Priapism is a persistent abnormal erection of the penis, usually without sexual desire, and is accompanied by pain and tenderness. A lack of detumescence is referred to as a prolonged erection if the duration of the rigidity is less than 4 to 6 hours and priapism if the erection lasts longer than 4 to 6 hours.
If the erection lasts longer than 6 to 8 hours, it is often associated with pain. Priapism may occur from too much blood flow into the penis (high flow), or it may be a result of too little bood flow out of the penis (low flow). High-flow priapism may occur after there has been an injury to the penis that causes damage to an artery that results iunregulated blood flow into the penis. Because there is an increase in arterial blood (which carries oxygen) into the penis, high-flow priapism does not cause pain. In high-flow priapism, there is venous drainage out of the penis, so the erection does not tend to be as rigid as in a full erection. Low-flow priapism occurs more in men with sickle cell disease and trait – a condition in which the red blood cells take on an abnormal (sickle) shape in response to decreased unregulated blood flow into the penis. Because there is an increase in arterial blood (which carries oxygen) into the penis, high-flow priapism does not cause pain. In high-flow priapism, there is venous drainage out of the penis, so the erection does not tend to be as rigid as in a full erection. Low-flow priapism occurs more in men with sickle cell disease and trait – a condition in which the red blood cells take on an abnormal (sickle) shape in response to decreased oxygenation, dehydration, and acidosis – and cancers of the blood, such as leukemia. It may also occur with injection therapy for erectile dysfunction and with certain psychiatric medications, such as trazodone. Because the problem consists of a problem with drainage of blood from the penis, which has little oxygen in it, this form of priapism is associated with pain and full rigidity.