Canine Urethral Sphincter Mechanism Incompetence

During urine storage, urethral resistance is maintained by a complex mechanism of factors. These include tone in the urethral smooth muscle (the "internal" sphincter), tone in the urethral striated muscle (the "external" sphincter), the natural elasticity of the urethral wall tissues (not only musculature), the physical properties of the urethra (length and diameter), and the degree of engorgement of suburothelial venous plexuses. Because 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" was suggested to describe a weakness of the urinary sphincter [1]. This term has been widely adopted.

The neuromuscular function of the lower urinary tract in the dog is complex and not completely understood. A summary is provided in Fig. 69.1.

Figure 69.1. The neuromuscular control of urinary continence. During storage, the sympathetic innervation exerts a β (relaxation) effect on the bladder and an α-adrenergic (contractile) effect on urethral smooth muscle. In some species such as humans and cats, the sympathetic innervation also acts on the urethral striated muscle, but this has not yet been shown to occur in the dog. Urethral striated muscle provides additional tone during times of stress and is under voluntary control via the pudendal nerve.

Urethral sphincter mechanism incompetence occurs in dogs and cats, although it is rare in the latter. In dogs referred for the investigation of urinary incontinence, urethral sphincter mechanism incompetence is the most common diagnosis made in adults and is second only to ureteral ectopia in juveniles, particularly in females [2-4].

It can present as a congenital or acquired condition. The acquired form is rare in the cat; in that species, most cases of urethral sphincter mechanism incompetence are congenital [5]. Although urethral pressure profilometry can be used to demonstrate incompetence of the urethral sphincter mechanism [6], this technique is not readily available in general practice and is predisposed to a number of artefacts that can make interpretation difficult [7,8]. In general practice, therefore, the diagnosis is usually made on the basis of the breed and the history, and by the elimination of other possible diagnoses using imaging and laboratory techniques.

Congenital Urethral Sphincter Mechanism Incompetence in Bitches

This tends to be a problem of large breeds of dog, predominantly bitches. Leakage of urine is more copious compared with animals with ureteral ectopia and occurs predominantly when the dogs are recumbent. The urethra may be abnormally short or even absent, especially in cats, and urethral diverticula and dilations may be present in juvenile male dogs [9]. In many bitches, no gross abnormalities are detected on contrast radiographic investigations, apart from a caudally positioned bladder. 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 estrus. Those that do not may be candidates for medical or surgical management. Estrogens are contraindicated 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 estrus.

Acquired Urethral Sphincter Mechanism Incompetence in Bitches

Acquired urethral sphincter mechanism incompetence usually (but not always) follows spaying in the bitch [10,11] and castration in male dogs [12]. Before treatment can be contemplated, an understanding of the pathophysiology of urethral sphincter mechanism incompetence is required. The exact abnormality leading to urethral sphincter mechanism incompetence and the region of the urethra in which it occurs are unknown. It is a multifactorial problem and various 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/ovariectomy, hormones, and obesity.

Urethral Tone

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 owing to urethral sphincter mechanism incompetence [6]. 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.

Urethral Length

Considerable variation exists in urethral length among bitches of different sizes. However, taking body size into consideration, bitches with urethral sphincter mechanism incompetence tend to have shorter urethras than do continent animals [13].

Bladder Neck Position

Several 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 good evidence exists that an intrapelvic bladder neck contributes significantly to urinary incontinence owing to urethral sphincter mechanism incompetence [13]. 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 [14]. The role of bladder neck position in bitches with urethral sphincter mechanism incompetence is thought to be due to changes in transference of abdominal pressures to the urethra (Fig. 69.2) [13,15,16].

Figure 69.2. The effect of bladder neck position on continence control. In an animal with an intra-abdominal bladder neck (A), increases in abdominal pressure are transmitted not only to the bladder but simultaneously to the bladder neck and proximal urethra. Thus any increase in intravesical pressure may be counteracted by an increase in urethral resistance. If the bladder neck is intrapelvic (B), however, abdominal pressure increases act predominantly on the bladder and less efficiently on the bladder neck and proximal urethra. If urethral tone is adequate, bladder neck position is of little relevance but, if urethral sphincter mechanism incompetence exists, the bitch with the anatomy illustrated in Figure 69-2B is more likely to leak urine at times of intra-abdominal pressure increases (e.g., recumbency). Unfortunately for affected bitches and male dogs, the intrapelvic bladder neck is a common finding in animals with urethral sphincter mechanism incompetence. Retroperitoneal accumulations of fat in obese animals are likely to worsen this situation by displacing the caudal extent of the abdominal cavity cranially. v – vagina; b – bladder; the dotted line represents the caudal extent of the abdominal (peritoneal) cavity.

Body Size and Breed

Body size appears to be a factor because large and giant breeds are particularly at risk. Urethral sphincter mechanism incompetence is most common in the United Kingdom in Dobermans and Old English sheepdogs, and evidence exists that these breeds as well as Rottweilers, Weimaraners, springer spaniels, and Irish setters are particularly at risk [10].

Ovariohysterectomy/Ovariectomy and Hormones

An association exists between spaying and urinary incontinence [11]. This is probably caused by a lack of circulating estrogens [17], although one study suggests that an excess of gonadotrophins may also be a factor [18]. In general terms, spayed animals are nearly 8 times as likely to develop this form of urinary incontinence as are intact bitches [11]. However this should be considered in terms of dog years. In simple terms, if 100 bitches were not spayed, 10 years later, two of them would be incontinent. If 100 bitches were spayed, about 16 of them would be incontinent after 10 years. Spaying before the first season may increase the risk, although this could not be proved conclusively in a prospective study [11], probably because of the low number of animals examined. One study suggested that, if early gonadectomy of female dogs is practiced, it should be delayed until the animal is at least three months of age [19]. The function and morphometry of the lower urinary tract are affected by the changes in hormone balance that occur during the estrus cycle [20].

Obesity

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

Vestibulovaginal Stenosis in Relation to Urethral Sphincter Mechanism Incompetence in Bitches

Vestibulovaginal stenosis (owing to a true stricture or persistent perforate hymen) has been linked to urinary incontinence by some authors [21-23]. Holt and Sayle suggested that the incontinence might be associated with urine accumulating cranial to the stenosis during micturition and subsequently dribbling out [21]. However, Holt later demonstrated no difference in the prevalence of vestibulovaginal stenosis between normal and incontinent bitches [13]. The author now believes that vestibulovaginal stenosis per se is not a cause of incontinence, although if a bitch is already incontinent and leaking urine into the vagina, some may accumulate cranial to the stenosis, exacerbating the signs and predisposing to vaginitis. Experience suggests that the incontinence is a result of other causes such as urethral sphincter mechanism incompetence and that, if the incontinence owing to these other causes can be cured, the vestibulovaginal stenosis is irrelevant.

Urethral Sphincter Mechanism Incompetence in Male Dogs

Urethral sphincter mechanism incompetence in male dogs is uncommon [9]. 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 deformities (urethral dilatations and prostatic urethral diverticulae) and is unlikely to be successfully treated. As in the bitch, the acquired form is associated with neutering [12], and larger breeds appear to be at risk. Incontinence is likely to occur when intra-abdominal pressure increases (e.g., during recumbency). Affected animals tend to have intrapelvic bladder necks, although a short urethra does not appear to be a factor in males [12]. 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 its management include androgens, estrogens, 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 [9]. Attempts have been made to relocate the intrapelvic bladder neck to an intraabdominal position. These have mainly involved pexy of the deferent ducts to the abdominal wall [24,25]. although the author has used prostatopexy (in castrated males) to the prepubic tendon [26]. 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.

Practical Applications

The treatment of sphincter mechanism incompetence may be difficult and most therapies correct only one of the factors discussed. It is unlikely, therefore, that any one 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 (Fig. 69.1), but the results of clinical trials are variable and possible long-term adverse systemic effects have not been evaluated. Currently, the most popular medical treatment is the alpha-adrenergic, phenylpropanolamine [27-30]. Affected spayed bitches may also respond to therapy with estrogens such as estriol [31]. In some animals that respond initially to alpha-adrenergics or estrogens, the response ceases eventually. In the case of estrogens, this is possibly a result of desensitization of estrogen receptors. Estrogens sensitize the urethral smooth muscle to alpha-adrenergic stimulation and so a combination of estrogen 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 author's experience, the results are disappointing.

The main options for surgical treatment are to attempt to:

- Increase urethral resistance - e.g., peri-urethral surgical slings [32,33] or artificial sphincters [34]; intra-urethral injection of bulking agents [35-37];
- Increase urethral length, using bladder neck reconstruction techniques [38];
- Re-locate the bladder neck to an intraabdominal position by means of colposuspension [39-43], vas deferentopexy [24,25], urethropexy [44,45] or prostatopexy [26].

The potential problem with techniques intended to increase urethral resistance is that they may increase the morbidity by making an incontinent animal dysuric. Urethral bulking agents have been used most commonly to increase urethral resistance. 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 relocation of the bladder neck to an intraabdominal 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. It may also prevent the caudal movement of the bladder that occurs during recumbency and is more pronounced in bitches with urethral sphincter mechanism incompetence [14]. However, a review of urethropexy as a treatment for urethral sphincter mechanism incompetence in 100 bitches revealed similar results to colposuspension [], although the prevalence and severity of urethral complications was higher. The reviewer believes that cystopexy is contraindicated in these cases because 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 intraabdominal 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 (Fig. 69.2).

Although great progress has been made over the past three decades in our understanding of the pathophysiology and, thus, the treatment of canine urethral sphincter mechanism incompetence, a recent editorial by one of the world's most respected urologists suggests we still have a great deal to learn [46]. Despite over 200 procedures being described for the treatment of human stress incontinence, the author was left asking, when it comes to his own patients, what should he do?