Practical Matters: Desmopressin is safer than water deprivation to identify the cause of polyuria and polydipsia in dogs

Polyuria and polydipsia have many causes in dogs (Table 1). Obtaining a thorough history and performing a physical examination, a complete blood count, a serum chemistry profile, a full urinalysis including urine bacterial culture, and imaging (e.g. ultrasonographic examination of the kidneys, liver, and adrenal glands) will help rule out many of the common causes.

Marjorie L. Chandler, DVM, MS, MACVSc, DACVN, DACVIM, DECVIM, MRCVS

Measuring the basal serum cortisol concentration may help rule out hypoadrenocorticism since this disorder is unlikely if the value is > 2 μg/dl (~70 mmol/L). If the value is ≤ 2 μg/dl, hypoadrenocorticism must be confirmed with an ACTH stimulation test. If hyperadrenocorticism is suspected, perform ACTH stimulation and low-dose dexamethasone suppression tests, imaging, and other diagnostic tests as needed.

If these test results are inconclusive or within the reference range, the remaining differential diagnoses include central diabetes insipidus, psychogenic polydipsia, nephrogenic diabetes insipidus, and early-stage chronic kidney disease. A congenital form of nephrogenic diabetes insipidus exists, but it is rare and manifests in puppies. Most causes of acquired nephrogenic diabetes insipidus are among the listed differential diagnoses in Table 1 and need to be definitively diagnosed.

Table 1: Differential Diagnoses in Dogs with Polyuria and Polydipsia

A water deprivation test can be used to differentiate central diabetes insipidus from psychogenic polydipsia; however, this test involves intentionally dehydrating a patient. Ideally, the glomerular filtration rate should be evaluated before water deprivation testing is considered in a polyuric, nonazotemic patient for which the remaining differential diagnoses are central or nephrogenic diabetes insipidus, psychogenic polydipsia, and early-stage chronic kidney disease. But evaluating the glomerular filtration rate is not usually practical for general practitioners. To avoid the risks associated with a water deprivation test in an animal that potentially has an abnormal glomerular filtration rate, consider conducting a desmopressin trial, a safer method for diagnosing central diabetes insipidus.

To conduct this trial, the owner measures the patient's free-choice 24-hour water intake for two or three days and collects and labels daily urine samples. The specific gravity of the urine samples should be checked. The owner then administers 0.05 to 0.2 mg desmopressin orally every eight hours or 1 to 4 drops of the 100 μg/ml desmopressin nasal spray in one conjunctival sac every 12 hours for five to seven days. The owner monitors the dog's water consumption and collects and labels urine samples during the treatment period. Dogs with central diabetes insipidus should respond well to this treatment, forming more concentrated urine and drinking much less water than they were previously drinking. A moderate response may occur in dogs with partial central diabetes insipidus or with hyperadrenocorticism.

Even with a desmopressin trial, caution should be exercised in geriatric patients and those having isosthenuria since these patients are more likely to have early-stage chronic kidney disease, and excessive dehydration could be damaging. Further, if an animal with unexplained polyuria and polydipsia presents to the hospital in a dehydrated state and its urine specific gravity is < 1.030, a water deprivation test is unnecessary and is contraindicated.

SUGGESTED READING

1. Feldman EC, Nelson RW. Water metabolism and diabetes insipidus. In: Canine and feline endocrinology and reproduction. 3rd ed. St Louis, Mo: Saunders, 2004;2-44.

Marjorie L. Chandler, DVM, MS, MACVSc, DACVN, DACVIM, DECVIM, MRCVS

Hospital for Small Animals

University of Edinburgh

Roslin, Midlothian, Scotland