Note 10 - Summary of Stock Options - Summary of Option Activity Under All Plans (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Options outstanding (in shares)	4,014,475
Options outstanding, weighted average exercise price (in dollars per share)	$ 4.56
Options outstanding, weighted average remaining contractual term (Year)	8 years 73 days	7 years 146 days
Options granted (in shares)	2,145,000
Options granted, weighted average exercise price (in dollars per share)	$ 1.24
Options exercised (in shares)
Options exercised, weighted average exercise price (in dollars per share)
Options canceled (in shares)	(566,345)
Options canceled, weighted average exercise price (in dollars per share)	$ 4.63
Options outstanding (in shares)	5,593,130	4,014,475
Options outstanding, weighted average exercise price (in dollars per share)	$ 3.28	$ 4.56
Options outstanding, aggregate intrinsic value
Vested and exercisable and expected to vest, end of period (in shares)	5,291,876
Vested and exercisable and expected to vest, end of period, weighted average exercise price (in dollars per share)	$ 3.33
Vested and exercisable and expected to vest, end of period, weighted average remaining contractual term (Year)	8 years 36 days
Vested and exercisable and expected to vest, end of period, aggregate intrinsic value
Vested and exercisable, end of period (in shares)	1,725,352
Vested and exercisable, end of period, weighted average exercise price (in dollars per share)	$ 5.04
Vested and exercisable, end of period, weighted average remaining contractual term (Year)	5 years 219 days
Vested and exercisable, end of period, aggregate intrinsic value