Disorders of micturition characterized by urine retention and bladder distention are usually due to hypocontractility of the bladder or to urethral obstruction. Prolonged bladder distention leads to breakdown of the tight junctions between detrusor muscle cells of the bladder, thereby preventing normal depolarization and contraction of the detrusor muscles.
Vesico-urethral reflex dyssynergia (VURD) is due to insufficient relaxation of the internal urethral sphincter during contraction of the detrusor muscle. VURD often occurs in middle-aged, large-breed male dogs.
Prazosin is an alpha-1-adrenergic antagonist that is recommended to relax the sympathetic internal urethral sphincter.
If a dog with VURD does not show clinical improvement in urine outflow after being treated with prazosin, overdistention has probably impaired bladder contractility. Suggested additional treatment of bladder atony includes both frequent emptying of the bladder by catheterization and administration of bethanechol to increase contraction of the detrusor muscle.
An adrenergic antagonist may be indicated when manual expression or voluntary voiding is nonproductive because urethral sphincter tone is excessive, as is often the case in cats after relief of obstruction. Phenoxybenzamine, an irreversible alpha-adrenergic antagonist, has been used with some success.
Diazepam is a benzodiazepine anxiolytic that is also a central muscle relaxant. Dosages sufficient to enable urethral relaxation may also cause sedation. Diazepam administered orally may cause idiosyncratic acute hepatic necrosis in cats.
In patients with detrusor hyporeflexia or bladder atony, bethanechol chloride may be of some benefit. This cholinergic agonist stimulates the initiation of detrusor muscle contraction.
Bethanechol is potentially useful in horses with urine retention that is due to neurologic equine herpesvirus 1. An injectable formulation is available only through compounding. Although apparently less clinically effective, oral doses of bethanechol are also suggested for horses.
