ISSN 2398-2969      

Urinary incontinence: urethral sphincter mechanism incompetence (USMI)


Kathy Lunn

Pieter Nelissen

Synonym(s): hormone responsive urinary incontinence, sphincter mechanism incompetence, idiopathic incontinence and spay incontinence


  • Urinary incontinence, loss of voluntary control over the retention and expulsion of urine, is a common medical problem in small animal patients. Incontinence occurs when pressure within the bladder exceeds urethral pressure.
  • USMI is the most common cause of urinary incontinence in adult female dogs ~80% of cases.
  • Cause: inadequate function of external urethral sphincter. May be linked to bladder neck position and neutering.
  • Signs: dribbling urine when relaxed - particularly lying down or incontinence at night.
  • Diagnosis: signs, response to treatment. USMI is a diagnosis of exclusion once all other disorders have been ruled out.
  • Treatment: medical: phenylpropanolamine, estrogen. Surgical: colposuspension, urethropexy and colposuspension and static hydrolic occluder.
  • Prognosis: continence rates of between 75-90% are reported with phenylpropanolamine, but unfortunately not all of these are durable. Estriol has a complete response rate of 65% reported, with partial response in a further 17%. Some patients are refractory to therapy. Continence rates between 60-70% have been reported with various surgical options.
    Follow the diagnostic trees for the investigation of Canine urinary incontinence and Congenital canine urinary incontinence.
Use the interactive tool from ROYAL CANIN® UK  to explain dog anatomy and disease conditions to your client. Visit ROYAL CANIN Natom Explorer to find out more.​



  • Symptoms arise due to a combination of factors:
    • Caudally positioned bladder neck Pelvic bladder.
    • Decreased sensitivity of alpha-adrenergic receptors.
    • Reduced number of adrenergic receptors.
    • Increased intra-abdominal pressure.
    • Obesity.
    • Breed/weight.
    • Lack of sex hormones.

Predisposing factors

  • Neutering increases risk of development in susceptible dogs due to reduction in circulating hormones (estrogen or testosterone) which help to maintain sensitivity of alpha-adrenergic receptors. Early neutering may increase the risk by reducing the number of receptors in the bladder neck.
  • Approximately 20% of female dogs have been reported to develop some degree of USMI after OVH performed between the first and second heat cycles. In dogs spayed before first estrus, the incidence is reported to be 9.7%. The incidence of incontinence may be as high as 30% in large-breed female dogs (>20 kg).
  • A positive correlation between docking and USMI has been shown.
  • Polydipsia (drinking more) results in the bladder being more full more often, increases outflow pressure.


  • May be a congenital or acquired condition.
  • Congenital: the urethra may be shortened, hypoplastic or even absent, and both diverticula and dilation can develop.
  • Acquired: develops after neutering.
  • Storage of urine and successful voiding at the appropriate time is a complex procedure that requires co-ordination of the detrusor muscle of the bladder wall, the internal and the external urethral sphincters.
  • Nervous control of the micturition reflex involves sympathetic nerves (hypogastric from the first to fourth lumbar spinal segments), parasympathetic nerves (pelvic nerves from the first to third sacral spinal sements) and also the somatic nervous system (pudendal nerves, from the first to third sacral spinal segments).
  • The detrusor muscle (bladder) contracts in response to motor innervation from the parasympathetic system.
  • Beta-adrenergic receptors cause relaxation of the bladder detrusor muscle to allow the bladder to fill.
  • The internal urethral sphincter is normally controlled by the action of the sympathetic nervous system on the alpha-adrenergic receptors in the urethral smooth muscle.
  • Noradrenaline acts on the alpha receptors in the bladder neck and urethra promoting contraction of smooth muscle which increases outflow resistance.
  • The external urethral sphincter receives separate motor innervation from the somatic system via cholinergic (nicotinic) receptors.
  • Urination involves both storage and voiding phases. During storage, sympathetic stimulation relaxes the detrusor muscle and contracts the internal urethral sphincter. Contraction of the external sphincter provides extra support as required.
  • Increases in intra-abdominal pressure (such as when the animal lies down, coughs or barks) or relaxation of the voluntary control of the bladder sphincters (when the animal is asleep) causes urine to leak from the bladder.
  • The problem is exacerbated if the bladder neck is positioned caudally in the body ('pelvic' bladder Pelvic bladder).


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Martinoli S, Nelissen P, White R A (2014) The outcome of combined urethropexy and colposuspension for management of bitches with urinary incontinence associated with urethral sphincter mechanism. Vet Surg 43 (1), 52-57 PubMed.
  • Currao R L, Berent A C, Weisse C et al (2013) Use of a percutaneously controlled urethral hydraulic occluder for treatment of refractory urinary incontinence in 18 female dogs. Vet Surg 42 (4), 440-447 PubMed.
  • Delisser P J, Friend E J, Chanoit G P et al (2012) Static hydraulic urethral sphincter for treatment of urethral sphincter mechanism incompetence in 11 dogs. JSAP 53 (6), 338-343 PubMed.
  • Claeys S, De Leval J, Hamaide A (2010) Transobturator vaginal tape inside out for treatment of urethral sphincter mechanism incompetence: preliminary results in 7 female dogs. Vet Surg 39 (8), 969-79 PubMed.
  • Rose S A, Adin C A, Ellison G W et al (2009) Long‐Term Efficacy of a Percutaneously adjustable Hydraulic Urethral Sphincter for Treatment of Urinary Incontinence in Four Dogs. Vet Surg 38 (6), 747-753 PubMed.
  • Holt P E, Coe R J & Hotston Moore A (2005) Prostatopexy as a treatment for urethral sphincter mechanism incompetence in male dogs. JSAP 46 (12), 567-570 PubMed.
  • Adin C A, Farese J P, Cross A R et al (2004) Urodynamic effects of a percutaneously controlled static hydraulic urethral sphincter in canine cadavers. Am J Vet Res 65 (3), 283-288 PubMed.
  • Lane I F (2003) Treating urinary incontinence. Vet Med 98 (1), 58-65 VetMedResource.
  • Bacon N J, Oni O, White R A (2002) Treatment of urethral sphincter mechanism incompetence in 11 bitches with a sustained release formulation of phenylpropanolamine hydrochloride. Vet Rec 151 (13), 373-376 PubMed.
  • Rawlings C A (2002) Colposuspension as a treatment for urinary incontinence in spayed dogs. J Am Anim Hosp Assoc 38 (2), 107-110 PubMed.
  • Scott L, Leddy M, Bernay F et al (2002) Evaluation of phenylpropanolamine in the treatment of urethral sphincter mechanism incompetence in the bitch. JSAP 43 (11), 493-496 PubMed.
  • Rawlings C, Barsanti J A, Mahaffrey M B et al (2001) Evaluation of colposuspension for treatment of incontinence in spayed female dogs. J Am Vet Med Assoc 219 (6), 770-775 PubMed.
  • White R N (2001) Urethropexy for the management of urethral sphincter mechanism incompetence in the bitch. J Small Anim Pract 42 (10), 481-486 PubMed.
  • Gregory S P, Holt P E, Parkinson T J & Wathes C M (1999) Vaginal position and length in the bitch - relationship to spaying and urinary incontinence. JSAP 40 (4), 180-184 PubMed.
  • Marchevsky A, Edwards G A, Lavelle R B et al (1999) Colposuspension in 60 bitches with incompetence of the urethral sphincter mechanism. Aust Vet Pract 29 (1), 2-8 VetMedResource.
  • Atalan G, Holt P E, Barr F J (1998) Ultrasonographic assessment of bladder neck mobility in continent bitches and bitches with urinary incontinence attributable to urethral sphincter mechanism incompetence. Am J Vet Res 59 (6), 673-679 PubMed.
  • Nickel R F, Wiegand U & van den Brom W E (1998) Evaluation of a transpelvic sling procedure with and without colposuspension for treatment of female dogs with refractory urethral sphincter mechanism incompetence. Vet Surg 27 (2), 94-104 PubMed.
  • Power S C, Eggleton K E, Aaron A J et al (1998) Urethral sphincter mechanism incompetence in the male dog - importance of bladder neck position, proximal urethral length and castration. JSAP 39 (2), 69-72 PubMed.
  • Thrusfield M V, Holt P E & Muirhead R H (1998) Acquired urinary incontinence in bitches - its incidence and relationship to neutering practices. JSAP 39 (12), 559-566 PubMed.
  • Weber U T, Arnold S, Hubler M et al (1997) Surgical treatment of male dogs with urinary incontinence due to urethral sphincter mechanism incompetence. Vet Surg 26 (1), 51-56 PubMed.
  • Aaron A, Eggleton K, Power C et al (1996) Urethral sphincter mechanism incompetence in male dogs - a retrospective analysis of 54 cases. Vet Rec 139 (22), 542-546 PubMed.
  • Gregory S P, Cripps P J, Holt P E (1996) Comparison of urethral pressure profilometry and contrast radiography in the diagnosis of incompetence of the urethral sphincter mechanism in bitches. Vet Rec 138 (3), 58-61 PubMed.
  • Gregory S P (1994) Developments in the understanding of the pathophysiology of urethral sphincter mechanism incompetence in the bitch. Br Vet J 150 (2), 135-150 PubMed.
  • Gregory S P (1994) The immediate effect of colposuspension on resting and stressed urethral pressure profiles in anesthetized incontinent bitches. Vet Surg 23 (5), 330-340 Wiley Online Library.
  • Lamb C R, Gregory S P (1994) Ultrasonography of the ureterovesicular junction in the dog: a preliminary report. Vet Rec 134 (2), 36-38 PubMed.
  • Holt P E & Thrusfield M V (1993) Association in bitches between breed, size, neutering, docking and acquired urinary incontinence due to incompetence of the urethral sphincter mechanism. Vet Rec 133 (8), 177-80 PubMed.
  • Holt P E (1990) Long term evaluation of colposuspensions in the treatment of urinary incontinence due to incompetence of the urethral sphincter mechanism in the bitch. Vet Rec 127 (22), 537-542 PubMed.
  • White R A, Pomeroy C J (1989) Phenylpropanolamine: an alpha-adrenergic agent for the management of urinary incontinence in the bitch associated with urethral sphincter mechanism incompetence. Vet Rec 125 (19), 478-480 PubMed.
  • Holt P E (1988) 'Simultaneous' urethral pressue profilometry - comparisons between continent and incontinent bitches. JSAP 29 (12), 761-9 Wiley Online Library.
  • Holt P E (1985) Importance of urethral length, bladder neck position and vestibulovaginal stenosis in sphincter mechanism incompetence in the incontinent bitch. Res Vet Sci 39 (3), 364-372 PubMed.
  • Holt P E (1985) Urinary incontinence in the bitch due to sphincter mechanism incompetence: prevalence in referred dogs and retrospective analysis of sixty cases. J Small Anim Pract 26 (4), 181-190 VetMedResource.
  • Richter K P, Ling GV (1985) Clinical response and urethral pressure profile changes after phenylpropanolamine in dogs with primary sphincter incompetence. J Am Vet Med Assoc 187 (6), 605-611 PubMed.

Other sources of information

  • Bjorling D E (2003) The urethra. In: D Slatter (Ed.) Textbook of small animal surgery. 3rd edition. WB Saunders, Philadelphia, pp1638–1651.
  • Stone E A, Barsanti J A (1992) Urologic surgery of the dog and cat. Lea & Febiger, Philadelphia.

Further Reading

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