Urethral Sphincter Mechanism Incompetence

Also known as: SMI

Introduction

This is a common cause of urinary incontinence in dogs, especially neutered bitches, although castrated male dogs and entire animals can also be affected.

SMI is most commonly seen as an acquired condition, but may also be seen as a congenital problem in young animals.

Congenital SMI may occur in conjunction with anatomical defects such as ureteral ectopia and intersexuality. It occurs most commonly in large-breed bitches. The urethra may be abnormally short or absent, but often no abnormalities are detected apart from a caudally positioned bladder.

50% of congenital SMI bitches become continent following their first oestrus.

Acquired SMI usually follows spaying or castrating. It is a multifactorial disease involving: urethral tone, urethral length, bladder neck position, body size and breed, spaying, obesity and hormones. The urethra is usually shorter in affected bitches, and the bladder neck is usually intra pelvic.

Large and giant breeds are again at risk, in particular the Old English Sheepdog, Rottweiler, Doberman, Weimaraner and Irish Setter.

The link between spaying and acquired SMI is probably due to the lack of circulating oestrogens. Spaying before the first season may increase the risk of developing the disease, and spayed animals are 8 times more likely to develop SMI than entire animals.

Obesity may be involved, and some incontinent dogs will improve following weight loss.

Clinical Signs

The presenting sign is usually passive urinary incontinence when the animal is recumbent. Typically, the animal urinates normally between episodes of leakage. The incontinence varies in severity and can improve or get progressively worse over time.

Diagnosis

Diagnosis is based on the history, clinical signs, and in some cases a response to treatment.

Physical examination is usually unremarkable.

Full investigation include:

plain radiography: intra-pelvic bladder and short urethra

contrast radiography: intravenous urogram and retrograde vaginourethrogram

ultrasound: to confirm emptying of ureters into the bladder

urinalysis and bacteriology

urodynamic testing: urethral pressure profilometry

Diagnosis is usually based on the exclusion of all other likely causes (urinary tract infection, ectopic ureters, neoplasia) and the presence of a caudal bladder neck on radiography.

A positive response to treatment may be the only diagnosis possible if finances preclude further testing.

Treatment

Treatment can be medical or surgical.

Medical treatment aims to increase urethral tone and functional length. Drugs include:

sympathomimetic drugs such as phenylpropanolamine or ephedrine

exogenous oestrogens: oestriol

a combination of the above drugs

Phenylpropanolamine is the first choice and has also been used in male dogs.

In juvenile bitches with congenital SMI, 50% will spontaneously get better following their first season, and therefore these dogs should not be spayed before their first season and should not be given exogenous oestrogens. Phenylpropanolamine can be used instead.

There are numerous surgical treatments available, and involve:

• increasing urethral resistance: peri-urethral surgical slings, artificial sphincters, intra-urethral bulking agents
• increasing urethral length: bladder neck reconstruction techniques
• relocating the bladder to an intraabdominal position: colposuspension

Colposuspension carries the lowest complication rate, and the technique is as follows: via a caudal midline laparotomy, using the vagina as a sling, the bladder neck is brought cranially and kept in the intra-abdominal position by sutures placed from the pre-pubic tendon to the lateral walls of the vagina.

50% of animals are cured following colposuspension, and a further 40% are much improved. The rest are not generally made worse.

The choice of treatment is based on the owner's wishes, the age of the animal and its response to previous treatments.

Urethral Sphincter Mechanism Incompetence Learning Resources
Flashcards Test your knowledge using flashcard type questions	Small Animal Abdominal and Metabolic Disorders Q&A 02

References

Holt, P. (1999) Diagnosis and management of canine urethral sphincter mechanism incompetence Waltham Focus Vol 9 No 4

Gregory, S. (2010) Urinary Incontinence RVC student notes

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