Jodi L. Westropp, DVM, PhD, DACVIM
University of California, Davis, School of Veterinary Medicine, Dept. of Veterinary Medicine and Epidemiology
Davis, CA, USA
Anatomy and Physiology
Micturition depends on the coordinated action among the sympathetic, parasympathetic and somatic nervous systems and central control centers. The coordination among these systems in animals appears to take place in the pontine tegmentum in the brainstem in the pontine micturition center (PMC), also known as Barrington's nucleus. The PMC receives input from other sensory stimuli to determine the onset of micturition. Not only is the fullness of the bladder important, but also the evaluation of the environment, territory demarcation and signaling will all contribute to the act of micturition. For a review of the micturition pathways, the reader is referred to the literature cited.1,2
History
Differentiations must be made between pollakiuria, polyuria, and urinary incontinence (UI) because the diagnostic plan for each complaint is quite different. An animal can also present with multiple problems such as UI and polyuria. Depending on the etiology for the incontinence, correcting the polyuric disorder can oftentimes lead to significant improvement in UI.
Urodynamic Testing
Urodynamics can be useful tests for animals that present with urinary incontinence, particularly for those dogs refractory to standard treatment. A review of urodynamic procedures can be found in the section entitled, 'Advanced diagnostics for canine and feline lower urinary tract disorders'.
Anatomic Causes for Urinary Incontinence
Ectopic ureters (EUs) are the most common cause of UI in young dogs. An ectopic ureter is defined as a ureteral opening in any area other than the normal position in the trigone of the bladder. UI is the most common clinical sign in dogs with EUs and is usually diagnosed in dogs prior to one year of age; however EUs should be considered in any dog with UI, particularly when the history is unknown. Breeds reported to be at risk include the golden retriever, Labrador retriever, Siberian husky, Newfoundland and English bulldog. EUs are uncommon in male dogs and these animals are often asymptomatic.3
A diagnosis of EUs can be made by excretory urography, fluoroscopic urethrography or ureterography, abdominal ultrasound, cystoscopy, helical computed tomography (CT) or a combination of these diagnostic procedures. The latter two appear to be the diagnostics of choice.4,5 Other congenital abnormalities can also occur in dogs with EUs, therefore it is essential to evaluate the entire urinary system prior to surgery. Urine cultures should always be performed in dogs with suspected EUs because urinary tract infections appear to be quite common with this disorder.
The standard treatment for dogs with ureteral ectopia is surgical correction, but postoperative success rates vary between 50-75%. It has been reported that dogs weighing <20kg have a better outcome postoperatively. The poor success rate could be due to a variety of causes including incorrectly identifying the terminal portion of the EU, the presence of multiple ureteral openings, concurrent urethral sphincter mechanism incompetence or a combination of these. Newer less invasive therapies have also been utilized in dogs with EUs such as cystoscopic-guided laser ablation for the ectopic ureter. Preliminary reports suggest that urinary continence after this procedure is comparable or better than after surgery, but too few cases have been done to fully evaluate long-term outcome.6
Urinary Incontinence Caused by Decreased Urethral Closure Pressure
Decreased urethral closure pressure can occur due to lumbosacral disorders such as intervertebral disc disease, degenerative myelopathy, trauma, malformations of the spinal vertebrae (e.g., Manx cats), and rare disorders such as dysautonomia. A thorough neurologic examination should be performed on all patients who present for UI. Urethral sphincter mechanism incompetence (USMI) is a diagnosis of exclusion once all other disorders have been ruled out.
UI can occur after spaying the female dog, and the onset of clinical signs can vary from immediately after spaying to 10 years after the surgery. Nocturia appears to be the most common complaint noted. UI can be daily or episodic and range from mild to very severe. There appears to be a higher risk for UI in larger breed dogs after spaying as compared to small breeds.
The exact cause of USMI is unclear and estrogen deficiency is unlikely to be the sole cause of the UI because estrogen concentrations are similar between continent anestrous dogs and incontinent spayed dogs.7 It has been reported that progesterone has antagonistic effects on estrogen and this hormone imbalance appeared to affect urethral sphincter pressure values in healthy, intact beagles.8 Estradiol has been shown to increase urethral sphincter tone in sexually intact and spayed female dogs without UI, but the urodynamic effects of estradiol are still not completely understood.9 Although some studies have documented normal activity of the external urethral sphincter others have shown reduced numbers and decreased total cross-sectional area of the type I fibers in spayed dogs which suggest a weakening of the type I portion of the urethralis muscle.10
The diagnosis of USMI can be made based on signalment, history and the absence of any other cause(s) of UI found on diagnostics. A urethral pressure profile is the gold standard and further urodynamics should be considered in refractory patients. Other neurologic abnormalities pertaining to the bladder such as detrusor overactivity may occur simultaneously and contribute to UI.
Medical management of USMI includes the use of drugs aimed at improving urethral tone via the alpha-1 adrenoceptors (α1-ARs), such as phenylpropanolamine (PPA 1-2 mg/kg BID) or pseudoephedrine (PD). Changes in maximum urethral closure pressure and functional area after PPA therapy were significantly higher in dogs with USMI on PPA compared to PD therapy.11 Side effects in dogs include restlessness, anxiety, hypertension and tachycardia. Alpha1-AR agonists are not recommended in patients with cardiac disease or hypertension.
Estrogens may also be used for USMI and these hormones are thought to sensitize the α1-AR to nor-epinephrine and indirectly result in an improvement in the closure pressure.7 However, a recent study analyzing urethral closure pressures in healthy intact beagle dogs found no additional increase in urethral pressure when PPA was added to animals already receiving estradiol. The reasons for this are not clear.12 Although estrogen therapy is usually not as successful as the therapy utilizing alpha agonists, these drugs can often be administered 2-3 times per week compared to 1-2 times per day which is significantly more convenient for the client. Bone marrow suppression has been described in dogs receiving older generation depot estrogens and in those receiving much higher doses of DES.
Submucosal urethral collagen injections are available for animals that are refractory to medications or for owners who do not wish to continually medicate their pet. Patients are placed under general anesthesia and three to four collagen deposits (Contingen®, Bard collagen implant) are injected in a circular fashion approximately 1.5 cm distal to the trigone via the cystoscope. Some dogs still require medications after this procedure, but greater continence is usually gained following the implants.
Novel treatment of USMI in ovariectomized dogs using gonadotropin releasing hormone (GnRH) analogues has been reported. It was found in the most recent published data on the GnRH analogues that although they were helpful for dogs with USMI, alpha agonists were superior. The authors theorized a possible direct effect of GnRH analogues on the bladder for the improvement that was noted.13
Urinary Incontinence Caused by Increased Urethral Closure Pressure
Mechanical causes for urinary retention include urethroliths, bladder and/or urethral neoplasia, proliferative urethritis, urethral strictures, urethral foreign bodies, urethral plugs in cats, prostatic diseases (abscess, paraprostatic cyst, benign prostatic hypertrophy), and extraluminal compressions. Functional obstructions can be seen with suprasacral or brainstem disease (upper motor neuron bladders), urethral spasms that usually occur secondary to urethritis or a mechanical obstruction, idiopathic detrusor-urethral dyssynergia, and administration of α-AR agonists. Bladder dysfunction can lead to detrusor atony and secondary 'overflow incontinence'.
The diagnosis of overflow incontinence is made based on history, thorough physical and neurologic examination, and complete imaging of the urinary tract. Most dogs will have a history of intermittent to persistent stranguria prior to the UI. Plain radiographs, cystourethrograms, and cystoscopy can all be beneficial to evaluate the patient for mechanical obstructions. An enema should be performed prior to radiographic studies in order to fully evaluate the distal urethra. Cystoscopy is useful to evaluate the urethral mucosa, obtain samples for biopsy and culture, and aid in placement of urethral stents if needed. If no mechanical obstructions are present, urodynamic studies may help provide insight for functional obstructions.
Treatment for mechanical obstructions is to either remove the obstruction if possible or treat the lesion pharmacologically. Alpha-1 adrenoceptor antagonists (phenoxybenzamine, prazosin) can be used to help relax the internal urethral sphincter. In some cases, such as reflex dyssynergia, a skeletal muscle relaxant such as valium is also beneficial. Once these drugs have taken effect, or a urinary catheter is in place, parasympathomimetics can be started. Bethanechol, a muscarinic agent, helps to restore bladder tone and facilitate bladder emptying. In severe cases, a tube cystotomy or urethral stent may be needed for urine evacuation.
Urinary Incontinence Caused by Detrusor Instability
An overactive bladder occasionally results in UI, although more commonly patients have pollakiuria. Most often animals with detrusor hyperreflexia have an underlying cystitis caused by bacteria, cystic calculi, neoplasia, polyps, or drugs. Occasionally, idiopathic detrusor hyperreflexia can occur and medical management can be beneficial in controlling clinical signs. Oxybutynin, tolterodine and some newer anticholinergics as well as tricyclic antidepressants (imipramine, clomipramine) have anticholinergic properties which can be considered for treatment of patients suspected of having detrusor hyperreflexia.
References
1. de Groat WC, Booth AM, Yshimura N. Neurophysiology of Micturition and its Modification in Animal Models of Human Disease. In: C.A. M, ed. Nervous Control of the Urogenital System. The Autonomic Nervous System. Chur, Harwood; 1993,:227-290.
2. Blok BF, Holstege G. The central nervous system control of micturition in cats and humans. Behav Brain Res 1998;92:119-125.
3. Lautzenhiser SJ, Bjorling DE. Urinary incontinence in a dog with an ectopic ureterocele. J Am Anim Hosp Assoc 2002;38:29-32.
4. Cannizzo KL, McLoughlin MA, Mattoon JS, et al. Evaluation of transurethral cystoscopy and excretory urography for diagnosis of ectopic ureters in female dogs: 25 cases (1992-2000). J Am Vet Med Assoc 2003;223:475-481.
5. Samii VF, McLoughlin MA, Mattoon JS, et al. Digital fluoroscopic excretory urography, digital fluoroscopic urethrography, helical computed tomography, and cystoscopy in 24 dogs with suspected ureteral ectopia. J Vet Intern Med 2004;18:271-281.
6. Berent A, Mayhew P. Cystoscopic-guided laser ablation of ectopic ureters in 12 dogs. JVIM 2007;21:600, abstract #101.
7. Reichler IM, Pfeiffer E, Piche CA, et al. Changes in plasma gonadotropin concentrations and urethral closure pressure in the bitch during the 12 months following ovariectomy. Theriogenology 2004;62:1391-1402.
8. Hamaide AJ, Verstegen JP, Snaps FR, et al. Influence of the estrous cycle on urodynamic and morphometric measurements of the lower portion of the urogenital tract in dogs. Am J Vet Res 2005;66:1075-1083.
9. Hamaide AJ, Grand JG, Farnir F, et al. Urodynamic and morphologic changes in the lower portion of the urogenital tract after administration of estriol alone and in combination with phenylpropanolamine in sexually intact and spayed female dogs. Am J Vet Res 2006;67:901-908.
10. Augsburger HR, Cruz-Orive LM. Influence of ovariectomy on the canine striated external urethral sphincter (M. urethralis): a stereological analysis of slow and fast twitch fibres. Urol Res 1998;26:417-422.
11. Byron JK, March PA, Chew DJ, et al. Effect of phenylpropanolamine and pseudoephedrine on the urethral pressure profile and continence scores of incontinent female dogs. J Vet Intern Med 2007;21:47-53.
12. Carofiglio F, Hamaide AJ, Farnir F, et al. Evaluation of the urodynamic and hemodynamic effects of orally administered phenylpropanolamine and ephedrine in female dogs. Am J Vet Res 2006;67:723-730.
13. Reichler IM, Jochle W, Piche CA, et al. Effect of a long acting GnRH analogue or placebo on plasma LH/FSH, urethral pressure profiles and clinical signs of urinary incontinence due to sphincter mechanism incompetence in bitches. Theriogenology 2006;66:1227-1236.