Page 772 - Small Animal Internal Medicine, 6th Edition

744 PART VI Endocrine Disorders

an adult dog in which CDI and psychogenic PD have risen A thorough review of the diagnostic evaluation of the
to the top of the differentials and always before initiating patient, owner compliance in treating their pet, and adjust-
VetBooks.ir DDAVP treatment. ments in the DDAVP treatment protocol should be under-
taken in dogs and cats that fail to respond to DDAVP before
RESPONSE TO TRIAL THERAPY WITH
DESMOPRESSIN ACETATE considering the modified water deprivation test.
CDI, primary NDI, and psychogenic PD are uncommon to MODIFIED WATER DEPRIVATION TEST
rare causes of PU and PD in dogs and cats, and of these three The technique, interpretation, contraindications, and com-
differentials, partial CDI and psychogenic PD are the most plications of the modified water deprivation test are described
common. Because CDI is treated with DDAVP, a viable in Chapter 39. The test consists of two phases. In phase I,
approach to establishing the diagnosis is to evaluate the ani- AVP secretory capabilities and renal distal and collecting
mal’s response to trial therapy with DDAVP (Desmopressin tubule responsiveness to AVP are evaluated by assessing the
®
acetate , Aventis Pharmaceuticals). Oral DDAVP tablets or effects of dehydration (i.e., water restriction until the animal
conjunctival drops of DDAVP nasal spray (see Treatment loses 3%-5% of its body weight) on urine specific gravity. The
section) should be administered every 12 hours for 7 days. normal dog and cat, as well as those with psychogenic water
The effect of DDAVP should not be critically evaluated until consumption, should be able to concentrate urine to greater
after 5 to 7 days of therapy because renal medullary washout than 1.030 (1.035 in the cat) if dehydrated. Dogs and cats
may prevent the dog or cat with CDI from concentrating its with partial and complete CDI and primary NDI have an
urine and decreasing water intake after only 1 or 2 days of impaired ability to concentrate urine in the face of dehydra-
DDAVP treatment. Clients should notice a definite improve- tion (Table 46.3 and Fig. 46.2). The time required to attain
ment in the severity of PU and PD by the end of the treat- 3% to 5% dehydration can sometimes be helpful in establish-
ment period if the PU and PD are caused by CDI. Urine ing the diagnosis. It often takes less than 6 hours for dogs
specific gravity should be measured on several urine samples and cats with complete CDI to attain 3% to 5% dehydration,
collected by the client on the last couple of days of trial whereas it often takes longer than 8 to 10 hours for dogs and
therapy. An increase in urine specific gravity by 50% or cats with partial CDI, especially those with psychogenic
more, compared with pretreatment specific gravities, sup- water consumption, to attain 3% to 5% dehydration.
ports the diagnosis of CDI, especially if the urine specific Phase II of the water deprivation test is indicated for dogs
gravity exceeds 1.030. There should be only minimal and cats that do not concentrate urine to greater than 1.030
improvement in dogs and cats with primary NDI, although during phase I of the test. Phase II determines the effect, if
a response may be observed with very high doses of DDAVP. any, that exogenous AVP has on the renal tubular ability to
Dogs and cats with psychogenic water consumption may concentrate urine in the face of dehydration (see Fig. 46.2).
exhibit a mild decline in urine output and water intake This phase differentiates impaired AVP secretion from
because the chronically low serum osmolality tends to impaired renal tubular responsiveness to AVP (see Table
depress AVP production. Theoretically, dogs with psycho- 46.3).
genic PD could develop clinical signs of hyponatremia
during DDAVP therapy, but we have only identified this RANDOM PLASMA OSMOLALITY
complication with excess DDAVAP administered by SC Measurement of random plasma osmolality may help iden-
injection. This approach to diagnosis requires that all other tify primary or psychogenic PD. Plasma osmolality in normal
causes of PU and PD, except CDI, primary NDI, and psy- dogs and cats is approximately 280 to 300 mOsm/kg. Diabetes
chogenic PD, must be previously ruled out. insipidus is a primary polyuric disorder, with compensatory

TABLE 46.3

Guidelines for Interpretation of the Modified Water Deprivation Test
TIME TO 5%
URINE SPECIFIC GRAVITY DEHYDRATION
DISORDER INITIAL 5% DEHYDRATION POST ADH MEAN (h) RANGE (h)

Central Di
Complete <1.006 <1.006 >1.008 4 3-7
Partial <1.006 1.008-1.020 >1.015 8 6-11
Primary nephrogenic DI <1.006 <1.006 <1.006 5 3-9
Primary polydipsia 1.002-1.020 >1.030 NA 13 8-20
ADH, Antidiuretic hormone; DI, diabetes insipidus; NA, not applicable.