Sustained (> 4h) erection not associated with sexual stimulation
May lead to erectile dysfunction and penile necrosis
High rate of sexual dysfunction if present > 24hrs
Rare
Associated with trauma or instrumentation
Usually painless
Increased arterial flow
Usually self-resolves and does not require intervention
Usually does not cause ischemia or sexual dysfunction
Most common type
Veno-occlusion causing pooling of deoxygenated blood in cavernous tissue
Painful
Urologic emergency
May progress to ischemia and necrosis without intervention
May progress to ischemia and necrosis without intervention
High-flow
Arterio-cavernosal shunt due to groin or straddle injury
High spinal injury
Arterio-cavernosal shunt due to groin or straddle injury
High spinal injury
Low-flow
Sickle Cell Disease
Medications
SSRIs/trazodone
Antipsychotics
Erectile dysfunction medications (e.g. sildenafil)
Drugs of abuse
Cocaine use
Leukemia
Infection
Latrodectus envenomation (Black widow) 
Idiopathic
Sickle Cell Disease
Medications
SSRIs/trazodone
Antipsychotics
Erectile dysfunction medications (e.g. sildenafil)
Drugs of abuse
Cocaine use
SSRIs/trazodone
Antipsychotics
Erectile dysfunction medications (e.g. sildenafil)
Cocaine use
Priapism
Peyronie's Disease
Urinary incontinence
Urethral foreign body
Dysgeusia
Penile surgical implant
Ataxia
Erection from sexual arousal