Page 252 - Problem-Based Feline Medicine

244 PART 4 CAT WITH URINARY TRACT SIGNS

hyperadrenocorticosteroidism, hypokalemia and If central DI, administer synthetic vasopressin
drugs causing PU/PD (see page 244) must be ruled (desmopressin, DDAVP, SC, intranasal or conjuncti-
out. val). Thiazide diuretics are less expensive, but less
effective than desmopressin.
Trial synthetic vasopressin (DDAVP) therapy to
determine if PU/PD resolves. Pretreatment water
intake is measured for 2–3 days, and then again follow- Prognosis
ing DDAVP therapy. A greater than 50% reduction of
Prognosis depends on cause. The best prognoses occur
water intake suggests central or partial nephrogenic DI,
with reversible nephrogenic DI (e.g. hyperthyroidism)
but is rarely indicated.
and non-neoplastic central DI.
A modified water deprivation test can be performed
if the cat is not dehydrated, azotemic and laboratory
DRUG INDUCED*
tests do not indicate nephrogenic DI.
● Ignoring these contraindications may kill the
Classical signs
cat, or make renal failure worse. The cat must be
carefully monitored. ● PU/PD following initiation of drug therapy.
● Rarely is a water deprivation test required in
feline medicine, because central diabetes
insipidus is very rare and psychogenic polydipsia Clinical signs
has not been reported.
Common drugs causing polyuria and polydispsia
● The main use of a water deprivation test is to deter-
include: diuretics, anticonvulsants (e.g. phenobarbital),
mine if the kidney can concentrate urine when
thyroxine, salt supplementation, sodium bicarbonate,
water is unavailable (indicating psychogenic poly-
amphotericin B (from renal toxicity and hypokalemia),
dipsia), and whether vasopressin therapy can
corticosteroids and megestrol acetate.
increase urine concentration (central or partial
nephrogenic DI) after medullary washout has Polyuria and polydipsia occur following initiation of
resolved with water deprivation. drug therapy.
● Only consider performing a water deprivation test if
Other clinical signs relate to the underlying disease
there is massive polyuria and polydipsia, urine SG
process.
is < 1.008, and nephrogenic diabetes inspidus has
been ruled out.
● Details of the test and interpretation are found in Diagnosis
many standard textbooks.
Diagnosis is rarely a problem. If polyuria and polydip-
sia occur with phenobarbital or thyroxine, check
Differential diagnosis dose is not excessive by measuring trough serum
concentration.
It is important to differentiate central DI from second-
ary nephrogenic DI, which is much more common. Test Although cats are more resistant to the side effects of
for renal failure, hyperthyroidism, hyperadrenocor- corticosteroids, polyuria and polydipsia does occur, but
ticosteriodism, pyelonephritis, pyometra, hypercal- is usually less dramatic than in dogs.
cemia and hypokalemia.

Treatment HYPOKALEMIA*

Ensure water is freely available at all times. Classical signs
If nephrogenic DI, manage the underlying disease, ● Acute onset of muscle weakness, neck
e.g. renal failure, hyperthyroidism. Thiazide diuretics ventroflexion, a stiff, stilted gait, and
may be useful for reducing water consumption if the muscle pain.
underlying disease is not treatable.