Diabetes Insipidus
versus Diabetes Mellitus
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.
Normal Fluid
Regulation in the Body
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.
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.
Central
DI
Damage to
the pituitary gland can be caused by different diseases as
well as by head injuries, neurosurgery, or genetic disorders.
To treat the ADH deficiency that results from any kind of
damage to the hypothalamus or pituitary, 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.
Nephrogenic
DI
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 another drug called amiloride. The combination of HCTZ
and amiloride is sold under the brand name Moduretic. Again,
with this combination of drugs, you should drink fluids only
when you are thirsty and not at other
times.
Dipsogenic
DI
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.
Gestational
DI
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.
Diagnosis
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 measuring how many particles are in a kilogram of
water (osmolality) or by comparing the weight of the urine to
an equal volume of distilled water (specific
gravity).
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.
For More
Information
The
Diabetes Insipidus Foundation, Inc.
Patient Support and
Information
Mary Evans-Lee
3742 Woodland Drive
Columbus, GA 31907
Phone: 706–323–7576
Email:
info@diabetesinsipidus.org
Internet:
www.diabetesinsipidus.org
National
Organization for Rare Disorders (NORD)
55 Kenosia
Avenue
P.O. Box 1968
Danbury, CT 06813–1968
Phone:
1–800–999–6673 (voicemail) or 203–744–0100
Email:
orphan@rarediseases.org
Internet:
www.rarediseases.org
Nephrogenic
Diabetes Insipidus Foundation
Main Street
P.O. Box
1390
Eastsound, WA 98245
Phone: 1–888–376–6343
Fax:
1–888–376–6356
Email: info@ndif.org
Internet:
www.ndif.org
