Note 5 - Convertible Preferred Stock, Common Stock and Common Stock Warrants - Warrants Outstanding to Acquire Shares of Its Common Stock (Details) - $ / shares	Jun. 30, 2018	Jan. 31, 2018	Jan. 22, 2018
Common stock warrants, outstanding (in shares)	31,312,512
Warrants issued before 2016 [Member]
Common stock warrants, outstanding (in shares)	36,000
Warrants issued before 2016 [Member] | Minimum [Member]
Range of exercise price (in dollars per share)	$ 37.50
Warrants issued before 2016 [Member] | Maximum [Member]
Range of exercise price (in dollars per share)	$ 49
Warrants Attached to Series A Convertible Preferred Stock [Member]
Common stock warrants, outstanding (in shares)	13,555,887
Range of exercise price (in dollars per share)	$ 2.22
Warrants Issued in Connection with the January 2018 Offering [Member]
Common stock warrants, outstanding (in shares)	17,720,625
Range of exercise price (in dollars per share)		$ 0.80	$ 0.80
Warrants Issued in Connection with the January 2018 Offering [Member] | Minimum [Member]
Range of exercise price (in dollars per share)	0.80
Warrants Issued in Connection with the January 2018 Offering [Member] | Maximum [Member]
Range of exercise price (in dollars per share)	$ 1