Urinary incontinence in dogs (Proceedings)
Urinary incontinence is a common problem that is very vexing to clients, and frequently to veterinarians. Micturition is defined as the process by which the bladder empties after becoming filled. Micturition disorders may involve failure of the filling stage or failure of the voiding stage. In order to understand the disorders and the potential therapies, a review of the anatomy and neurologic control of micturition is necessary.
AnatomyThe bladder contains a layer of smooth muscle, the detrusor muscle, which contracts during micturition. The internal urethral sphincter is located at the neck of the bladder in female dogs, and in the prostatic urethra (also at the neck of the bladder) in male dogs. The internal sphincter is smooth muscle, and is therefore not under voluntary control. The external urethral sphincter is composed of striated muscle, and thus is the voluntary portion. It is short in female dogs and is located in the midurethra portion. The external sphincter in male dogs is in the membranous urethra, and because it is longer, male dogs have more voluntary control in micturition, which is undoubtedly important in marking behavior.
Innervation: The regions of the central nervous center that are involved in micturition include the frontal cerebral cortex (for consciousness of micturition events), thalamus (conditioned micturition), brain stem micturition center (coordination of micturition), sympathetic nervous system at L1-L4, parasympathetic nervous system at S1-S3, and somatic system at S1-S3. The sympathetic nervous system (SNS) is served by the hypogastric nerve. The parasympathetic nervous system (PSNS) is served by the pelvic nerve. The pudendal nerve carries somatic information.
Sensory innervation of the bladder detects fullness and stretch. These impulses are relayed via the pelvic nerve to the brain stem micturition center and to higher center via the thalamus. Receptors in the submucosa detect extreme distension and relay pain sensation via the pelvic and hypogastric nerves. Sensory information from the urethra is carried by the pudendal nerve.
Micturition Reflex: During the storage phase, α-adrenergic stimulation via the hypogastric nerve (SNS) maintains internal urethral sphincter contraction. The external urethral sphincter maintains constant tone. Beta-adrenergic stimulation (SNS) inhibits detrusor contraction, allowing low pressure filling. When the bladder fills, stretch receptors signal the brain stem micturition center. Voluntary micturition involves abdominal contraction and relaxation of the perineal musculature. Increased parasympathetic tone to the bladder leads to contraction of the detrusor muscle. Initiation of contraction leads to inhibition of α-adrenergic tone to the internal urethral sphincter, and -adrenergic stimulation for detrusor inhibition. Storage is under sympathetic control, voiding is a parasympathetic activity.
Causes of Incontinence
Failure of urine storage usually results in the animal maintaining a small bladder and displaying incontinence. Failure of voiding usually results in the animal maintaining a large bladder or having incomplete emptying and may result in incontinence.
Small Bladder Incontinence
Primary Urethral Sphincter Mechanism Incontinence: This disorder is also known as spay incontinence or estrogen responsive incontinence. The actual cause is not clearly known, but theories include estrogen deficiency, vaginal stump adhesions that interfere with sphincter activity, damage to supporting structures of the bladder, pelvic bladder, and obesity. This is the most common cause of incontinence in the spayed female dog. It occurs within days to years of ovariohysterectomy, and it also occurs in unspayed dogs. Large breed dogs are more likely to be affected than small breed dogs. The incontinence is usually worse when the dog is lying down or asleep.
Ectopic Ureter: Continual incontinence starting at birth or a very early age is typical of ectopic ureters. The ureters bypass the bladder and open into the urethra, usually at a point distal to the internal sphincter. Occasionally, intermittent or positional incontinence has been described.
Patent Urachus: Drips urine from ventral abdomen from birth.
Vaginal Anomalies: Some vaginal anomalies, like vestibulovaginal constrictions, vaginal bands or rectal/vaginal-urethral fistulas have been associated with urine pooling and incontinence. Positional changes (i.e., standing up from a recumbent position) cause urine leakage. Most of these dogs also have a functional disturbance that accounts for the incontinence.
Urinary Tract Infection: The inflammation associated with a urinary tract infection can cause a sensory hyperreflexia in the bladder, leading to involuntary hypercontractility. Other signs of urinary tract infection, including pollakiuria and stranguria are usually present.