Urinary Incontinence in Spayed Bitches: New Insights into the Pathophysiology and Options for Medical Treatment

INCIDENCE OF INCONTINENCE AFTER OVARECTOMY
Urinary incontinence is the involuntary loss of urine (1). In intact bitches urinary incontinence is rare (0-1 %) (2), whereas in spayed bitches the incidence is up to 20% (3).
The underlying pathophysiological mechanism is mainly an acquired insufficient closure of the urethra after spaying (4). Therefore urinary incontinence after spaying is called urethral sphincter mechanism incompetence (USMI). Within one year after spaying the urethral closure pressure is significantly reduced. Because many bitches only become incontinent years after surgery it took a long time until spaying was considered to be the cause. In one study, 83 of 412 (20%) bitches became incontinent 3 to 10 years after surgery (3).
Even 40 years ago urinary incontinence was described as a rare side effect of spaying (5). However, it took 20 years to verify the causal relationship between the removal of the ovaries and urinary incontinence (6). It is still unclear what mechanism is the trigger.
Neuronal damage can most likely be disregarded, as the risk of urinary incontinence is the same in ovariectomizsed and ovariohysterectomizsed bitches and in many cases the urinary incontinence occurs years after the surgery (3).

THE ROLE OF ESTROGEN DEFICIENCY
It is generally assumed that USMI after spaying is due to an estrogen deficiency (7,8). In view of other facts it appears unlikely that estrogen deficiency alone accounts for USMI after spaying. For example, bitches treated with long acting gestagens, to suppress the sexual cycle, have no increased risk for urinary incontinence, although this treatment leads to suppressed ovarian function (9) and a serum estradiol concentration that remains at a basal level (10). In addition, the daily supplementation of estrogen only results in 61-65% of incontinent bitches becoming continent (3,11,12). Also, the plasma estrogen concentration of spayed incontinent bitches is the same (13) or slightly lower (14) than that of intact, continent bitches.

MEDICAL TREATMENT OF USMI
The medical treatment of USMI is the method of choice and should always precede a surgical therapy. The action of the used substances is aimed at increasing the urethral closure pressure.

Alpha-adrenergic drugs
In the first line alpha-adrenergic agonists are used. The effect of these sympathomimetic drugs is explained by the fact, that 50% of the urethral closure pressure is generated by the sympathetic nervous system. Alpha-adrenergic agonists improve the urethral closure pressure by stimulation of the alpha-receptors of the smooth urethral musculature (13, 15-19). The treatment with alpha-adrenergic agonists results in continence in 75% of incontinent bitches.

The alpha-receptors are divided into alpha1- and alpha2-subtypes. These receptor subtypes are distributed differently in each single effector. Alpha-1 receptors are found in many target organs of the sympathetic nervous system. With a few exceptions, alpha-2 receptors are not present in target organs of the sympathetic nervous system, but in neuronal synapses. It is now known, that the alpha-receptors at the bladder neck and proximal urethra of the bitch, which are responsible for continence, belong to the subtype 1 (20). [...]