In dogs referred for the investigation of urinary incontinence, urethral sphincter mechanism incompetence is the commonest diagnosis made in adults and is second only to ureteral ectopia in juveniles, particularly in females.

It can present as a congenital or acquired condition. Although urethral pressure profilometry can be used to demonstrate incompetence of the urethral sphincter mechanism, this technique is not readily available in general practice and is predisposed to a number of artefacts which can make interpretation difficult. In general practice, therefore, the diagnosis is usually made on the basis of the breed, history and by the elimination of other possible diagnoses using imaging and laboratory techniques.

Congenital urethral sphincter mechanism incompetence tends to be a problem of large breeds of dog, predominantly bitches. Leakage of urine is more copious compared to animals with ureteral ectopia and occurs predominantly when the dogs are recumbent. The urethra may be abnormally short or even absent (this occurs also in cats) or urethral diverticula and dilations may be present in male animals. In many bitches, no gross abnormalities are detected on contrast radiographic investigations, apart from a caudally positioned bladder and the diagnosis frequently relies on the history and elimination of other possible causes of incontinence. Approximately half of affected bitches become continent following their first oestrus and should thus not be spayed earlier! Those that do not may be candidates for medical or surgical management (see acquired sphincter mechanism incompetence). Oestrogens are contra-indicated in juvenile bitches with this condition because of possible adverse 'feedback' effects on the pituitary but alpha-adrenergics such as phenylpropanolamine could be used, pending oestrus.

Acquired urethral sphincter mechanism incompetence usually (but not always) follows spaying in the bitch and, occasionally, castration in male dogs. Before treatment can be contemplated, an understanding of the pathophysiology of urethral sphincter mechanism incompetence is required. Since no true bladder neck sphincter exists in the bitch and continence is maintained by a complex mechanism of interacting factors, the term "urethral sphincter mechanism incompetence" has been used to describe a weakness of the urinary sphincter. The exact abnormality leading to urethral sphincter mechanism incompetence and the region of the urethra in which it occurs are unknown. It is a multi-factorial problem and a variety of factors are known or suspected to contribute to its clinical manifestation of urinary incontinence. These factors include urethral tone, urethral length, bladder neck position, body size and breed, ovariohysterectomy, hormones and obesity.

The introduction of urethral pressure profilometry as an investigative aid in the bitch has allowed confirmation of the belief that poor urethral tone is implicated in urinary incontinence due to sphincter mechanism incompetence. Urethral tone is maintained by a complex interaction of neuromuscular, vascular and passive elastic components and it is unclear which of these is deficient in sphincter mechanism incompetence.

There is considerable variation in urethral length between bitches of different sizes. However, taking body size into consideration, bitches with sphincter mechanism incompetence tend to have shorter urethras than continent animals.

A number of authors recorded the radiographic finding of a "pelvic bladder" during the investigation of incontinent animals. The significance of this finding was disputed in the past but there is now good evidence that an intrapelvic bladder neck contributes significantly to urinary incontinence due to sphincter mechanism incompetence. The caudal bladder position in affected dogs is associated with the shorter urethral length and also the fact that the bladder moves caudally when a bitch moves from a standing to a relaxed recumbent position. This movement is more pronounced in bitches with urethral sphincter mechanism incompetence than in normal animals, suggesting a deficiency in supporting mechanisms in the lower urinary tract of affected animals.

Body size appears to be a factor since large and giant breeds are particularly at risk. Sphincter mechanism incompetence is commonest in the UK in Dobermans and Old English Sheepdogs, and recent work has shown that these breeds and Rottweilers, Weimaraners, Springer Spaniels and Irish Setters are at significant risk.

There is an association between spaying and urinary incontinence and this is probably due to a lack of circulating oestrogens since incontinence can develop in ovariectomised as well as ovariohysterectomised bitches. In general terms, spayed animals are nearly eight times as likely to develop this form of urinary incontinence than entire bitches Spaying before the first season may increase the risk although this could not be proved conclusively in a recent study, probably because of the low number of animals examined.

Hormones, particularly sex hormones, can influence vesico-urethral function and so may be implicated in sphincter mechanism incompetence.

Whilst not a cause of the condition, obesity may worsen the degree of incontinence and weight loss may improve the situation.

The treatment of cases of sphincter mechanism incompetence may be difficult and most therapies correct only one of the above factors. It is unlikely, therefore, that anyone form of treatment alone will cure 100% of cases in the long-term.

In theory, sympathomimetic or parasympatholytic agents should improve continence control by increasing urethral tone or reducing intravesical pressure respectively but the results of clinical trials are variable and possible long-term adverse systemic effects have not been evaluated. Currently, the two medical treatments licensed for the treatment of incontinent dogs are oestrogens (Incurin, Intervet) and the alpha-adrenergic phenylpropanolamine (Propalin, Vétoquinol). Affected spayed bitches may respond well to oestrogen therapy but in some animals the response ceases eventually, despite increasing the dosage of oestrogens, possibly due to desensitisation of oestrogen receptors. Some animals show no response. A similar variation in response is also seen with the use of alpha-adrenergic drugs Oestrogens sensitise the urethral smooth muscle to alpha-adrenergic stimulation and so a combination of oestrogen and alpha-adrenergic therapy may be useful and reduce the dose of each individual drug, lessening the chances of side effects. Androgens have been employed in castrated male dogs but, in the reviewer's experience, the results are disappointing.

The main options for surgical treatment are to attempt to:

 Increase urethral resistance--e.g., periurethral surgical slings or artificial sphincters; intra-urethral injection of bulking agents;

 Increase urethral length, using bladder neck reconstruction techniques;

 Re-locate the bladder neck to an intraabdominal position by means of colposuspension or urethropexy.

The problem with techniques intended to increase urethral resistance is that they may increase the morbidity by making an incontinent animal dysuric. Similarly, increasing urethral length carries potentially serious surgical risks and, in the author's view, should be reserved for animals with severe congenital urethral hypoplasia. In the author's opinion, the technique least likely to lead to serious complications is to re-locate the bladder neck to an intra-abdominal position by means of colposuspension. The greater experiences of medical urologists treating incontinent women suggests that colposuspension provides firmer anchorage of the lower urogenital tract than urethropexy and avoids urethral trauma. The author believes that cystopexy is contraindicated in these cases since it may result in detrusor instability, a further cause of urinary incontinence!

Colposuspension is intended to move the intrapelvic bladder neck of bitches with urethral sphincter mechanism incompetence to an intra-abdominal position so that increases in intra-abdominal pressure can act simultaneously on the bladder and urethra. Thus, any increase in intravesical pressure is counteracted by an increase in urethral resistance. During the procedure, the vagina is stretched cranially and anchored on either side of the urethra to the prepubic tendon. This cranial movement of the vagina results in similar displacement of the urethra and bladder. Care is taken to avoid compression of the urethra during the technique. A long-term evaluation of cases treated by the author indicates that a cure-rate of approximately 50% can be expected with most of the remaining bitches improved. Ten percent of bitches fail to respond at all to colposuspension and the complication rate is low. Similar results have been obtained with urethropexy (White 2001).

Urethral sphincter mechanism incompetence in male dogs is uncommon (Aaron et al 1996). As in the bitch, the condition can occur as either a congenital or acquired condition. The congenital condition is often associated with gross prostatic/pelvic urethral deformity and is unlikely to be successfully treated. As in the bitch, the acquired form often follows neutering and larger breeds appear to be at risk. Incontinence is likely to occur when intra-abdominal pressure increases (e.g., during recumbency) and affected animals tend to have intrapelvic bladder necks, although a short urethra does not appear to be a factor in males (Power et al 1998). Conservative management is frequently disappointing. In comparison with the bitch, the condition is less likely to respond to medical therapy. The pathophysiology of male urethral sphincter mechanism incompetence is poorly understood, making rational treatment difficult.

Drugs used in it's management include androgens, oestrogens and alpha-adrenergics. Of these, alpha-adrenergics gave the best results but, even then, more than half of the dogs failed to respond to therapy. Attempts have been made to relocate the intrapelvic bladder neck to an intra-abdominal position. These have mainly involved pexy of the deferent ducts to the abdominal wall (Weber et al 1997) although the author has used prostatopexy (in castrated males) to the prepubic tendon. As with medical treatment, the impression in a limited number of cases is that surgical treatment alone of urethral sphincter mechanism incompetence is less successful in males than in bitches.

References

1.  Aaron, A.J., Eggleton, K.E., Power, S.C. & Holt, P.E. (1996) Urethral sphincter mechanism incompetence in male dogs: a retrospective analysis of 54 cases. Vet. Rec. 139, 542-546.

2.  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, 673-679.

3.  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, 364-372.

4.  Holt, P.E. (1990) Long-term evaluation of colposuspension in the treatment of urinary incontinence due to incompetence of the urethral sphincter mechanism in the bitch. Vet. Rec. 127, 537-542.

5.  Holt,P.E .&Thrusfield,M.V.(1993)The association between breed, size, neutering, docking and acquired urinary incontinence due to urethral sphincter mechanism incompetence in bitches. Vet. Rec. 133, 177-180.

6.  Power, S.C., Eggleton, K.E., Aaron, A.J., Holt, P.E. & Cripps, P.J. (1998) Urethral sphincter mechanism incompetence in the male dog: importance of bladder neck position, proximal urethral length and castration. J. small Anim. Pract. 39, 69-72.

7.  Rawlings, C.A., Mahaffey, M.B., Chernosky, A.C. & Huzella, L. (2000) Immediate urodynamic and anatomic response to colposuspension in female Beagles. Am. J. Vet. Res. 61, 1353-1357.

8.  Scott, L., Leddy, M., Bernay, F. & Davot, J.L . (2002) Evaluation of phenylpropanolamine in the treatment of urethral sphincter mechanism incompetence in the bitch. Journal of Small Animal Practice 43, 493-496.

9.  Thrusfield, M.V., Holt, P.E .& Muirhead, R.H. (1998) Acquired urinary incontinence in bitches: its incidence and relationship to neutering practices. J. small Anim. Pract. 39, 559-566.

10. Weber, U. T., Arnold, S., Hubler, M. & Kupper, J.R. (1997) Surgical treatment of male dogs with urinary incontinence due to urethral sphincter mechanism incompetence. Veterinary Surgery 26, 51-56.

11. White, R.A.S. & 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, 478-480.

12. White, R.N. (2001) Urethropexy for the management of urethral sphincter mechanism incompetence in the bitch. Journal of Small Animal Practice 42, 481-486.