taxes $ (8,583) $ (7,023) $ (6,750)
Tax (benefit) at statutory tax rate (1,802) (2,388) (2,295)
Effects of:      
Exclusion of incentive stock option expense 47 86 117
R&D tax credits (238) (283) (252)
Increase (decrease) in valuation allowance 1,845 (1,447) 3,001
FMV of warrants 0 (658) (582)
Prior year adjustments 0 5 6
Carryforward adjustment 2 (252) 5
Stock option forfeitures 146 0 0
Rate change on net deferred tax asset 0 4,937 0
Provision for income taxes $ 0 $ 0 $ 0