Etiology
Ischemic priapism
Characterized by low or absence of cavernosal blood flow. Known causes include the following.
Thromboembolic/hypercoagulable states:
Neoplastic disease:
Neurologic disease:
As a consequence of "spinal shock" in acute spinal cord injury, cauda equina syndrome, multiple sclerosis, spinal cord tumor with compression, or neurotoxins.
Infection:
Rarely, ischemic priapism may be seen in pelvic infection.
Vasoactive agents, medications, and legal/illegal drugs:
These include erectile dysfunction pharmacotherapies (e.g., phosphodiesterase-5 [PDE5] inhibitors and intracavernous alprostadil), antihypertensives (e.g., hydralazine, prazosin), antipsychotics (e.g., chlorpromazine), and antidepressants (e.g., trazodone). In addition, alcohol and cocaine may predispose to ischemic priapism.[1][10]
Second-generation antipsychotics (33.8%), other medications (11.3%), and alpha-adrenergic antagonists (8.8%) account for most reported cases of drug-induced priapism.[10]
Nonischemic priapism
This is much rarer than ischemic priapism, and is predominantly caused by traumatic injuries.[4] However, in some patients no underlying cause is found. The most common injuries reported are to the corporal bodies or perineum.
Mechanisms include straddle injury, coital trauma, kicks to the penis or perineum, pelvic fractures, birth canal trauma to the newborn male, and vascular erosions complicating metastatic cancer infiltration of the corpora cavernosa.[17][18][19][20]
While blunt trauma is still the most commonly reported etiology, high-flow priapism has also been described following surgical interventions such as cold-knife urethrotomy, Nesbit corporoplasty, and deep dorsal vein arterializations.[3]
Stuttering or recurrent priapism
Stuttering priapism can result from episodes of ischemic priapism and vice versa. Therefore, it shares many of the same etiologic factors as those previously discussed for ischemic priapism.[1]
While the cause of stuttering priapism remains idiopathic in some cases, studies have shown recurrent priapism to be related to a defective PDE5 regulatory function in the penis, resulting from altered nitric oxide and cyclic guanosine monophosphate signaling mechanisms that control erectile function.[3][21]
Pathophysiology
Ischemic priapism is thought to be the result of an imbalance in the vasoconstrictive and vasorelaxatory mechanisms governing penile erection. Studies suggest that both vascular stasis within the penis and reduced venous outflow are the primary pathophysiologic features in ischemic priapism.[22] Deoxygenated blood in the penis becomes trapped, creating venous congestion. The tissue ischemia and increased pressure generated within the corporal bodies lead to the pain and penile rigidity clinically seen with ischemic priapism.[21]
Vascular stasis may result from an underlying hypercoagulable state. Venous outflow obstruction may be a consequence of tumor, infection, or reduced neural function. It is believed these ischemic changes cause apoptosis, failure of corporal smooth muscle contraction when activated by appropriate stimulus, and upregulation of hypoxia-induced growth factors in the penis. This results in damage to the corporal smooth muscle and vascular endothelium leading to progressive hypoxia, hypercarbia, and acidosis, which can be seen on cavernous blood gas analysis. Permanent erectile dysfunction can follow.[1][2][3][21] For these reasons, ischemic priapism is a medical emergency that demands prompt medical treatment.
Nonischemic priapism is much rarer and has a different pathophysiologic basis. It is the result of unregulated cavernous arterial inflow. Most episodes are trauma-induced, whereby the cavernous artery or one of its tributaries within the corpora becomes lacerated, creating an arteriolar-sinusoidal fistula.[21] The disruption of penile arterial circulation results in excessive arterial inflow to the penis.[17][23] This produces unregulated pooling of blood in the sinusoidal spaces, with consequent erection.[3] Cavernous blood gases do not show hypoxia, hypercarbia, or acidosis.[1][2] Therefore, nonischemic priapism is not a medical emergency.
Stuttering priapism shares many of the same pathophysiologic mechanisms as ischemic priapism. Studies also suggest that dysregulation at the level of the penis, and at other regulatory levels, including central or peripheral levels of the nervous system, have a role in abnormal penile erection.[24] Recurrent priapism is thought to be related to a defective phosphodiesterase-5 regulatory function in the penis, resulting from altered nitric oxide (NO) and cyclic guanosine monophosphate signaling mechanisms that control erectile function.[24][25][26] NO is a chemical released from the endothelium lining the trabeculae of the corpora cavernosa that, together with its neuronal sources, results in vasodilation during erection. A leading proposal in this regard is that NO becomes dysfunctional in association with underlying disease states.[25] From this understanding, novel modalities of treatment are evolving. Research now suggests that factors involved in pathways affecting inflammation, cellular adhesion, NO metabolism, vascular reactivity, and coagulation may have roles in the pathophysiology of sickle-cell disease-associated stuttering priapism.
Classification
Physiologic/clinical classification[1]
A classification system exists to assist with the practical understanding of priapism and facilitate its clinical management. Priapism is divided into three main categories: ischemic, nonischemic, and stuttering priapism.
Ischemic priapism
Veno-occlusive or low-flow priapism.
Typically features little or absent intracorporal blood flow.
Cavernous blood gases are abnormal.
May be associated with hematologic abnormalities (e.g., sickle cell disease), malignancy, medication or illicit drug use (psychiatric medications, cocaine, amphetamines), and vasoactive drug use.
Is a true compartment syndrome involving the penis, in which there are characteristic changes in cavernous blood gases and excessive increases in intracorporal pressure.
Penis is fully rigid.
Penile pain is present.
Is a medical emergency and must be treated.
Nonischemic priapism
Arterial or high-flow priapism.
Typically features elevated vascular flow within the corpora cavernosa.
Not associated with systemic disease or pharmacotherapy.
Commonly follows an episode of trauma to the perineum or the genitalia.
Penis shows a chronic tolerated tumescence without full rigidity.
Penile pain is not typically present.
Is not a medical emergency.
Recurrent (stuttering) priapism
Usually ischemic/low-flow in nature.
Typically features recurrent attacks of short-lived (<4 hours) and painful priapic episodes.
Often occurs during nocturnal sleep, after morning erections, or preceding or following sexual stimulation.
May be associated with hematologic abnormalities (sickle cell disease).
Is considered a medical emergency and must be treated with a goal of prevention in mind.
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