USMI most often occurs in bitches after spaying since the urethral closure pressure is significantly reduced within 1 year. Even 40 years ago, urinary incontinence was described as a rare side effect of spaying.1 However, it took 20 years to verify the relationship between the removal of the ovaries and urinary incontinence.2 Urinary incontinence is a common problem in spayed bitches,3-5 affecting up to 20%.6,7 Larger bitches and those from breeds such as boxer, giant schnauzer, Doberman, and rottweiler seem to be at increased risk. In bitches with USMI, incontinence typically occurs during sleep.
Urinary incontinence due to USMI also occurs in male dogs; occurrence is less frequent than in bitches, however, and castration does not seem to increase the risk. Affected male dogs tend to be incontinent when awake.
After a thorough history is obtained, a neurologic examination should be performed to rule out a neurologic cause of urinary incontinence. Development of incontinence may be solely due to USMI or can result from the additive effect of reduced sphincter tone, detrusor instability secondary to the inflammation of bacterial cystitis, and conditions that induce polyuria.
Bacterial cystitis can be ruled out by bacteriologic examination of a urine sample obtained by cystocentesis. If the bacteriologic result is positive, the patient is treated with an antibiotic according to the sensitivity. In many patients, urinary incontinence will resolve once bladder inflammation subsides. Similarly, with diseases accompanied by polyuria, resolution of or at least reduction in urine volume can decrease or eliminate clinical signs. Biochemical blood and urine analyses are useful to determine the cause of polyuria.
In patients in which the sole reason for inappropriate urination is urinary tract infection, pollakiuria and stranguria are usually observed, and urine is passed when the animal is conscious and adopts a normal urination posture. Similarly, patients with inappropriate urination secondary to polyuria adopt a normal urination stance. In most patients, careful history regarding the circumstances of inappropriate urination allows differentiation of USMI from pollakiuria and polyuria; in USMI, loss of urine is involuntary.
Iatrogenic Ureterovaginal Fistula
If urinary incontinence occurs immediately after ovariohysterectomy, an iatrogenic ureterovaginal fistula must be ruled out by a contrast imaging study. Incontinent male dogs must be examined for ectopic ureters. In contrast to bitches with this congenital malformation, which mostly results in urinary incontinence during puppy-hood, male dogs often become incontinent at an advanced age.
In typical cases of urinary incontinence due to USMI, the results of the examinations and analyses mentioned previously will be normal. For the practitioner, a diagnosis of USMI made by ruling out other causes is sufficient. In questionable cases, USMI can be directly proven by recording a urethral pressure profile, for which a special urodynamic unit is required. The urethral pressure profile allows measurement of the maximal urethral closure pressure. While absolute values vary with the type of sedation used to obtain a urethral pressure profile, in spayed continent bitches the maximum urethral closure pressure is about 10 cm H2O; in spayed incontinent bitches this parameter is about 4 cm H2O.
Levels of Care
Medical treatment of USMI is the method of choice and should always precede surgery. For first-line therapy, α-adrenergic agonists are used. These sympathomimetic drugs are effective because 50% of the urethral closure pressure is generated by the sympathetic nervous system. Alpha-adrenergic agonists increase urethral closure pressure, which results in continence in at least 75% of incontinent bitches. In males with USMI, the success rate of this treatment is less than 50%.
A recent study indicates that phenylpropanolamine may be superior to pseudoephedrine for increasing urethral closure pressure.8 If this therapy is unsuccessful, additional therapy with flavoxate or estrogens (in bitches only) is indicated. However, estrogens may cause unwanted side effects, such as swelling of the vulva and attractiveness to male dogs. Only short-acting estrogens, diethylstilbestrol, or conjugated estrogens are now used. The depot preparations used in the past are obsolete, in part because they may cause bone marrow depression.
For refractory cases, different surgical therapies are available, of which colposuspension,9 urethropexy,10 and the endoscopic injection of collagen11 are mainly used, with a success rate of 50% to 75%.
When to Consider Referral
• If the dog does not respond to combined medication, or the effect is insufficient
• If side effects such as diarrhea, vomiting, or nervousness occur
• If the dog has a disease that is a contraindication for α-adrenergic substances (renal insufficiency, hypertonia, cardiac arrhythmias, glaucoma)
• If regular administration of medication is not possible
When Referral Is Unnecessary
• If the dog has cystitis
• If the bitch or the male dog responds well to the therapy
• If no side effects occur
• If regular oral administration of the tablets is not a problem
The Referral Process
The referring veterinarian should provide a thorough history and the results of hematologic tests, biochemistry analysis, and urinalysis. The result of the bacteriologic analysis of the urine should be negative, and the urine sample should have been collected no more than 1 week previously. The referred patient should have an empty stomach.
Collagen is injected under general anesthesia. In bitches in which the cystoscope can be introduced via the urethra, the whole procedure takes about 20 minutes. Three collagen deposits are injected in a circular manner beneath the mucosa of the urethra, approximately 1.5 cm caudal to the bladder neck. The amount of collagen required depends on the bulking effect because the procedure is considered complete when the urethral lumen is closed by the collagen deposits (Figure 1).
Collagen may also be injected into males with good results. The procedure takes longer because it requires both laparotomy and cystotomy. Since the continence zone of the male urethra is mainly the pars prostatica, the collagen is injected at the level of the vas deferens openings. The injection of foreign material at this position increases the risk for prostatitis; as a result, sexually intact dogs with USMI should be castrated 3 weeks before the procedure.
Experience has shown that in the first 12 months after the injection, the initial result may deteriorate. When incontinence recurs, the clinician should first rule out cystitis. If the patient does not have cystitis, then the collagen injection can be repeated or medical therapy can begin. It is often possible for incontinence to improve with conservative therapy, even if there had been no or an unsatisfactory response before the collagen injection.