I did some search on the net.
“Sickle Cell Information Center Protocols
by James Eckman, M.D. and Allan Platt, PA-C
The diagnosis of priapism is usually obvious from the physical examination. Patients usually present with the spontaneous onset of erection which frequently occurs in the middle of the night. Can be precipitated by prolonged intercourse or masturbation. Penis remains erect and is very painful. If there is a history of nocturnal erection, pain and persistence for more than thirty minutes must be present to differentiate priapism from normal nocturnal erections. There are three basic patterns seen with priapism.”