The duration of the erection considered to represent priapism is generally over four hours.
Priapism can be classified into 2 main types; ischemic (synonymous with venocclusive, low-flow) and non-ischemic (arterial, high-flow). Ischemic priapism results from the failure of blood to drain from the erect penis. This can be thought of as a compartment syndrome of the penis. Over time, the lack of oxygenated blood flow to the penis results in irreversible structural changes with in the erectile tissue leading to tissue death and scarring. It is believed that even after 6-8 hours of ischemic priapism, irreversible damage begins to occur.
Non-ischemic priapism results form unregulated inflow of blood into the erect penis. This differs form non-ischemic priapism in that the blood is oxygenated and does not get trapped within the erectile tissue.
Proper categorization is essential, as the management of the two conditions is very different, particularly as ischemic priapism is a urologic emergency, while non-ischemic is not.