Diabetes insipidus

For the disease most commonly associated with the generic term "diabetes", see Diabetes mellitus.

Diabetes insipidus (DI) is a diseasecharacterized 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 kidneysto that hormone.

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 growthas 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, bicarbonateand calciumneed to be tested. Electrolytescan show substantial derangement; hypernatremia(excess sodiumlevels) are common in severe cases. Urinalysisshows 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, desmopressinstimulation 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 homeostasisis a complex mechanism that balances the body's requirements for blood pressureand the main electrolytes sodiumand 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.

The main effector organ for fluidhomeostasis is the kidney. In response to ADH, it concentrates urine in the distal tubule.

There are several forms of DI:

• Central diabetes insipidus is due to damage to the hypothalamus of pituitary due to a tumor, stroke, neurosurgeryor some rather rare causes (which include hemochromatosis, sarcoidosisand some genetic disorders). If the hypothalamus is damaged, the feeling of thirst may be completely absent.
• Nephrogenic diabetes insipidus is due to the inability of the kidney to respond normally to ADH. There are hereditary causes (90% are due to mutations of the ADH V2 receptor, and 10% mutations of the aquaporin 2water channel), but these are rare (incidence is around 4 per million live births). Most are male, because V2 receptor mutations are x-linked recessive defects. More common are acquired forms of NDI, which occur as a side-effect to some medications(such as lithium citrateand amphotericin B), as well as in polycystic kidney disease(PKD) and sickle-cell disease, and electrolyte disturbances such as hypokalaemia and hypercalcaemia. In some cases, no cause is found.
• Dipsogenic DI is due to a defect or damage to the thirst mechanism, which is located in the hypothalamus. This defect results in an abnormal increase in thirst and fluid intake that suppresses ADH secretion and increases urine output. Desmopressin is ineffective, and can lead to fluid overload as the thirst remains.
• Gestational DI only occurs during pregnancy. While all pregnant women produce vasopressinase in the placenta, which breaks down ADH, this can assume extreme forms in GDI. Most cases of gestational DI can be treated with desmopressin. In rare cases, however, an abnormality in the thirst mechanism causes gestational DI, and desmopressin should not be used.

Treatment

Central DI and gestational DI respond to desmopressin. In dipsogenic DI, desmopressin is not usually an option.

Desmopressin will be ineffective in nephrogenic DI. Instead, the diuretichydrochlorothiazide(HCT or HCTZ) or indomethacincan improve NDI; HCT is sometimes combined with amilorideto prevent hyperkalemia. Again, the patient should be reminded only to drink fluids when thirsty, and not at other times.

Sources

• The public domaindocument "Diabetes Insipidus", NIHPublication No. 01-4620, December 2000.da:Diabetes insipidus

de:Diabetes insipidus es:Diabetes insípida fr:Diabète insipide pt:Diabetes insipidus sv:Diabetes insipidus

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Diabetes+insipidus Wikipedia article Diabetes insipidus.