Julie K. Byron,     Columbus. Ohio

Dennis J. Chew,     Columbus, Ohio

Mary A. McLoughlin,     Columbus, Ohio

Cause

Maintenance of urinary continence relies on many factors. Normally the major component of urethral tone is made up of smooth muscle. Mucosal integrity, vasculature, and connective tissue surrounding the urethra also play important roles in preserving continence. In addition, healthy urothelium, surface tension from glandular secretions, and the pliability of the mucosa contribute significantly to coaptation of the urethral walls.

Bladder position and the exposure of the vesicourethral junction and proximal urethra to intraabdominal pressure also are important in preserving continence. When sudden increases in abdominal pressure occur, an increase in pressure occurs within the bladder and in the proximal and midurethra, termed pressure transmission (Gregory and Holt, 1994). Several studies have evaluated the relationship of urethral position and length, as well as the vesicourethral angle to incontinence (Gregory, 1994; Gregory, Cripps, and Holt, 1996). However, these measurements overlap in normal and affected dogs; many incontinent dogs have short urethras and intrapelvic bladders. Despite lack of definitive evidence for a direct cause-and-effect relationship, bladder and urethral position appear to be an important risk factor for incontinence.

The relationship of hormone status to incontinence has long been recognized. As many as 20% of neutered female dogs are expected to develop some degree of urinary incontinence during their lives, and 75% of these dogs will do so within 3 years of neutering. Decreased estrogen concentrations in women are associated with loss of urethral muscle tone, urethral vascular atrophy, and decreased glandular secretions, affecting major components of the continence mechanism. Decreased estrogen concentrations are likely a factor in the development of incontinence in dogs, but the lack of clinically recognized incontinence in anestrus-intact females (and the many ovariectomized dogs without incontinence) supports a more complex mechanism than a simple lack of trophic effect (Richter and Ling, 1985). Recent evidence suggests that increases in luteinizing hormone and follicle-stimulating hormone associated with estrogen decrease also may play a role in the development of incontinence in dogs, but the mechanism has yet to be determined (Reichler et al, 2005).

Other risk factors for incontinence are breed, body weight, and tail docking. Among the breeds at increased risk for urinary incontinence are Old English sheepdogs, Doberman pinschers, German shepherd dogs, boxers, weimaraners, rottweilers, and Irish setters. Of the breeds evaluated, Labrador retrievers appeared to have a decreased risk of incontinence, particularly among large-breed dogs (Holt and Thrusfield, 1993). Large- and giant-breed dogs and dogs weighing more than 20 kg have a significantly increased risk of developing incontinence, whereas small-breed dogs have decreased risk. Tail docking is a suspected contributor to the onset of incontinence in dogs because damage to the muscles of the pelvic floor is considered to be a contributing factor in the development of stress incontinence in women, but no studies have related tail docking directly to incontinence. However, tail docking is common; and among the breeds with increased risk of incontinence many females are docked as neonates but develop the disorder only after they are neutered, sometimes as older adults.

Although the risk factors remain unclear, the most common type of urinary incontinence recognized in female dogs is USMI. Incontinent female dogs have lower urethral closure pressures than do continent dogs, and closure pressure increases in patients successfully managed with medical therapy. As noted previously, the closure pressure of the urethra depends on multiple factors; however, α-adrenergic receptor number and stimulation have been identified as primary therapeutic targets.

Diagnosis

Treatment

Treatment of urinary incontinence has taken several avenues. Pelvic floor muscle training and pharmacologic treatments are considered primary therapy in humans, but surgical procedures for stress incontinence in women have been described for more than 100 years and still are applied extensively. Medical and surgical treatments have been adapted for small animals, including sympathomimetic drugs, estrogens, periurethral injections, and colposuspension and artificial urethral sphincters (see Chapter 202). The use of injectable bulking agents is becoming more widespread and, with the advent of newer materials, shows significant promise in treating incontinence in dogs.