Page 771 - Small Animal Internal Medicine, 6th Edition

CHAPTER 46 Disorders of the Hypothalamus and Pituitary Gland 743

of observed problems with urine dribbling, especially when Results of these screening tests are normal in dogs and
resting or asleep. Owners of cats with diabetes insipidus cats with CDI, primary NDI, and psychogenic water con-
VetBooks.ir often complain that they need to change the kitty litter more sumption, although a low-normal serum urea nitrogen con-
centration (5-10 mg/dL), and mild hyponatremia and
frequently than expected. Additional clinical signs may be
found in dogs and cats with secondary causes of diabetes
is usually less than 1.006 and is often as low as 1.001 if the
insipidus. The most worrisome are neurologic signs and hypokalemia may be found. Random urine specific gravity
inappetence, which may indicate the presence of an expand- dog or cat has unlimited access to water. The urine osmolal-
ing hypothalamic or pituitary tumor in the adult dog or cat ity is less than 300 mOsm/kg. A urine specific gravity in the
that has not had head trauma. isosthenuric range (i.e., 1.008-1.015) does not rule out dia-
betes insipidus (Fig. 46.1), especially if the urine has been
PHYSICAL EXAMINATION obtained after water is knowingly or inadvertently withheld
The physical examination findings are usually unremarkable (e.g., after a long car ride and wait in the veterinary office).
in animals with CDI, although some dogs and cats are thin, The urine of dogs and cats with partial diabetes insipidus can
presumably because the pet’s strong desire for water over- be concentrated into the isosthenuric range if they become
rides its normal appetite. As long as access to water is not dehydrated. Erythrocytosis (packed cell volume of 50%-
restricted, the animal’s hydration status, mucous membrane 60%), hyperproteinemia, hypernatremia, and azotemia may
color, and capillary refill time remain normal. The presence be found in animals whose access to water has been restricted.
of neurologic abnormalities is variable in dogs and cats with Diagnostic tests to confirm and differentiate among CDI,
trauma-induced CDI or neoplastic destruction of the hypo- primary NDI, and psychogenic water consumption include
thalamus or pituitary gland. When present, neurologic signs the modified water deprivation test, random plasma osmo-
may include stupor, disorientation, ataxia, circling, pacing, lality determination, and the response to AVP supplementa-
and convulsions. Severe hypernatremia may also cause neu- tion. Results of these tests can be interpreted only after known
rologic signs in the traumatized dog or cat with undiagnosed causes for acquired secondary NDI have been ruled out, most
CDI given inadequate fluid therapy (see Chapter 53) and the notably hyperadrenocorticism. Pituitary-dependent hyper-
dog or cat with undiagnosed CDI that has prolonged adrenocorticism can mimic CDI in the adult dog. Pituitary-
restricted access to water. Hyposthenuria in the presence of dependent hyperadrenocorticism commonly causes severe
persistent hypernatremia should raise suspicion for diabetes PU and PD, and occasionally dogs have no other clinical
insipidus. signs, do not have the typical abnormalities (e.g., increased
serum alkaline phosphatase activity, hypercholesterolemia)
Diagnosis associated with the disease, and adrenal gland size is in
The diagnostic workup for PU and PD should initially rule the reference interval with ultrasonography. Results of the
out causes of acquired secondary NDI (see Chapter 38). modified water deprivation test in dogs with hyperadreno-
Recommended initial diagnostic studies include CBC; a corticism are similar to results in dogs with partial CDI
serum biochemistry panel; serum T 4 concentration (older and, sometimes, dogs with psychogenic PD. Severity of PU
cat); urinalysis with bacterial culture; abdominal ultrasonog- and PD may improve noticeably to the owner after initiat-
raphy; and in dogs, a urine cortisol/creatinine ratio or low- ing treatment with DDAVP in these dogs, but improvement
dose dexamethasone suppression test, or both; a baseline tends to be transient, lasting only a few months, and the
serum cortisol concentration if hypoadrenocorticism is sus- dog typically presents to the hospital with owner concerns
pected; and pre- and postprandial bile acids if hepatic insuf- that the DDAVP is no longer working. For these reasons, we
ficiency is suspected. always perform screening tests for hyperadrenocorticism in

10

8
Number of dogs 6 FIG 46.1
Urine specific gravity measured in 30
dogs with central diabetes insipidus at
the time of initial presentation to the
4
veterinarian. (From Feldman EC,
Nelson RW: Canine and feline
endocrinology and reproduction, ed 3,
2 St Louis, 2004, WB Saunders.)

0
1.001 1.005 1.010 1.015 1.020
Urine specific gravity