Polyuria and Polydipsia. Problems Associated with Patient Evaluation

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Title: Polyuria and Polydipsia. Problems Associated with Patient Evaluation
Author: Nichols, DVM, Rhett
Publisher: Problems in Veterinary Medicine
Date Published: December 01, 1990
Reference Number: 435
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Primary disorders of water balance (central diabetes insipidus, congenital nephrogenic diabetes insipidus, and psychogenic polydipsia) should always be considered in the differential diagnosis of polyuria and polydipsia. In general, animals with these disorders have only one laboratory abnormality, a low urine specific gravity. The more common causes of polyuria and polydipsia (eg, hypercalcemia, chronic renal insufficiency, pyelonephritis, hyperadrenocorticism), in most instances, have specific and obvious abnormalities associated with the complete blood count (CBC), serum chemistry profile, and urinalysis. However, in some cases, a low urine specific gravity may initially be the only abnormality in these more common ruleouts. The workup for polyuria and polydipsia, especially in those cases with normal or near normal blood work, can be tedious, time consuming, confusing, and not without significant patient morbidity. This chapter will focus on the problems associated with diagnostic testing used to evaluate animals with disorders of water balance.

This translation by the NDI Foundation is to assist the lay reader. To provide a clear, accessible interpretation of the original article, we eliminated or simplified some technical detail and complicated scientific language. We concentrated our translation on those aspects of the article dealing directly with NDI. The NDI Foundation thanks the researchers for their work toward understanding and more effectively treating this disorder.
© Copyright NDI Foundation 2007 (JC)

All three forms of diabetes insipidus (DI) -- pituitary DI (PDI), nephrogenic DI (NDI), and primary polydipsia (PP) -- can occur in animals as well as humans. As in humans, DI can either be acquired or inherited. The methods of differentiating among the three forms is similar in human and animal. In this report, Nichols, a doctor of veterinary medicine, describes the methods and challenges involved in diagnosing to distinguish among these three forms of DI.

DI is a disorder of water balance characterized by polyuria (chronic passage of large amounts of urine), polydipsia (chronic, excessive thirst), and urine of a low specific gravity or osmolality. (Urine specific gravity is a measure of the density of urine or the ratio of the weight of urine to an equal volume of water. Urine osmolality is a measure of the number of particles, e.g. sodium, in the urine. The urine of DI patients is generally dilute, i.e., having a lower number of particles in the water component of urine than normal.)

PDI is caused by an inability to secrete the required amount of antidiuretic hormone (ADH) from the pituitary. As a result, there is nothing to give the kidney the message to reabsorb body water in order to concentrate urine. NDI is caused by the kidney's inability to respond to ADH. Here, there is enough ADH in the system, but it cannot get the kidneys to respond to its message. PP is caused by an excessive intake of water resulting from a behavior problem. (If the excessive intake is the result of a defect in the thirst mechanism, the DI is called dipsogenic DI.)

Differentiating among these disorders is more straightforward when the disorders present themselves in a complete manner. First, the veterinarian must rule out other causes of DI. (NDI may arise as a result of a number of other disorders.) Then the animal is placed on a water fast for three to 10 hours to dehydrate it. The animal's urine concentration is measured after the water fast. They measure this because the measure of how concentrated the animal's urine is provides an indirect index of ADH secretion or effect. Urine osmolality (Os) is the most direct and precise measure of urine concentration. Urine specific gravity (SG) is less so. However, the SG measure is more commonly used as it is less expensive and easily obtained.

The water deprivation test determines whether ADH is released in response to dehydration and whether the kidneys can respond to it. If the animal fails to produce concentrated urine as a consequence of the water fast, the veterinarian will administer ADH, then again measure urine concentration. Animals with complete PDI cannot concentrate urine osmolality greater than plasma osmolality. That is, the urine will not be more concentrated than the blood. However, the PDI animal's urine concentration increases dramatically after administration of ADH. Animals with partial PDI can increase their urine osmolality somewhat when dehydrated, and if injected with ADH afterwards, they can concentrate their urine around 10 - 50% more.

Animals with complete NDI cannot concentrate urine to levels greater than plasma osmolality after dehydration. Neither can they increase urine osmolality or specific gravity after ADH administration. Animals with PP can usually increase their urine osmolality significantly after dehydration, whereas injections with ADH will cause less than a 10% increase in urine concentration.

The water deprivation test can be an excellent way to differentiate complete NDI from complete PDI, but it may not differentiate between partial PDI and PP. Making the correct distinction is especially important because treatment for partial PDI in a patient who actually has PP can lead to marked water retention and possibly life-threatening increases in blood sodium. Though expensive, measurements of plasma ADH levels can be used to distinguish between these disorders. If this route is not available, then the veterinarian can begin a therapeutic trial with ADH. If the animal is rapidly relieved of its polyuria and polydipsia, then it most likely has PDI. During this test it is important to monitor the blood to check for excessive sodium buildup.

One caution to take with the water deprivation test involves checking for Cushing's syndrome, as this syndrome is difficult to impossible to distinguish from partial ID or PP. Other things to watch for include severe dehydration and acute water overload and cerebral edema following fluid restriction. Also, the patient must be monitored to prevent marked volume depletion. (Most animals with PDI and NDI lose five percent of their body weight in three to 10 hours during the water deprivation test.) Some people recommend progressive water restriction before abrupt water deprivation to minimize washing out the electrolytes in the innermost part of the kidney that can sometimes occur.

The Hickey-Hare test, though not as safe or easy as the water deprivation test or the ADH therapeutic trial, helps distinguish polyuric states which are complicated by the type of washout mentioned above. This involves administering hypertonic saline to a water-loaded animal and then measuring changes in urine production. A significant reduction in urine flow rate indicates a normal pituitary-kidney concentrating axis, therefore animals with PP should have reduced urine volume in response to the test whereas those with PDI and NDI should not.

Testing the concentrations of ADH in the plasma has been extremely useful in distinguishing the different types of DI in human patients, especially when incorporated into the water deprivation test. However, because of high cost and limited availability, direct measurement of plasma ADH levels is impractical for most veterinarians.