Urinary incontinence is a common disorder in older female dogs, as many as 20% of neutered bitches develop it at some point in their lives. The reason is usually urethral sphincter mechanism incompetence (USMI). It can happen to any breed, though Boxers, Dobermanns and Collie breeds are particularly susceptible. In this article we review the current state of knowledge on this condition, focusing on the management and treatment options.

What is Urethral Sphincter Mechanism Incompetence (USMI)?

Affected bitches that are affected by this have poor tone to the smooth muscle surrounding their urethra (the tube that connects the bladder with the vagina), and often a shorter urethra than normal, and a bladder neck that is positioned too far back in the abdomen.

There are a number of contributing factors to this condition developing, but the main ones are breed susceptibility, obesity, body size, bladder position and spaying (neutering). Spaying is thought to have an effect by decreasing the estrogen levels, which affects the local muscle tone. However, most spayed bitches do not develop the condition and it should certainly not be used as an excuse not to neuter, as the increased likelihood of cancer in non neutered bitches is far more worrying.

What are the clinical signs?

The disorder is characterized by the passive leakage of urine without the dog noticing, often during sleep or rest while the dog is lying down. The owner usually remarks how there is always a stench of urine coming from the dogs bedding, and how the dog seems permanently wet and smelly around the back end. Continual leakage can also cause scalding of the skin around the vulva and groin.

How is it diagnosed?

Diagnosis is usually made based on the history. The pattern of urination is important to rule out other causes of incontinence. Urinalysis, radiography and ultrasonography can be performed for confirmation if necessary (e.g. to assess the location of the bladder neck within the abdomen).

Treatment

The goal of treatment is to increase the tone of the urethral smooth muscle. This can be done medically, surgically or by both. Medical treatment is either by giving estrogens or adrenergic drugs.

Estrogens

Estrogens have been used for a long time as HRT in post menopausal women. Estriol, a naturally occurring estrogen, is available as a licensed veterinary product and works by restoring the normal function of the urethral sphincter muscle, and increasing the sensitivity of the urogenital tract to adrenergic stimulation. Side effects traditionally associated with long term estrogen use include bone marrow depletion, but this has not been observed with Incurin, the licensed veterinary drug.

Adrenergic drugs

Adrenergic drugs stimulate relaxation of the bladder wall and contraction of the urethral sphincter simultaneously. Both of these actions facilitate urine storage. Phenylpropanolamine (Propalin, Vetoquinol) is the most frequently used drug for this, and is extremely effective in the majority of cases.

Surgery

Surgery is usually only appropriate if medical treatment has failed to control the incontinence. The procedure most often used performed is colposuspension, which involves pulling the bladder neck and vagina forwards from within the pelvis to an abdominal position. This has a cure rate of about 50%.

Another technique is the transpelvic sling, where a ribbon is passed through the obturator foramen in the pelvis and between the urethra and vagina. The ribbon is pulled tight and fixed to itself. This may be performed in addition to colposuspension.

A less invasive technique is endoscopic injection of Teflon or collagen into the urethra about 1.5cm away from the bladder neck, artificially increasing the tone of the smooth muscle there. This has a reported success rate of just over 50%, and is sometimes done alongside adrenergic medication.