Central Diabetes Insipidus
Central diabetes insipidus is a lack of antidiuretic hormone that causes excessive production of very dilute urine (polyuria).
Causes
Central diabetes insipidus usually results from the decreased production of antidiuretic hormone (vasopressin), the hormone that helps regulate the amount of water in the body (see Section 12, Chapter 158). Antidiuretic hormone is unique in that it is produced in the hypothalamus but is then stored and released into the bloodstream by the pituitary gland.
Central diabetes insipidus may be caused by insufficient production of antidiuretic hormone by the hypothalamus. Alternatively, the disorder may be caused by failure of the pituitary gland to release antidiuretic hormone into the bloodstream. Other causes of central diabetes insipidus include damage done during surgery on the hypothalamus or pituitary gland; a brain injury, particularly a fracture of the base of the skull; a tumor; sarcoidosis or tuberculosis; an aneurysm (a bulge in the wall of an artery) or blockage in the arteries leading to the brain; some forms of encephalitis or meningitis; and the rare disease Langerhans' cell granulomatosis (histiocytosis X). Another type of diabetes insipidus, nephrogenic diabetes insipidus, may be caused by abnormalities in the kidneys (see Section 11, Chapter 146).
Symptoms and Diagnosis
Symptoms may begin gradually or suddenly at any age. Often the only symptoms are excessive thirst and excessive urine production. A person may drink huge amounts of fluid--4 to 40 quarts a day--to compensate for the fluid lost in urine. Ice-cold water is often the preferred drink. When compensation is not possible, dehydration can quickly follow, resulting in low blood pressure and shock. The person continues to urinate large quantities of dilute urine, and this is particularly noticeable during the night.
Doctors suspect diabetes insipidus in people who produce large amounts of urine. They first test the urine for sugar to rule out diabetes mellitus. Blood tests show abnormal levels of many electrolytes, including a high level of sodium. The best test is a water deprivation test, in which urine production, blood electrolyte (sodium) levels, and weight are measured regularly for a period of about 12 hours, during which the person is not allowed to drink. A doctor monitors the person's condition throughout the course of the test. At the end of the 12 hours--or sooner if the person's blood pressure falls or heart rate increases or if he loses more than 5% of his body weight--the doctor stops the test and injects antidiuretic hormone. The diagnosis of central diabetes insipidus is confirmed if, in response to antidiuretic hormone, the person's excessive urination stops, the urine becomes more concentrated, the blood pressure rises, and the heart beats more normally. The diagnosis of nephrogenic diabetes insipidus is made if, after the injection, the excessive urination continues, the urine remains dilute, and blood pressure and heart rate do not change.
Treatment
Vasopressin or desmopressin, which are modified forms of antidiuretic hormone, may be taken as a nasal spray several times a day. The dose is adjusted to maintain the body's water balance and a normal urine output. Taking too much of these drugs can lead to fluid retention, swelling, and other problems. People with central diabetes insipidus who are undergoing surgery or are unconscious are generally given injections of antidiuretic hormone.
Sometimes central diabetes insipidus can be controlled with drugs that stimulate production of antidiuretic hormone, such as chlorpropamide, carbamazepine, clofibrate, and various diuretics (thiazides). These drugs are unlikely to relieve symptoms completely in people whose diabetes insipidus is severe.
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