Water Metabolism and Diabetes Insipidus

Desmopressin acetate-induced water intoxication in a dog with psychogenic polydipsia

“…Primary PD is confirmed by ruling out other known causes of PU/PD and demonstrating that renal concentrating ability is intact. After exclusion of causes for acquired nephrogenic DI, the differentiation between central DI, primary nephrogenic DI and psychogenic PD is based on results of the MWDT, response to DDAVP trial therapy and measurement of plasma osmolality 1. Measurement of plasma vasopressin concentrations under osmotic stimulation has been called into question, as findings were inconsistent and could not distinguish between different disorders 17.…”

“…Although the MWDT is considered the best differentiating tool, it is time‐consuming, expensive and unpleasant for the patient. Without close observation, the test can pose the animal at several risks and results can be confusing, especially with partial disorders 1 3. A viable alternative is a therapeutic DDAVP trial.…”

“…An increase of USG and decrease of water consumption by >50% should be noticed in dogs with central DI, and only minimal or no response can be seen with primary nephrogenic DI. However, there may be some improvement in dogs with psychogenic PD, because chronically low plasma osmolality suppresses ADH production 1 3. Dogs with psychogenic PD often exhibit subclinical hypervolaemia, normonatraemia to mild hyponatraemia, normal or low plasma osmolality and low urine osmolality.…”

“…During the diagnostic work‐up of a patient with PU/PD, the more common causes leading to osmotic diuresis or secondary (acquired) nephrogenic DI must be ruled out first 1. Traditionally, the modified water deprivation test (MWDT) has been the test of choice for differentiating central DI, primary nephrogenic DI and psychogenic PD 1 2. However, the MWDT can be time‐consuming, expensive and may be uncomfortable for the patient.…”

“…A viable alternative is to assess the animal’s response to trial therapy with DDAVP. During the treatment period, water consumption and urine specific gravity (USG) must be carefully evaluated, and DDAVP doses must be adjusted if necessary 1 3. Without adequate treatment response, a MWDT should be performed.…”