Polycystic Kidney Disorder:

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 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, also called 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

• What is autosomal dominant PKD?

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 (see Genetic Diseases). 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.

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. High blood pressure occurs early in the disease, often before cysts appear.

• What are the symptoms of autosomal dominant PKD?

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)

• How is autosomal dominant PKD diagnosed?

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.

• How is autosomal dominant PKD treated?

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 usually does not slow the disease's progression 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

• What is autosomal recessive PKD?

• What are the symptoms of autosomal recessive PKD?

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.

• How is autosomal recessive PKD diagnosed?

  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.

• How is autosomal recessive PKD treated?

  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 (see End-stage renal disease).

Acquired Cystic Kidney Disease

• What is ACKD?

  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.

• How is ACKD diagnosed?

  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.

• How is ACKD treated?

  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.

Hope Through Research

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.

Genetic analyses of most families with PKD confirm mutations in either the PKD1 or PKD2 gene. In rare cases, however, families with PKD have been found to have normal PKD1 and PKD2 genes. As a result, researchers theorize that a PKD3 gene exists, but that gene has not been mapped or identified.

Researchers recently identified the autosomal recessive PKD gene (called PKHD1) on chromosome 6. No genetic test kit is yet 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. In 2003, scientists also demonstrated that another compound, one that blocks function of a kidney receptor, inhibits cyst formation in mice with the ADPKD or ARPKD gene. The scientists hope that further testing will lead to safe and effective treatments for humans.

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