Diabetes Insipidus
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Diabetes insipidus (DI) is characterized by excretion of large
amounts of dilute urine, which disrupts your body's water
regulation. To make up for lost water, you may feel the need to
drink large amounts of water. You are likely to urinate frequently,
even at night, which can disrupt sleep or, on occasion, cause
bedwetting. Because of the excretion of abnormally large volumes of
dilute urine, you may quickly become dehydrated if you do not drink
enough water. Children with DI may be irritable or listless and, in
some cases, may have fever, vomiting, or diarrhea. In its clinically
significant forms, it is a rare disease.
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DI should not be confused with diabetes mellitus, which results
from insulin deficiency or resistance leading to high blood glucose.
Diabetes insipidus and diabetes mellitus are unrelated, although
they can have similar signs and symptoms, like excessive thirst and
excessive urination.
Diabetes mellitus (DM) is far more common than DI and receives
more news coverage. DM has two forms, referred to as type 1 diabetes
(formerly called juvenile diabetes, or insulin-dependent diabetes
mellitus, or IDDM) and type 2 diabetes (formerly called adult-onset
diabetes, or noninsulin-dependent diabetes mellitus, or NIDDM). DI
is a different form of illness altogether.
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Your body has a complex system for balancing the volume and
composition of body fluids. Your kidneys remove extra body fluids
from your bloodstream. This fluid waste is stored in the bladder as
urine. If your fluid regulation system is working properly, your
kidneys make less urine to conserve fluid when the body is losing
water. Your kidneys also make less urine at night when the body's
metabolic processes are slower.
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The hypothalamus makes antidiuretic hormone
(ADH), which directs the kidneys to make less
urine. |
In order to keep the volume and composition of body fluids
balanced, the rate of fluid intake is governed by thirst, and the
rate of excretion is governed by the production of antidiuretic
hormone (ADH), also called vasopressin. This hormone is made in the
hypothalamus, a small gland located in the base of the brain. ADH is
stored in the nearby pituitary gland and released from it into the
bloodstream when necessary. When ADH reaches the kidneys, it directs
the kidneys to concentrate the urine by returning excess water to
the bloodstream and therefore make less urine.
DI occurs when this precise system for regulating the kidneys'
handling of fluids is disrupted. The most common form of clinically
serious DI, central DI, results from damage to the pituitary gland,
which disrupts the normal storage and release of ADH. Another form,
nephrogenic DI, results when the kidneys are unable to respond to
ADH. Rarer forms occur because of a defect in the thirst mechanism
(dipsogenic DI) or during pregnancy (gestational DI).
A specialist should determine which form of DI is present before
starting any treatment.
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Damage to the pituitary gland can be caused by different diseases
as well as by head injuries, neurosurgery, or genetic disorders. To
treat the resulting ADH deficiency, a synthetic hormone called
desmopressin can be taken by an injection, a nasal spray, or a pill.
While taking desmopressin, you should drink fluids or water only
when you are thirsty and not at other times. This is because the
drug prevents water excretion and water can build up now that your
kidneys are making less urine and are less responsive to changes in
body fluids.
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The kidneys' ability to respond to ADH can be impaired by drugs
(like lithium, for example) and by chronic disorders including
polycystic kidney disease, sickle cell disease, kidney failure,
partial blockage of the ureters, and inherited genetic disorders.
Sometimes the cause of nephrogenic DI is never discovered.
Desmopressin will not work for this form of DI. Instead, you may
be given a drug called hydrochlorothiazide (also called HCTZ) or
indomethacin. HCTZ is sometimes combined with amiloride. Again, you
should drink fluids only when you are thirsty and not at other
times.
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A third type of DI is caused by a defect in 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 or
other drugs should not be used to treat dipsogenic DI because they
may decrease urine output but not thirst and fluid intake. This
fluid "overload" can lead to water intoxication, a condition that
lowers the concentration of sodium in the blood and can seriously
damage the brain.
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A fourth type of DI occurs only during pregnancy. Gestational DI
occurs when an enzyme made by the placenta destroys ADH in the
mother. The placenta is the system of blood vessels and other tissue
that develops with the fetus. The placenta allows exchange of
nutrients and waste products between mother and fetus.
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.
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Because DM is more common and because DM and DI have similar
symptoms, a health care provider may suspect that a patient with DI
has DM. But testing should make the diagnosis clear.
Your physician must determine which type of DI is involved before
proper treatment can begin. Diagnosis is based on a series of tests,
including urinalysis and a fluid deprivation test.
Urinalysis is the physical and chemical examination of urine. The
urine of a person with DI will be less concentrated. Therefore, the
salt and waste concentrations are low, and the amount of water
excreted is high. A physician evaluates the concentration of urine
by testing its specific gravity or osmolality.
A fluid deprivation test helps determine whether DI is caused by
(1) excessive intake of fluid, (2) a defect in ADH production, or
(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.
In some patients, an MRI (magnetic resonance imaging) of the
brain may be necessary as well.
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The Diabetes Insipidus Foundation,
Inc.
The Diabetes Insipidus
and Related Disorders Network
National Organization for Rare Disorders
(NORD)
Nephrogenic Diabetes Insipidus Foundation
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