Treat Canis Urinary incontinence: urethral sphincter mechanism incompetence (USMI)

Urinary incontinence: urethral sphincter mechanism incompetence (USMI)

i canis

Contributor(s):

Kathy Lunn

Pieter Nelissen

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

Introduction

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.

Pathogenesis

Etiology

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.

Pathophysiology

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).

Diagnosis

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Treatment

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Outcomes

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Urinary incontinence: urethral sphincter mechanism incompetence (USMI)

Introduction