Disorders of micturition characterized by urine retention and a distended bladder are usually caused by hypocontractility of the bladder or by urethral obstruction. Prolonged bladder distention leads to breakdown of the tight junctions between detrusor muscle cells of the bladder, which prevents normal depolarization and contraction of the detrusor muscles.
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 antagonist, has been used with some success. The dosage for dogs or cats is 0.25 mg/kg, PO, bid.
Diazepam is a benzodiazepine anxiolytic that is also a central muscle relaxant. Dosages sufficient to allow for urethral relaxation may also cause sedation. The dosage in dogs is 0.2 mg/kg, PO, tid, and in cats is 0.5 mg/kg, IV. Diazepam given PO may cause idiosyncratic acute hepatic necrosis in cats.
In animals with detrusor hyporeflexia or bladder atony, bethanechol chloride may be of some benefit. This cholinergic agonist stimulates the initiation of detrusor muscle contraction. The dosage for dogs is 5–25 mg/dog, PO, tid, and for cats is 2.5–7.5 mg/cat, PO, tid.