Polycystic Kidney
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Polycystic kidney disease (PKD) is a genetic disorder characterized by the
growth of numerous cysts in the kidneys. The cysts are filled with fluid.
PKD cysts can slowly replace much of the mass of the kidneys, reducing kidney
function and leading to kidney failure.
The kidneys are two organs, each about the size of a fist, located in the
upper part of a person's abdomen, toward the back. The kidneys filter wastes
from the blood to form urine. They also regulate amounts of certain vital
substances in the body.
When PKD causes kidneys to fail--which usually happens only after many years--the
patient requires dialysis or kidney transplantation. About one-half of people
with the major type of PKD progress to kidney failure, i.e., end-stage renal
disease (ESRD).
PKD can cause cysts in the liver and problems in other organs, such as the
heart and blood vessels in the brain. These complications help doctors distinguish
PKD from the usually harmless "simple" cysts that often form in the kidneys
in later years of life.
In the United States, about 500,000 people have PKD, and it is the fourth
leading cause of kidney failure. Medical professionals describe two major
inherited forms of PKD and a noninherited form:
- Autosomal dominant PKD is the most common, inherited form. Symptoms
usually develop between the ages of 30 and 40, but they can begin earlier,
even in childhood. About 90 percent of all PKD cases are autosomal dominant
PKD.
- Autosomal recessive PKD is a rare, inherited form. Symptoms of
autosomal recessive PKD begin in the earliest months of life, even in the
womb.
- Acquired cystic kidney disease (ACKD) develops in association
with long-term kidney problems, especially in patients who have kidney
failure and who have been on dialysis for a long time. Therefore it tends
to occur in later years of life. It is not an inherited form of PKD.
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Autosomal dominant PKD is one of the most common inherited disorders. The
phrase "autosomal dominant" means that if one parent has the disease, there
is a 50-percent chance that the disease will pass to a child. At least one
parent must have the disease for a child to inherit it. Either the mother
or father can pass it along, but new mutations may account for one-fourth
of new cases. In some rare cases, the cause of autosomal dominant PKD occurs
spontaneously in the child soon after conception--in these cases the parents
are not the source of this disease.
Many people with autosomal dominant PKD live for decades without developing
symptoms. For this reason, autosomal dominant PKD is often called "adult
polycystic kidney disease." Yet, in some cases, cysts may form earlier, even
in the first years of life.
The disease is thought to occur equally in men and women and equally in
people of all races. However, some studies suggest that it occurs more often
in whites than in blacks and more often in females than in males. High blood
pressure occurs early in the disease, often before cysts appear.
The cysts grow out of nephrons, the tiny filtering units inside the kidneys.
The cysts eventually separate from the nephrons and continue to enlarge.
The kidneys enlarge along with the cysts (which can number in the thousands),
while retaining roughly their kidney shape. In fully developed PKD, a cyst-filled
kidney can weigh as much as 22 pounds.
The most common symptoms are pain in the back and the sides (between the
ribs and hips), and headaches. The dull pain can be temporary or persistent,
mild or severe.
People with autosomal dominant PKD also can experience the following:
- urinary tract infections
- hematuria (blood in the urine)
- liver and pancreatic cysts
- abnormal heart valves
- high blood pressure
- kidney stones
- aneurysms (bulges in the walls of blood vessels) in the brain
- diverticulosis (small sacs on the colon)
To diagnose autosomal dominant PKD, a doctor typically observes three or
more kidney cysts using ultrasound imaging. The diagnosis is strengthened
by a family history of autosomal dominant PKD and the presence of cysts in
other organs.
In most cases of autosomal dominant PKD, the person's physical condition
appears normal for many years, even decades, so the disease can go unnoticed.
Physical checkups and blood and urine tests may not lead to diagnosis. The
slow, undetected progression is why some people live for many years without
knowing they have autosomal dominant PKD.
Once cysts have formed, however, diagnosis is possible with imaging technology.
Ultrasound, which passes sound waves through the body to create a picture
of the kidneys, is used most often. Ultrasound imaging employs no injected
dyes or radiation and is safe for all patients including pregnant women.
It can also detect cysts in the kidneys of a fetus.
More powerful and expensive imaging methods such as computed tomography
(CT scan) and magnetic resonance imaging (MRI) also can detect cysts, but
these methods usually are not required because ultrasound provides adequate
information. CT scans require x rays and sometimes injected dyes.
A genetic test has recently been developed and is commercially available
to detect mutations in the PKD1 and PKD2 genes. This test can detect the
presence of the autosomal dominant PKD mutations before cysts develop, but
its usefulness is limited by the fact that the test cannot predict ultimate
severity or onset of disease, and by the fact that no absolute cure is available
to prevent the onset of the disease. A young person who knows of a PKD gene
mutation may be able to forestall the disease through diet and blood pressure
control. The test may also be used to determine whether a young member of
a PKD family can safely donate a kidney to a parent. Anyone considering genetic
testing should receive counseling to understand all the implications of the
test.
Although a cure for autosomal dominant PKD is not available, treatment can
ease the symptoms and prolong life.
Pain. A doctor will first suggest over-the-counter pain medications,
such as aspirin or Tylenol. For most but not all cases of severe pain, surgery
to shrink cysts can relieve pain in the back and flanks. However, surgery
provides only temporary relief and does not slow the disease's progression,
in many cases, toward kidney failure.
Headaches that are severe or that seem to feel different from other headaches
might be caused by aneurysms, or swollen blood vessels, in the brain. Headaches
also can be caused by high blood pressure. People with autosomal dominant
PKD should see a doctor if they have severe or recurring headaches--even
before considering over-the-counter pain medications.
Urinary tract infections. Patients with autosomal dominant PKD tend
to have frequent urinary tract infections, which can be treated with antibiotics.
People with the disease should seek treatment for urinary tract infections
immediately, because infection can spread from the urinary tract to the cysts
in the kidneys. Cyst infections are difficult to treat because many antibiotics
do not penetrate into the cysts. However, some antibiotics are effective.
High blood pressure. Keeping blood pressure under control can slow
the effects of autosomal dominant PKD. Lifestyle changes and various medications
can lower high blood pressure. Patients should ask their doctors about such
treatments. Sometimes proper diet and exercise are enough to keep blood pressure
low.
End-stage renal disease. Because kidneys are essential
for life, people with ESRD must seek one of two options for replacing kidney
functions: dialysis or transplantation. In hemodialysis, blood is circulated
into an external machine, where it is cleaned before reentering the body;
in peritoneal dialysis, a fluid is introduced into the abdomen, where it
absorbs wastes, and it is then removed. Transplantation of healthy kidneys
into ESRD patients has become a common and successful procedure. Healthy
(non-PKD) kidneys transplanted into PKD patients do not develop cysts.
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Autosomal recessive PKD is caused by a particular genetic flaw that is different
from the genetic flaw that causes autosomal dominant PKD. Parents who do
not have the disease can have a child with the disease if both parents carry
the abnormal gene and both pass the gene to their baby. The chance of this
happening (when both parents carry the abnormal gene) is one in four. If
only one parent carries the abnormal gene, the baby cannot get the disease.
The symptoms of autosomal recessive PKD can begin before birth, so it is
often called "infantile PKD." Children born with autosomal recessive PKD
usually develop kidney failure within a few years. Severity of the disease
varies. Babies with the worst cases die hours or days after birth. Children
with an infantile version may have sufficient renal function for normal activities
for a few years. People with the juvenile version may live into their teens
and twenties and usually will have liver problems as well.
Children with autosomal recessive PKD experience high blood pressure, urinary
tract infections, and frequent urination. The disease usually affects the
liver, spleen, and pancreas, resulting in low blood-cell counts, varicose
veins, and hemorrhoids. Because kidney function is crucial for early physical
development, children with autosomal recessive PKD are usually smaller than
average size.
Ultrasound imaging of the fetus or newborn baby reveals cysts in the kidneys
but does not distinguish between the cysts of autosomal recessive and autosomal
dominant PKD. Ultrasound examination of kidneys of relatives can be helpful;
for example, a parent or grandparent with autosomal dominant PKD cysts could
help confirm diagnosis of autosomal dominant PKD in a fetus or child. (It
is extremely rare, although not impossible, for a person with autosomal recessive
PKD to become a parent.) Because autosomal recessive PKD tends to scar the
liver, ultrasound imaging of the liver also aids in diagnosis.
Medicines can control high blood pressure in autosomal recessive PKD, and
antibiotics can control urinary tract infections. Eating increased amounts
of nutritious food improves growth in children with autosomal recessive PKD.
In some cases, growth hormones are used. In response to kidney failure, autosomal
recessive PKD patients must receive dialysis or transplantation
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Genes are segments of DNA, the long molecules that reside in the
nuclei of your body's cells. The genes, through complex processes,
cause chemical activities that lead to growth and maintenance of
the body. At conception, DNA (and therefore genes) from both parents
are passed to the child.
A genetic disease occurs when one or both parents pass abnormal
genes to a child at conception. If receiving an abnormal gene from
just one parent is enough to produce a disease in the child, the
disease is said to have dominant inheritance. If receiving abnormal
genes from both parents is needed to produce disease in the child,
the disease is said to be recessive.
The chance of acquiring a dominant disease (one gene copy is enough)
is higher than the chance of acquiring a recessive disease (two gene
copies are needed). A child who receives only one gene copy for a
recessive disease at conception will not develop the genetic disease
(such as autosomal recessive PKD), but could pass the gene to the
following generation.
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ACKD develops in kidneys with long-term damage and bad scarring, so it often
is associated with dialysis and end-stage renal disease. About 90 percent
of people on dialysis for 5 years develop ACKD. People with ACKD can have
any underlying kidney disease, such as glomerulonephritis or kidney disease
of diabetes.
The cysts of ACKD may bleed. Kidney tumors, including kidney (renal) cancer,
can develop in people with ACKD. Renal cancer is rare yet occurs at least
twice as often in ACKD patients as in the general population.
Patients with ACKD usually seek help because they notice blood in their
urine (hematuria). The cysts bleed into the urinary system, which discolors
urine. Diagnosis is confirmed using ultrasound, CT scan, or MRI of the kidneys.
Most ACKD patients are already receiving treatment for kidney problems.
In rare cases, surgery is used to stop bleeding of cysts and to remove tumors
or suspected tumors.
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Scientists have begun to identify the processes that trigger formation of
PKD cysts. Advances in the field of genetics have increased our understanding
of the abnormal genes responsible for autosomal dominant and autosomal recessive
PKD. Scientists have located two genes associated with autosomal dominant
PKD. The first was located in 1985 on chromosome 16 and labeled PKD1. PKD2
was localized to chromosome 4 in 1993. Within 3 years, the scientists had
isolated the proteins these two genes produce--polycystin-1 and polycystin-2.
When both of these genes are normal, the proteins they produce work together
to foster normal kidney development and inhibit cyst formation. A mutation
in either PKD1 or PKD2 can lead to cyst formation, but evidence suggests
that the disease development also requires other factors, in addition to
the mutation in one of the PKD genes.
Researchers recently identified the autosomal recessive PKD gene (called
PKHD1) on chromosome 6. A genetic test kit is not yet commercially available
to detect mutations in PKHD1.
Researchers have bred mice with a genetic disease that parallels both inherited
forms of human PKD. Studying these mice will lead to greater understanding
of the genetic and nongenetic mechanisms involved in cyst formation. In 2000,
scientists reported that a cancer drug was successful in inhibiting cyst
formation in mice with the PKD gene. The scientists hope that further testing
will lead to safe and effective treatments for humans.
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- two inherited forms:
- a common form that usually causes symptoms in midlife
- a rare form that usually causes symptoms in early childhood
- a noninherited form associated with long-term kidney problems, dialysis,
and old age
- pain in the back and lower sides
- headaches
- urinary tract infections
- blood in the urine
- cysts in the kidneys and other organs
- ultrasound imaging of kidney cysts
- ultrasound imaging of cysts in other organs
- family medical history (genetic testing)
- medicine and surgery to reduce pain
- antibiotics to resolve infections
- dialysis and transplantation to replace functions of failed kidneys
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Polycystic Kidney Disease
Foundation
American Association of
Kidney Patients
National Kidney Foundation
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