Polyuria in Childhood
| Title: | Polyuria in Childhood |
|---|---|
| Authors: | Leung, Alexander K.C.; Robson, Wm. Lane; Halperin, Mitchell L. |
| Publisher: | Clinical Pediatrics |
| Date Published: | November 01, 1991 |
| Reference Number: | 313 |
Polyuria may result from either a water or a solute diuresis. Although the history and physical examination may provide clues to the cause of the polyuria, the definitive diagnosis requires laboratory tests which focus on the osmolality of the urine and serum in combination with the urine volume and the rate of excretion of osmoles. An isoosmolar or hyperosmolar urine is found in children with a solute diuresis or in normal children, whereas a hypoosmolar urine is found in children with a water diuresis. In the latter case, a low serum osmolality suggests primary polydipsia whereas a high serum osmolality suggests antidiuretic hormone (ADH) deficiency or insensitivity. A water deprivation test is necessary when the initial evaluation fails to establish the cause of polyuria. A vasopressin test enables the differentiation between neurogenic and nephrogenic diabetes insipidus (DI).
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)
Osmosis is fundamental to the production of urine. During osmosis, water moves from a less concentrated solution across a semi-permeable membrane into a more concentrated solution. To concentrate the urine, water must move from the less concentrated urine solution in the tubules and collecting ducts of the kidney into the more concentrated plasma solution in the blood. Osmolality is the term used to describe the concentration of osmotically active particles in a solution such as urine or blood. A concentrated urine has a high osmolality.
The ability to concentrate urine in the kidney depends on the presence of arginine vasopressin (AVP), a chemical that is made in the hypothalamus of the brain, and acts in the collection ducts of the kidney to allow water to be reabsorbed from the urine. AVP acts on special receptors in the collecting duct cells and initiates a chemical reaction that allows water to flow from the urine to the blood.
Polyuria may be due to either drinking excess liquid, referred to as primary polydipsia, or failure of the kidney to reabsorb water, a condition known as diabetes insipidus (DI). DI is due to either a lack of AVP (neurogenic DI) or failure of the collecting ducts of the kidney to respond to AVP (nephrogenic DI). Both neurogenic and nephrogenic DI might be either congenital or acquired; occur as a primary disease or secondary to another disorder; or occur as a side effect of specific prescription medications.
A thorough history, complete physical examination and blood and urine tests are necessary to clarify the cause of polyuria. Patients with primary polydipsia can concentrate the urine and have an appropriate urine osmolality. Patients with DI cannot concentrate the urine and the urine osmolality is inappropriately low, even when the child is dehydrated. Patients with neurogenic DI can concentrate the urine when they are treated with DDAVP, a synthetic copy of AVP. Patients with nephrogenic DI cannot concentrate the urine even if they are treated with DDAVP. A water deprivation test is often used to differentiate the two basic kinds of DI. After restricting liquids for a safe period of time, the urine osmolality is measured. A low urine osmolality suggests DI. DDAVP is then administered. If the urine osmolality goes up, this suggests neurogenic DI. If the urine osmolality does not increase, this suggests nephrogenic DI. Blood osmolality and sometimes blood AVP levels are often measured during a water deprivation test. A water deprivation test should only be conducted by an experienced health professional since severe dehydration can develop.
Once the cause of the polyuria has been established, the underlying problem should be treated. Patients with DI should be allowed free access to water to avoid dehydration. Children with neurogenic DI should be treated with DDAVP. Children with nephrogenic DI might respond to a low salt diet and treatment with diuretic medications such as hydrochlorothiazide or amiloride.