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Diabetes insipidus

Diabetes insipidus (DI) is a disease characterized by excretion of large amounts of severely diluted urine, which cannot be reduced when fluid intake is reduced. It denotes inability of the kidney to concentrate urine. DI is caused by a deficiency of antidiuretic hormone, or by an insensitivity of the kidneys to that hormone. more...

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Signs and symptoms

Excessive urination and extreme thirst (especially for cold water) are typical for DI. Symptoms of diabetes insipidus are quite similar to those of severely deranged diabetes mellitus, with the distinction that the urine is not sweet and there is no hyperglycemia (elevated blood glucose). Blurred vision is a rarity.

The extreme urination continues throughout the day and the night. In children, DI can interfere with appetite, eating, weight gain, and growth as well. They may present with fever, vomiting, or diarrhea. Adults with untreated DI may remain healthy for decades as long as enough water is drunk to offset the urinary losses. However, there is a continuous risk of dehydration.

Diagnosis

In order to distinguish DI from other causes of excess urination, blood glucose, bicarbonate and calcium need to be tested. Electrolytes can show substantial derangement; hypernatremia (excess sodium levels) are common in severe cases. Urinalysis shows low electrolyte levels, and measurement of urine osmolarity (or specific gravity) is generally low.

A fluid deprivation test helps determine whether DI is caused by:

  1. excessive intake of fluid
  2. a defect in ADH production
  3. a defect in the kidneys' response to ADH

This test measures changes in body weight, urine output, and urine composition when fluids are withheld. Sometimes measuring blood levels of ADH during this test is also necessary.

To distinguish between the main forms, desmopressin stimulation is also used; desmopressin can be taken by injection, a nasal spray, or a tablet. While taking desmopressin, a patient should drink fluids or water only when thirsty and not at other times, as this can lead to sudden fluid accumulation in central DI. If desmopressin reduces urine output and increases osmolarity, the pituitary production of ADH is deficient, and the kidney responds normally. If the DI is due to renal pathology, desmopressin does not change either urine output or osmolarity.

If central DI is suspected, testing of other hormones of the pituitary, as well as magnetic resonance imaging (MRI), is necessary to discover if a disease process (such as a prolactinoma) is affecting pituitary function.

Pathophysiology

Electrolyte and volume homeostasis is a complex mechanism that balances the body's requirements for blood pressure and the main electrolytes sodium and potassium. In general, electrolyte regulation precedes volume regulation. When the volume is severely depleted, however, the body will retain water at the expense of deranging electrolyte levels.

The regulation of urine production is the hypothalamus, which produces antidiuretic hormone (ADH or vasopressin) in the posterior lobe of the pituitary gland. In addition, it regulates the sensation of thirst as perceived by the cortex.

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Myths & facts:...About diabetes insipidus
From Nursing, 6/1/03 by McConnell, Edwina A

MYTH: Diabetes insipidus (DI) is a syndrome characterized by excessive glucose in the urine.

FACT: Diabetes insipidus is characterized by production of abnormally large volumes of dilute urine. A 24-hour urine output greater than 50 ml/kg is characteristic.

MYTH: Diabetes insipidus is caused by an actual or relative deficiency of insulin.

FACT: The three main types of diabetes insipidus-pituitary, nephrogenic, and primary polydipsia-are all related to antidiuretic hormone (ADH), not insulin. Decreased secretion or decreased action of ADH prevents the hormone from acting on the renal tubules to retain water and concentrate urine.

MYTH: Only a few conditions can precipitate DI.

FACT: Many conditions can cause DI, but common ones include trauma to the brain or head (pituitary DI), drugs such as lithium (nephrogenic DI), and psychogenic disorders or disorders associated with abnormal thirst (primary polydipsia).

MYTH: Diabetes insipidus is relatively benign.

FACT: Untreated DI can lead to death from electrolyte imbalances, dehydration, hemodynamic instability, central nervous system depression, and circulatory collapse.

MYTH: Medical management of DI includes fluid and electrolyte replacement, plus dexamethasone.

FACT: Medical management depends on the type of DI. For example, patients with pituitary DI may be treated with desmopressin, which increases urine concentration and decreases urine output. Patients with nephrogenic DI are treated with thiazide diuretics (with or without amiloride) and a low-sodium diet. Primary polydipsia is usually corrected with patient counseling.

MYTH: Vasopressin is the drug of choice in patients needing long-term treatment for pituitary DI.

FACT: Although vasopressin injection may be used in emergency treatment of DI, it's not practical for longterm therapy because of its short duration of action. Intranasal desmopressin has a longer duration of action and fewer adverse effects, so many experts consider it the drug of choice for long-term treatment of pituitary DI.

BY EDWINA A. McCONNELL, RN, PHD, FRCNA

At the time this article was written, Edwina A. McConnell was an independent nurse-- consultant in Gorham, Me. Selected references for this article are available on request

Copyright Springhouse Corporation Jun 2003
Provided by ProQuest Information and Learning Company. All rights Reserved

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