July 03, 2018

Canine Urinary Incontinence

The mechanisms behind this familiar condition are multilayered and poorly understood, and treatment must be individualized based on the cause of the problem.
By Julie K. Byron, DVM, MS, DACVIM
Illustration by Diogo Guerra, medical illustrator
Urinary incontinence (UI), a common problem encountered in small animal practice, can result from congenital anatomic abnormalities, urine retention and overflow incontinence, or sphincter incompetence. The timing of the onset of UI and the ability of a dog to empty its bladder are important in determining the underlying cause. Behavioral changes are important factors as well.

This review focuses on 2 of the most common causes of UI: urethral sphincter mechanism incompetence (USMI) and functional urethral obstruction, also known as detrusor urethral dyssynergia (DUD).

History and Diagnosis

A careful history is essential when discussing UI with pet owners. UI must be distinguished from behavioral conditions and systemic problems (eg, polyuria, pollakiuria), and it is important to establish that the animal is unaware of the passage of urine. Young dogs that have never been continent or have been “difficult to housetrain” should be evaluated for congenital abnormalities, such as ectopic ureter(s). Patients whose UI has developed concurrent with an increase in water intake should be evaluated for disorders causing polyuria and polydipsia.

Physical examination should include a rectal examination with careful palpation of the urethra. Obstructive processes such as urethral neoplasia and prostatic hyperplasia can often be diagnosed this way. If possible, observe the dog while it is urinating. This is particularly important in males because functional obstruction and overflow incontinence may be more common than previously thought. Residual urine volume should be measured after voiding in dogs that have a narrow urine stream, exhibit stranguria, or only drip urine. Finally, a neurologic and orthopedic evaluation should be conducted if the dog is unable to posture normally to urinate, as this can lead to incomplete emptying of the bladder and UI.

Urinalysis with sediment examination should be conducted for all patients with UI. Although urine leakage may be exacerbated by a urinary tract infection (UTI), the incontinence itself may predispose the patient to a UTI. There is debate as to whether treating dogs with bacteriuria that lack signs, such as stranguria or dysuria, is appropriate. Bloodwork is indicated if the patient has a low urine-specific gravity, particularly in the face of dehydration.

UI typically develops in young or middle-aged dogs.1 Those with older onset may require additional diagnostic investigation as to an underlying cause. Some dogs may have mild USMI but are rarely incontinent until a secondary factor, such as increased urine volume, comes into play. If the underlying reason for polyuria can be addressed, the UI may improve.

Urethral Sphincter Mechanism Incompetence

USMI is the most common cause of UI in dogs, affecting up to 20% of all neutered females and 30% of neutered females weighing over 20 kg.2,3 It is less commonly reported in neutered males and intact dogs of both sexes. In most dogs, incontinence occurs within 3 to 4 years of neutering,1 although it may not become a major problem until later in life when it can be complicated by diseases that cause polyuria and polydipsia. Among the most commonly affected breeds are the Old English sheepdog, Doberman pinscher, boxer, German shepherd, and Weimeraner.3 Results from a recent study indicate that earlier neutering may increase the risk of USMI development in dogs with a projected adult body weight greater than 15 kg.1

Although many risk factors for the development of UI in dogs have been identified, the mechanism of development remains unclear. The underlying mechanism is likely multifactorial and involves the pituitary-gonadal axis, the anatomic structure of the lower urinary tract, and the integrity and tissue characteristics of the lower urinary tract’s supporting structures. Several things are important in the maintenance of a closed urethra, including vascular tone (which can represent up to 30% of closure pressure), the strength of supporting structures in the pelvic region, and the position of the bladder. Some studies’ findings have implicated a rise in luteinizing hormone and gonadotropin-releasing hormone (GnRH) levels as well as increases in their receptors in female animals secondary to the decline in estrogen levels after neutering.4,5 These changes may affect smooth muscle contractility in the lower urinary tract. In support of this theory, data from one study have shown that the administration of GnRH analogues to dogs with USMI can lead to improvement of continence.5

In animal models, estrogen has trophic effects on the vasculature and tissue matrix of the lower urinary tract and its support structures. The decline in estrogen levels after neutering in female dogs may cause a decrease in the vasculature of the urethra and its supporting tissues, which is an important part of maintaining a closed urethra. There is also a possibility that the collagen content may be altered in females with USMI; however, further studies are needed.

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