Note 9 - Convertible Preferred Stock, Stockholders' Equity and Common Stock Warrants - Warrants Outstanding to Acquire Shares of Its Common Stock (Details) - $ / shares	Dec. 31, 2018	Jan. 31, 2018	Jan. 22, 2018
Common stock warrants, outstanding (in shares)	2,087,012
Warrants issued before 2016 [Member]
Common stock warrants, outstanding (in shares)	1,767
Warrants issued before 2016 [Member] | Minimum [Member]
Range of exercise price (in dollars per share)	$ 562.50
Warrants issued before 2016 [Member] | Maximum [Member]
Range of exercise price (in dollars per share)	$ 735
Warrants Attached to Series A Convertible Preferred Stock [Member]
Common stock warrants, outstanding (in shares)	903,870
Range of exercise price (in dollars per share)	$ 33.30
Warrants Issued in Connection with the January 2018 Offering [Member]
Common stock warrants, outstanding (in shares)	1,181,375
Range of exercise price (in dollars per share)		$ 12	$ 12
Warrants Issued in Connection with the January 2018 Offering [Member] | Minimum [Member]
Range of exercise price (in dollars per share)	$ 12
Warrants Issued in Connection with the January 2018 Offering [Member] | Maximum [Member]
Range of exercise price (in dollars per share)	$ 15