Diabetes Insipidus

Diabetes Insipidus
*Diabetes Insipidus (DI) is a disorder in which there is an abnormal increase in urine output, fluid intake and often thirst. It causes symptoms such as urinary frequency, nocturia (frequent awakening at night to urinate) or enuresis (involuntary urination during sleep or "bedwetting"). Urine output is increased because it is not concentrated normally. Consequently, instead of being a yellow color, the urine is pale, colorless or watery in appearance and the measured concentration (osmolality or specific gravity) is low.
*Diabetes Insipidus is not the same as diabetes mellitus ("sugar" diabetes). Diabetes Insipidus resembles diabetes mellitus because the symptoms of both diseases are increased urination and thirst. However, in every other respect, including the causes and treatment of the disorders, the diseases are completely unrelated. Sometimes diabetes insipidus is referred to as "water" diabetes to distinguish it from the more common diabetes mellitus or "sugar" diabetes.
Diabetes Insipidus is divided into four types,
each of which has a different cause and must be treated differently. The most common type of DI is caused by a lack of vasopressin, a hormone that normally acts upon the kidney to reduce urine output by increasing the concentration of the urine. This type of DI is usually due to the destruction of the back or "posterior" part of the pituitary gland where vasopressin is normally produced. Hence, it is commonly called pituitary DI. It is also known as central or neurogenic DI. The posterior pituitary can be destroyed by a variety of underlying diseases including tumors, infections, head injuries, infiltrations, and various inheritable defects. The latter can be recognized by the onset of the DI in early childhood and a family history of parents, siblings or other relatives with the same disorder. Nearly half the time, however, pituitary DI is "idiopathic" (that is, no cause can be found despite a thorough search including magnetic resonance imaging or MRI of the brain) and the underlying cause(s) is (are) still unknown. Pituitary DI is usually permanent and cannot be cured but the signs and symptoms (i.e. constant thirst, drinking and urination) can be largely or completely eliminated by treatment with various drugs including a modified from of vasopressin known as desmopressin or DDAVP. Because pituitary DI is sometimes associated with abnormalities in other pituitary hormones, tests and sometimes treatments for these other abnormalities are also needed.
*Occasionally, a lack of vasopressin can also develop during pregnancy if the pituitary is slightly damaged and/or the placenta destroys the hormone too rapidly. This second type of vasopressin deficiency is called gestagenic or gestational DI and is also treatable with DDAVP but, in this case, the deficiency and the DI often disappear 4 to 6 weeks after delivery at which time the DDAVP treatment can usually be stopped. Often, however, the signs and symptoms of DI recur with subsequent pregnancies.
*The third type of DI is caused by an inability of the kidneys to respond to the "antidiuretic effect" of normal amounts of vasopressin. This type of DI is usually referred to as nephrogenic DI and can result from a variety of drugs or kidney diseases including heritable genetic defects. It cannot be treated with DDAVP and, depending on the cause, may or may not be curable by eliminating the offending drug or disease. The heritable form, for example, lasts for life and cannot be cured at present. However, there are treatments that can partially relieve the signs and symptoms of nephrogenic DI.
*The fourth form of DI occurs when vasopressin is suppressed by excessive intake of fluids. The latter is usually referred to as primary polydipsia and is most often caused by an abnormality in the part of the brain that regulates thirst. This subtype is called dipsogenic DI and is difficult to differentiate from pituitary DI particularly since the two disorders can result form many of the same brain diseases. The only sure way to tell them apart is to measure vasopressin during a stimulus such as fluid deprivation or to observe the effects of DDAVP treatment. In dipsogenic DI, DDAVP also eliminates the excessive urination but, unlike pituitary DI, it does not completely eliminate the increased thirst and fluid intake. Thus, it also results in water intoxication, a condition associated with symptoms such as headache, loss of appetite, lethargy and nausea and signs such as an abnormally large decrease in the plasma sodium concentration (hyponatremia). Because of this and the current lack of a way to correct the underlying abnormality in thirst, dipsogenic DI cannot be treated at present, although the most troubling symptoms, nocturia, can be safely relieved by taking small doses of DDAVP at bedtime. The other subtype of primary polydipsia is due not to abnormal thirst but to psychosomatic causes and is often referred to as pyschogenic polydipsia. It cannot be treated at present.