Diagnosis and management of the majority of cases are routine; however, treatment of refractory urinary incontinence cases
are frustrating for both the veterinarian and owner. The diagnostic approach to dogs with refractory urinary incontinence
should include a thorough history (drugs, age of onset, and timing during the day of incontinence), physical examination (including
rectal and neurologic examination), serum biochemistry profile, urinalysis, urine culture, abdominal radiographs and ultrasonography.
Cystoscopy and urodynamic testing including urethral pressure profile (UPP) and cystometrogram (CMG) are recommended if available.
Urethral incompetence is the most common cause of incontinence in adult female dogs. Urethral incompetence is usually occurs
months to years after neutering. Congenital anatomic or functional abnormalities of the urethra may result in urethral incompetence
prior to neutering. The pathogenesis of urethral incompetence after neutering likely involves the permissive effects of estrogen
on α-adrenergic receptors of the internal urethral sphincter, thereby promoting increased urethral tone and continence. With
decreased estrogen concentration, α-adrenergic receptors require greater stimulation to maintain urethral tone. This provides
the basis of treatment with α-adrenergic agonists or estrogen. While the dog is awake, continence may be maintained by the
external urethral sphincter. When the dog is sleeping or with muscle relaxation, the internal sphincter fails maintain continence
resulting in incontinence. In older dogs, development of PUPD from any cause may result in clinical incontinence in dogs with
marginal urethral sphincter competence.
One common reason for refractory urinary incontinence is that some dogs become refractory to the effects of long-term administration
of α-adrenergic agonists. The α-adrenergic agonist phenylpropanolamine or PPA (1.1—1.5 mg/kg PO q 8 h) is initially effective
in approximately 85% of female dogs with urethral incompetence; however, prolonged administration of PPA may cause some down-regulation
of the α-adrenergic receptors and decreased efficacy. Pseudoephedrine is less effective than phenylpropanolamine. Phenylpropanolamine
should not be used in patients with pre-existing hypertension. The side effects of phenylpropanolamine in dogs include excitability,
panting, restlessness, irritability, and hypertension.
Estrogen therapy increases urethral closure pressure theoretically by increasing the density and responsiveness of α-adrenergic
receptors in urethral smooth muscle. Estrogen therapy is effective in approximately 65% of female dogs with urethral incompetence.
Excessive doses of estrogen may cause severe bone marrow suppression; therefore owners should be cautioned not to exceed recommended
doses and complete blood counts should be monitored in dogs receiving estrogen therapy. Diethylstilbestrol is administered
at 0.1—0.2 mg/kg PO daily (maximum dose 1 mg/dog) for 5 days followed by the same dose once to twice a week. The maximum maintenance
dose of diethylstilbestrol should be 0.1 mg/kg/week. The minimum effective dose should be used for maintenance therapy.
Oral estriol is an alternative to diethylstilbestrol. Estriol is administered at a dose of 2 mg PO daily for a week, then
the dose is reduced at weekly intervals to the minimal effective dose (0.5—2.0 mg/dog given daily or every other day).} Estriol
treatement resulted in continence in 61% of dogs with an additional 22% of dogs that were improved. Estriol has been marketed
for veterinary use in Europe since 2000, and the incidence of adverse effects associated with use of this product appears
to be low. Estriol therapy increased urethral pressure in normal dogs, but the effects of estriol on urodynamic measurements
have not been reported in dog with urinary incontinence.
Treatment of refractory urethral incompetence:
Estrogen therapy up-regulates the α-adrenergic receptors that phenylpropanolamine stimulates, thus combination therapy with
these medications is synergistic. Female dogs that are refractory to either medication alone may respond to combination therapy.
For dogs that are refractory to combination therapy, urodynamic evaluation is recommended. Alternative therapies for dogs
with refractory urinary incontinence due to confirmed urethral incompetence include cystoscopic injections of bulk-enhancing
agents (glutaraldehyde cross-linked collagen) or surgical methods to increase urethral resistance.
Periurethral injection of collagen narrows the urethral lumen and allows for more effective closure of the urethra by existing
urethral pressure. Periurethral injection of collagen resolved urinary incontinence in 53 to 69% of dogs without medication.
Overall, 75 to 93% of these dogs were improved after peri-urethral collagen injection with or without concurrent administration
of phenylpropanolamine. The mean duration of continence following peri-urethral collagen injection was 17 months in one study.
Prior surgical methods for treatment of refractory urinary incontinence included colposuspension and urethropexy. Surgery
by these techniques resolved incontinence in approximately 50% of dogs with an additional 25-40% being continent with concurrent
administration of phenylpropanolamine. However, long-term evaluation suggested that these techniques had much lower long-term
improvement with only 14% of dogs still continent one year after surgery.7 A novel approach to management of refractory urinary incontinence is the surgical placement of a hydraulic urethral sphincter
around the urethra. Results of this procedure for treatment of clinically affected dogs with incontinence indicates that it
appears more effective than previously reported surgical approaches.