Page 99 - Clinical Small Animal Internal Medicine

8 Polyuria and Polydipsia 67

polydipsia with secondary polyuria or polyuria with In addition to being time‐consuming and difficult to
VetBooks.ir compensatory polydipsia. perform, for patients with underlying undiagnosed ill-
ness, this test is contraindicated and may increase the
Primary polyuria (results in excess loss of free water) >
●
elevated Na concentration > osmoreceptors sense risk for complications, including death. This test
should never be performed in patients who are
increase in osmolality > trigger thirst response > com- azotemic or dehydrated. In addition, diseases such as
pensatory polydipsia hyperadrenocorticism may show some positive
Primary polydipsia (results in excess intake of free
● response, thereby complicating the diagnosis or result-
water) > decreased Na levels > osmoreceptors sense ing in misdiagnosis.
decrease in osmolality > decrease AVP release > com-
pensatory polyuria
Steps for Conducting the Modified WDT
It is important to note, however, that patients with Phase 1
hypoadrenocorticism will have a low sodium level due to Patients that have had PU/PD for a prolonged period of
mineralocorticoid deficiency despite primary polyuria. time may have wash‐out of their renal medullary concen-
Plasma osmolality can also be used to determine if a tration gradient and therefore, may be unable to concen-
patient has primary polydipsia. In clinical practice, this trate their urine in the face of water deprivation. For this
can be estimated by calculating serum osmolality using reason, it is recommended that once the pet’s water intake
the following formula: has been quantified, the total amount of water given
2 × [Na] + [BUN in mg/dL]/2.8+ [Glu in mg/dL]/18 should be gradually restricted over 3–5 days until the goal
of 60–80 mL/kg/day is reached. To avoid prolonged peri-
Low osmolality is suggestive of primary polydipsia and ods of water restriction, the total daily water should be
further evaluation for hepatic disease or hyperthyroid- calculated and provided in frequent small portions. A dry
ism are indicated. food diet should be fed during this phase. This testing will
require that the owners pay very close attention to the pet
Second‐Tier Diagnostic Tests for any signs of illness. Patients with severe polyuria are
at increased risk for hypertonic dehydration and hospi-
Further diagnostics will depend on physical and laboratory talization during this phase of the WDT should be con-
examination findings. For example, an abdominal ultra- sidered. A USG should be measured at the conclusion of
sound should be considered for patients with flank/abdom- this phase in case the patient has already reached a USG
inal pain in whom pyelonephritis is suspected. For patients >1.030 (supportive of a diagnosis of psychogenic polydip-
in whom adrenal disease is suspected, an adrenocortico- sia) and the WDT is no longer required.
trophic hormone (ACTH) stimulation test should be con-
sidered. It is important to remember that approximately
15% of dogs with pituitary dependent and 40% of dogs with Phase 2
adrenal‐dependent hyperadrenocorticism can have a nor- ● The patient should be admitted to the hospital for a
mal ACTH stimulation test. If hyperadrenocorticism is complete physical examination and assessment of
strongly suspected and the dog is otherwise healthy, a low‐ hydration and mentation status.
dose dexamethasone suppression test is a better option. ● An indwelling urinary catheter is placed. The bladder
Other tests that may be considered, depending on lab- should be emptied completely, and the USG
oratory findings, clinical signs, and signalment, include measured.
pre‐ and postprandial bile acids and imaging studies of ● A blood urea nitrogen (BUN), creatinine, packed cell
the chest and abdomen. Imaging will be particularly use- volume (PCV), total solids (TS), and electrolytes
ful for patients suspected of a neoplastic process and in should be measured.
any patient with hypercalcemia. Abdominal ultrasonog- ● Obtain patient’s body weight.
raphy may be useful in looking for masses, enlarged ● Patient should not have access to food or water.
abdominal lymph nodes, or adrenal enlargement. Patient should have the following performed every
●
1–2 hours.
– Physical examination to assess hydration and
Modified Water Deprivation Test (WDT) or mentation
DDAVP trial
– Completely empty bladder and check USG
A modified WDT should only be considered for patients – Obtain weight
in whom more common causes of PU/PD have been ● Every 4–6 hours check BUN, creatinine, PCV, TS, and
ruled out and central DI, primary NDI, or psychogenic electrolytes.
polydipsia is strongly suspected. Fortunately, very few ● The test should be stopped when any of the following
cases of PU/PD require performing this test. occur: