In ischemic priapism, the risk of permanent scarring of the penis begins to increase after four hours and definitely occurs after 48 hours. Priapism occurs in about 1 in 20,000 to 1 in 100,000 males per year. Classification
Priapism is classified into three groups: ischemic (low-flow), nonischemic (high-flow), and recurrent ischemic. The majority of cases (19 out of 20) are ischemic in nature.Some sources give a duration of four hours as a definition of priapism, but others give six. "The duration of a normal erection before it is classifiable as priapism is still controversial. Ongoing penile erections for more than 6 hours can be classified as priapism. "Priapism in females (continued, painful erection of the clitoris) is significantly rarer than priapism in men and is known as clitoral priapism or clitorism. It is associated with persistent genital arousal disorder (PGAD). Only a few case reports of women experiencing clitoral priapism exist. Signs and symptoms
Complications
Because ischemic priapism causes the blood to remain in the penis for unusually long periods of time, the blood becomes deprived of oxygen, which can cause damage to the penile tissue. Such damage can result in erectile dysfunction or disfigurement of the penis. In extreme cases, if the penis develops severe vascular disease, the priapism can result in penile gangrene. Low-flow priapism
Causes of low-flow priapism include sickle cell anemia (most common in children), leukemia, and other blood dyscrasias such as thalassemia and multiple myeloma, and the use of various drugs, as well as cancers.
A genome-wide association study on Brazilian patients with sickle cell disease identified four single nucleotide polymorphisms in LINC02537 and NAALADL2 significantly associated with priapism.Other conditions such as Fabrys disease, as well as neurologic disorders such as spinal cord lesions and spinal cord trauma (priapism has been reported in people who have been hanged; see death erection). Priapism can also be caused by reactions to medications. The most common medications that cause priapism are intra-cavernous injections for the treatment of erectile dysfunction (papaverine, alprostadil). Other medication groups reported are antihypertensives, antipsychotics (e.g., chlorpromazine, clozapine), antidepressants (most notably trazodone), anti-convulsant and mood stabilizer drugs such as sodium valproate. Anticoagulants, cantharides (Spanish Fly) and recreational drugs (alcohol, heroin and cocaine) have been associated. Priapism is also known to occur from bites of the Brazilian wandering spider. High-flow priapism
Causes of high-flow priapism include:

blunt trauma to the perineum or penis, with laceration of the cavernous artery, which can generate an arterial-lacunar fistula. Anticoagulants (heparin and warfarin). Antihypertensives (i.e., hydralazine, guanethidine and propranolol). Hormones (i.e., gonadotropin releasing hormone and testosterone). Diagnosis
The diagnosis is often based on the history of the condition as well as a physical exam.Blood gas testing the blood from the cavernosa of the penis can help in the diagnosis. If the low-flow type of priapism is present, the blood typically has a low pH, while if the high-flow type is present, the pH is typically normal. Color Doppler ultrasound may also help differentiate the two.