Drugs for management of urinary incontinence

Drugs for management of urinary incontinence

Urinary incontinence is broadly classified into disorders of storage and disorders of voiding. Storage disorders are associated with normal post-void residual volume and can be functional (e.g. urethral sphincter mechanism incompetence in dogs) or mechanical (ectopic ureters). Voiding disorders involve incomplete bladder emptying with increased post-void residual volume and may result from functional outflow obstruction or mechanical blockage. Functional causes affect the coordination of bladder contraction and urethral relaxation, while mechanical causes include anatomical abnormalities like strictures or masses. Classification helps guide appropriate diagnostic and treatment strategies. For details, please refer to textbooks and Kendall et al. (2024).

Storage phase

The bladder fills gradually under sympathetic control via the hypogastric nerve, which relaxes the detrusor muscle (beta-adrenergic effect) and contracts the internal urethral sphincter (alpha-adrenergic effect). Simultaneously, the pudendal nerve maintains tone in the external urethral sphincter (acetylcholine nicotinic receptors in the striated muscle). These mechanisms prevent urine leakage during bladder filling.

Voiding phase

Once the bladder is full, afferent signals from the bladder wall (via the pelvic nerve) reach the brainstem. When voluntary voiding is initiated, the parasympathetic system (pelvic nerve, acetylcholine muscarinic receptors) contracts the detrusor muscle while sympathetic and somatic outflow (hypogastric and pudendal nerves) are inhibited, relaxing both urethral sphincters. This coordination allows complete bladder emptying.

Detrusor muscle: acetylcholine muscarinic receptors are activated during voiding phase and beta-adrenergic receptors are activated during storage phase.

Internal sphincter muscle (smooth muscle sphincter): alpha-adrenergic receptors are activated during storage phase and inhibited during voiding phase.

External sphincter (striated muscle sphincter): acetylcholine nicotinic receptors are activated during storage phase and inhibited during voiding phase.

TABLE: Medications to treat storage disorders

Drug	Class and effect	Indication	Dose	Adverse effects
Phenylpropanolamine	Alpha agonist: directly stimulates alpha and some beta receptors Effect: increases internal sphincter/urethral tone	USMI	Dogs and cats: 1.5–2 mg/kg p.o. q8–12h	Hypertension, restlessness, GI effects, urine retention
Estriol	Oestrogen: binds to oestrogen receptors within sphincter cells, resulting in increased responsiveness to norepinephrine. May also sensitize the urethral sphincter to alpha-adrenergic stimulation Effect: increases internal sphincter/urethral tone	USMI (females)	Dogs: 2 mg per dog p.o. q24h for 14 days, then reduce to 1 mg per dog q24h	Attractiveness to males, mammary/vulvar swelling, behavioural changes
Diethylstilbestrol	Oestrogen (see Estriol)	USMI (females)	Dogs: 0.1–1 mg per dog p.o. q24h for 3–5 days, then weekly or prn	Myelosuppression (rare at these doses), attractiveness to males, mammary/ vulvar swelling, behavioural changes
Testosterone cypionate	Androgen: exact mechanism unknown. Suspected to maintain tone of urethra and bladder smooth muscle. May also cause hypertrophy of the prostate, which increases urethral resistance Effect: increases internal sphincter/urethral tone	USMI (males)	Dogs: 2.2 mg/kg i.m. q4–8wk	Behavioural changes, aggression, perianal adenoma, prostatic hyperplasia
Methyltestosterone	Androgen (see Testosterone cypionate)	USMI (males)	Dogs: 0.5 mg/kg p.o. q24h; if effective, change to testosterone cypionate	Behavioural changes, aggression, perianal adenoma, prostatic hyperplasia
Oxybutynin	Muscarinic receptor antagonist Effect: decreases detrusor contractility	Detrusor instability	Dogs: 0.2–0.3 mg/kg q8–12h in dogs	GI effects, urine retention, hypersalivation, sedation

USMI = Urethral sphincter mechanism incompetence.

TABLE: Medications to treat voiding disorders.

Drug	Class and effect	Indication	Dose	Adverse effects
Tamsulosin hydrochloride	Alpha antagonist Effect: decreases internal sphincter/urethral tone	Functional outflow obstruction	Dogs: 0.4–0.8 mg per dog p.o. q24h Cats: 4–6 micrograms/kg p.o. q12–24h	Hypotension
Prazosin	Alpha antagonist Effect: decreases internal sphincter/urethral tone	Functional outflow obstruction	Dogs: 1 mg per dog <15 kg, 2 mg per dog >15 kg p.o. q8–12h 30 minutes before blader expression Cats: 0.25–1 mg per cat p.o. q8–12h	Hypotension, weakness, syncope, GI upset
Phenoxybenzamine	Alpha antagonist Effect: decreases internal sphincter/urethral tone	Functional outflow obstruction	Dogs and cats: 0.25–1 mg/kg p.o. q8–24h	Hypotension, tachycardia, weakness
Diazepam	Benzodiazepine/skeletal muscle relaxant Effect: decreases external sphincter tone	Functional outflow obstruction	Dogs: 0.04–0.8 mg/kg/day, p.o., divided q8–12h; recommended 30 minutes prior to a walk or bladder expression Avoid in cats	Sedation, ataxia
Dantrolene sodium	Calcium release inhibitor/skeletal muscle relaxant Effect: decreases external sphincter tone	Functional outflow obstruction	Dogs: 1–2.7 mg/kg/day, p.o., divided q8–12h Cats: 0.5–2 mg/kg/day divided q8–12h	Sedation, weakness, hepatotoxicity
Alprazolam	Benzodiazepine/skeletal muscle relaxant Effect: decreases external sphincter tone	Functional outflow obstruction	Dogs: 0.02–0.1 mg/kg, p.o. q12h Cats: 0.125–0.25 mg/kg p.o. q12h	Sedation, increased appetite, paradoxical hyperactivity in cats
Bethanechol	Muscarinic receptor agonist Effect: increases detrusor muscle contraction	Detrusor atony	Dogs: 2.5–25 mg per dog p.o. q8h Cats: 1.25–5 mg per cat p.o. q8–12h Always use in combination with sphincter relaxants	Increased GI motility, vomiting, diarrhoea, hypersalivation, hypotension, bradycardia, dyspnoea
Cisapride	Increases acetylcholine release	Detrusor atony	Dogs: 0.5 mg/kg p.o. q8h Cats: 1.25–5 mg per cat p.o. q8–12h	Diarrhoea, abdominal pain
Metoclopramide	Increases acetylcholine release	Detrusor atony	Dogs and cats: 0.2–0.5 mg/kg p.o. q8h	Diarrhoea, abdominal pain, sedation or hyperactivity

References

Kendall A, Byron JK, Westropp JL et al. (2024) ACVIM consensus statement on diagnosis and management of urinary incontinence in dogs. Journal of Veterinary Internal Medicine 38, 878–903

Granger N, Olby NJ, Nout-Lomas YS and the Canine Spinal Cord Injury Consortium (2020) Bladder and bowel management in dogs with spinal cord injury. Frontiers in Veterinary Science 7, 583342

Coates JR and De Decker S (2026) Tail, anal and bladder dysfunction. In: BSAVA Manual of Canine and Feline Neurology, 5th edn, ed. SR Platt, N Olby and E Beltran. BSAVA Publications, Gloucester [in production]

Mitchell WC and Venable DD (1985) Effects of metoclopramide on detrusor function. The Journal of Urology 134, 791–794