Page 760 - Small Animal Internal Medicine, 6th Edition

732 PART V Urinary Tract Disorders

(USMI) or other concurrent anatomic abnormalities that BOX 45.1
may have been present in the dog.
VetBooks.ir URETHRAL SPHINCTER MECHANISM Considerations for Referral for Urethral Pressure Profile
and Cystometrography
INCOMPETENCE
Decreased urethral closure pressure can occur because of Before ectopic ureter correction
lumbosacral disorders such as intervertebral disk disease, If drug therapy with phenylpropanolamine (PPA) for
degenerative myelopathy, trauma, malformations of the urethral sphincter mechanism incompetence (USMI)
was not effective
spinal vertebrae (e.g., in Manx cats), and rare disorders such If drug therapy with diethylstilbestrol (DES) for USMI was
as dysautonomia. A thorough neurologic examination not effective
should be performed on all dogs and cats that present for UI. If the patient with suspected USMI has a higher risk of
USMI is a diagnosis of exclusion once all other disorders side effects with medical management for USMI, then
have been ruled out and can usually be made based on a confirming the diagnosis is advised.
compatible signalment and history, and absence of abnor-
malities on the physical examination.
USMI is similar to stress incontinence that occurs in
women. USMI is primarily a disease of dogs; all cats sus- A list of drugs and suggested dosages for managing
pected of having USMI should be tested for feline leukemia various micturition disorders is presented in Table 45.1.
virus (FeLV) because associations between these two prob- Medical management of USMI includes the use of drugs
lems have been suggested. USMI usually occurs in spayed aimed at improving urethral pressures via the α 1 -adrenoceptors
bitches but can occur in intact females and male dogs as well. (α 1 -ARs), such as phenylpropanolamine (PPA). Rarely pseu-
In spayed females, the onset of clinical signs can vary from doephedrine is used, but PPA is usually is more effective than
immediately after spaying to 10 years after the surgery. Noc- PSE and has fewer adverse effects. The therapeutic benefit with
turia appears to be the most common complaint from α-agonists is usually seen within 2 to 3 days. If no benefit is
owners. UI can be daily or episodic and can range from mild seen after 1 week, the dosage can be increased up to 1.5 mg/kg
to very severe. There appears to be a higher risk for USMI in orally (PO) q12h; if clinical signs are still present or side effects
larger-breed dogs after spaying as compared with small exist, another agent, such as an estrogen compound, should
breeds. Although male dogs can develop USMI, it is critical be considered. Adverse effects in dogs receiving α-agonists
to ascertain a residual urine volume because voiding disor- can include restlessness, anxiety, hyporexia, and possibly
ders associated with urinary retention and paradoxical UI is hypertension. A reflex bradycardia has also been reported.
more often seen in male dogs. In a study, hypertension was not reported in healthy dogs
The exact cause of USMI is unclear. Estrogen deficiency at dosages typically used for USMI. α 1 -AR agonists are not
is unlikely to be the sole cause of the UI because estrogen recommended for animals with cardiac disease, hyperten-
concentrations are similar between continent anestrous dogs sion, or kidney disease, and PPA has not been studied in
and incontinent spayed dogs. Estrogen has been shown to these groups of dogs. If required for dogs with kidney disease,
increase urethral sphincter closure pressure in sexually intact lower dosages should be administered and the dog’s blood
and spayed female dogs without UI, but the urodynamic pressure should be monitored regularly, ideally 2 to 4 hours
effects of estradiol are still not completely understood. Other post–PPA administration.
risk factors for USMI have been reported to include larger- Estrogens may also be administered for treatment of
breed dogs (>20 kg), tail docking, obesity, and neutered USMI because these hormones are thought to sensitize the
status. Timing of spay with regards to USMI risk is still con- α 1 -AR to NE and indirectly result in an improvement in the
troversial, and the literature is variable. There is some weak closure pressure, but the exact mechanism of action of these
evidence that “early” spay (“early” has not been well defined drugs is unclear. A U.S. Food and Drug Administration
in studies) may increase the risk of acquiring USMI, particu- (FDA)-approved estrogen compound, estriol (Incurin,
larly if dogs are >25 kg. Merck Animal Health, Summit, NJ) is available (see Table
Although a urethral pressure profile (UPP) is the gold 45.1). The same dosage is provided to every dog regardless
standard to document decreased urethral closure pres- of body weight (2 mg PO q24h per dog) and is initially
sure (Box 45.1), the diagnosis of USMI can often be made administered daily for 2 weeks and then titrated down to the
based on signalment, history, and the absence of any lowest effective dose. However, the author had success
other cause(s) for the UI. Response to drug therapy (see administering only 1 mg daily as a loading dosage to smaller-
later) provides further evidence for USMI. Urodynamic breed dogs (to avoid the potential side effect of vulvar hyper-
tests can be performed in cats and dogs, are usually per- plasia). The therapeutic effect is often noted within the first
formed with propofol sedation, and provide data related to week. If no benefit is seen this time, other therapeutic options
bladder and urethral function. A UPP evaluates the pres- should be considered. In some cases of USMI, a beneficial
sure along the entire length of the urethra; a cystometro- effect is noted when combining PPA and estrogen therapy.
gram evaluates the detrusor reflex, bladder fill volumes, Older estrogen compounds such as diethylstilbestrol (DES)
and compliance. are also usually available and can often be administered