Detrusor atony is a condition that occurs in previously obstructed cats after excessive expansion of the urinary bladder.19 In this condition, the tight junctions of the smooth muscle in the bladder become stretched or torn apart, resulting in
weak, uncoordinated, or completely absent contractions of the bladder muscle.2,19 Cats with this complication usually have a persistently large bladder that may or may not be easily expressed.
Treating atony. Once recognized, treatment for this condition is straightforward and involves keeping the bladder small to allow the tight
junctions to heal. This can be accomplished by regular manual expression of the bladder or placement of a urinary catheter.
Pharmacologic intervention with bethanechol (1.25 to 7.5 mg/cat orally, every eight hours) or cisapride (0.5 mg/kg orally,
b.i.d. or t.i.d.) until normal bladder function has returned (anywhere from a few days to a few weeks) can also be considered.
Bethanechol activates the muscarinic receptors in the detrusor muscle (parasympathetic stimulation) causing contraction. Cisapride
increases the release of acetylcholine to facilitate binding to muscarinic receptors and induce parasympathetic-induced contraction
of the bladder. In the normal micturition reflex, sympathetic innervation from the spinal cord simultaneously causes relaxation
of the internal urethral sphincter.19,20 Therefore, if bethanechol or cisapride is administered, it is imperative that the patient also be given a separate drug
to relax the urethral sphincter to facilitate bladder emptying. (See urethral hyperactivity below.)
In some cases, urethral spasms in cats may be a cause of functional obstruction.2 Clinically, these cats appear to be straining to urinate and pass only small amounts of urine. However, urine can be expressed
from these cats relatively easily because there is no physical obstruction.
Urethral edema and inflammation commonly develop in obstructed cats due to catheterization and the presence of the catheter,
and they are likely the cause of smooth and skeletal muscle spasms along the urethra's length.2,21 The concern in patients displaying urethral hyperactivity is that they are unable to completely express their bladders and
will require recatheterization.
Though it might seem that an obvious treatment for edema and inflammation would be an anti-inflammatory drug (a glucocorticoid
or NSAID), there is no published evidence that these medications reduce urethral inflammation. Also, with the well-documented
fact that both corticosteroids and NSAIDS can be detrimental in patients that are dehydrated or that have any sort of renal
dysfunction, there is no good support for the use of these drugs in cases of urethral obstruction.
Treating urethral hyperactivity. Therefore, treatments for urethral hyperactivity center on the use of the smooth muscle relaxants prazosin and phenoxybenzamine.
The preprostatic urethra in male cats is primarily smooth muscle, whereas the postprostatic region is primarily skeletal muscle.21 Prazosin (0.25 to 0.5 mg/cat orally, every 12 to 24 hours) is a selective alpha-1 antagonist, and phenoxybenzamine (2.5
to 10 mg/cat orally, divided one to two times daily) is a nonspecific alpha antagonist. Alpha receptors control the contraction
of smooth muscle, and therefore these medications are aimed at relaxing smooth muscle in the bladder neck and preprostatic
urethra to enhance urine flow.22 Both medications can be given orally to cats but can cause hypotension in sensitive cats or when given at higher dosages.
Clinical studies do not strongly support the efficacy of these medications in cats, but both are used commonly in clinical
cases.21,22 Some experimental evidence indicates that dantrolene, a skeletal muscle relaxant, in combination with prazosin, may reduce
urethral muscle tone in cats.21
Some cats experience reobstruction in short order after the urinary catheter is removed. Anecdotally, reobstruction occurs
in a large percentage of cats, most commonly in the first 24 hours after urinary catheter removal. One study describing 41
cats with urethral obstruction indicated that 14 cats (36%) had reobstruction within about two years' time (with the majority
experiencing reobstruction within six months).23
Handling reobstruction. No data directly support the best plan of action for cats that have recurrent obstruction. However, clinicians generally
recommend additional procedures to owners after a cat has become obstructed twice, especially in short order (i.e. within days to weeks). The most commonly recommended procedure for these cats is a perineal urethrostomy. This surgery is
aimed at permanently widening and shortening the urethra to prevent reobstruction. Anecdotally, patients undergoing this surgery
are at a very low risk of reobstruction. No reobstruction was reported within four years in a recent study of 59 cats that
had perineal urethrostomy.24
Urethral obstruction in male cats has a high morbidity rate. However, this condition should not have a high mortality rate.
While relieving the obstruction with catheterization is the definitive treatment, practitioners need to be prepared to identify
and treat the conditions that coexist with urethral obstruction. Anticipation of these complications, coupled with competent
treatment, results in a very low mortality rate in afflicted male cats.
Elizabeth J. Thomovsky, DVM, MS, DACVECC
Department of Medical Sciences
School of Veterinary Medicine
University of Wisconsin-Madison
Madison, WI 53706