To understand the pathophysiology and pharmacology of potential drugs given to treat priapism, a brief discussion of erectile
physiology is necessary. During the normal process of erection, relaxation of the sinusoidal smooth muscle and increased blood
flow through the arteries and arterioles facilitate rapid filling of the sinusoidal system, which in turn compresses the venous
channels and occludes outflow. During the process of detumescence, the trabecular smooth muscle contracts, enabling the venous
channels to reopen, and the trapped blood is expelled. The neurophysiology of erection includes sympathetic innervation provided
by the hypogastric nerve, parasympathetic innervation provided by the pelvic nerve, and somatic and sensory input provided
by the pudendal nerve.
Parasympathetic innervation is considered responsible for stimulating erection, and sympathetic innervation is responsible
for inhibiting erection and stimulating ejaculation. Priapism occurs rarely in dogs and cats.6 However, when it does occur, it must be corrected promptly because stagnated blood in the cavernous sinuses will eventually
clot, which will not resolve even when venous drainage is reestablished.
Some high-strung dogs transiently develop erections when they are excited for any reason. That is not priapism, and these
transient erections usually subside with age or neutering.7 Priapism is also different from the erection that persists in some dogs after copulation or semen collection. In these cases,
if the bitch is still present, it should be removed from the premises. The male dog should be taken out of the room where
the semen collection or copulation took place. If this is not sufficient, then sedation or application of cold compresses
could be considered.
In the case of true priapism, idiopathic priapism should be differentiated from other causes such as vascular or neoplastic
causes. Priapism should also be differentiated from other causes of penile swelling such as hematoma or edema. In the cases
reported in this article, there was no evidence of penile hematoma, but the presence of one in case report 1, which occurred
after neutering, cannot be ruled out. Paraphimosis, the condition that causes the inability of a dog's distended penis to
retract properly, causes edema, which can be a result of the extended erection or other causes of paraphimosis such as a narrowed
preputial opening, injury or fracture to the organ, or the tangling of hair around the base of the penis.
Simple visual inspection of the penis is usually sufficient to differentiate these conditions. In the cases reported in this
article, priapism was diagnosed through visual inspection and based on the history that the erection was still present long
after the initial stimulation had been removed. An ultrasonographic or color-flow Doppler examination can differentiate priapism
from other causes of penile swelling, but it was not performed in these cases.
Classification and etiology
In men, priapism is classified as high flow when an increase in arterial blood flow overrides venous drainage and low flow
when neural mediation of vascular tone is altered or primary vascular or hematologic alterations have occurred.6
The causes of priapism in men include spinal cord trauma, general anesthesia, phenothiazine administration, thromboembolism,
hematologic dyscrasias, nonhematologic malignancies, pelvic or perineal trauma, erectile dysfunction pharmacotherapy, pharmacologic
exposure to phosphodiesterase type 5 inhibitors, or idiopathic circumstances.8,9
The causes of priapism in dogs include spinal cord lesions, spinal surgery, constipation, trauma while mating, chronic distemper
encephalomyelitis, penile thromboembolism, meningomyeloceles, syringomyelia, and idiopathic causes.1,10,11
In the cases reported here, physical examination did not reveal any evidence of neurologic disease. In case report 1, the
dog was allowed to mount an intact female after neutering, which could have contributed to the condition. Furthermore, the
dog most likely received repeated doses of acepromazine and general anesthesia for the initial neutering and repeat operation.
(Acepromazine is a common preanesthetic tranquilizer used at the shelter facility where the patient was neutered.) Phenothiazine
tranquilizers are reported to cause priapism by blocking sympathetic impulses that initiate detumescence while causing paralysis
of the retractor penis muscle.12
Furthermore, in an experiment in 14 dogs, when administered through intracorporeal injection, chlorpromazine induced erection,
which is postulated to occur through a local effect.13