In one report, priapism was successfully treated in a dog by incising the penis over the bulbus glandis and pars longa glandis
and through the tunica albuginea, after which pressure was applied to expel free blood and thrombi from the corpus cavernosum
penis.10 In another case, small incisions were made over the tunica albuginea and corpus cavernosum penis, and then the penis was
decompressed and irrigated with heparinized saline solution.14 This treatment resulted in the successful replacement of the penis in its sheath.
Oral drug therapy
Various oral drugs have been used to successfully treat priapism, including benztropine,12 pseudoephedrine,1 and terbutaline.2-4
Benztropine. The antihistamine and anticholinergic drug benztropine mesylate (Cogentin—Merck) has been used to successfully
treat priapism in two horses and in men.12 It contains the active portions of atropine and diphenhydramine and is thought to be effective because of its central anticholinergic
properties. Treatment must be initiated within a few hours of the onset of the condition.12
Pseudoephedrine. Pseudoephedrine is a sympathomimetic amine that causes vasoconstriction through the release of endogenous
norepinephrine which stimulates adrenergic receptors in the lining of blood vessels.
Tertbutaline. Terbutaline, a beta2-adrenergic agonist, has also been used successfully to treat priapism in men.2-4 It is possible that it stimulates the sympathetic nervous system. It is reported to cause smooth muscle relaxation, particularly
of the bronchioles and uterus,3 and is widely prescribed to treat bronchial asthma.
Parenteral terbutaline has also been used successfully to treat an intraoperative penile erection.2 Its most likely mechanism of action for any persistent erection would be to relax the smooth muscle of cavernous tissues,
arteries, veins, and polsters in the blood vessels and Buck's fascia.3 In the normal situation, the cavernous smooth muscles are not only mechanically stretched because of the rapid filling of
blood, but they also contract (like a stretched spring) against the blood, providing rigidity. Terbutaline probably acts by
relaxing the stretched corporeal smooth muscles, relaxing the polsters in the penile veins, widening the diameter of the corporeal
draining veins, and removing the impediment of venous blood flow created by contracting polsters, resulting in penile flaccidity.3
A controlled, randomized study involving 68 men was conducted to study the effect of oral terbutaline on prolonged erection
after intracavernosal injection of a vasoactive agent. Detumescence was achieved in 42% of the men receiving terbutaline and
15% of the men receiving the placebo.3
In a controlled study, 75 patients with pharmacologically induced prolonged erections were randomized to receive tertbutaline,
pseudoephedrine, or placebo.4 Thirty-six percent of patients receiving terbutaline achieved detumescence, compared with 12% for placebo and 28% for pseudoephedrine.
The authors reported that because of the results of the study, they recommended that terbutaline should be the first-line
treatment in pharmacologically induced priapism.4
In the guidelines concerning priapism in men, terbutaline is not recommended for the treatment of ischemic priapism.15 If the cause of the priapism in a dog can be determined not to be thromboembolic, medical studies done in men indicate that
terbutaline is effective at relieving prolonged erection and may be useful in treating dogs with priapism.3
Pharmacologic therapy used in these cases. Low-flow priapism may respond to pharmacologic treatment. Initially, acepromazine
was considered as a treatment in case report 1 to cause vasodilation of the arterial supply, but because of its well-known
effect of causing irreversible priapism in horses, its use was quickly ruled out. Since the hospital formulary did not have
benztropine, it was decided that treatment with atropine sulfate and antihistamines (diphenhydramine, chlorpheniramine), which
have a central anticholinergic effect,1 should have a similar mechanism of action. Treatment with these agents in case report 1 caused a slight improvement by the
end of the second day, but it was feared that surgery still might be needed. The addition of terbutaline to the treatment
plan resolved the priapsim so that no surgery was needed. In case report 2, the atropine and diphenhydramine had a positive
effect more quickly than they did in case report 1. It is not known if the atropine-diphenhydramine combination, given immediately,
acted in a similar fashion to benztropine.
Treatment with terbutaline in the cases reported in this article did not cause any adverse effects that the owners reported,
and upon repeat physical examination while receiving the drug, the dog in case report 1 had a normal heart rate while the
dog in case report 2 had a mild tachycardia but no clinical signs of hyperexcitability. This drug had better results than
atropine and diphenhydramine-chlorpheniramine alone, which had little effect on the condition by the end of the second day
in case report 1. Atropine would also be impossible to administer on an outpatient basis.