Polycystic Kidney Disease (PKD)
Polycystic means “many fluid-filled sacs” or “many cysts”. Polycystic kidney disease (PKD) is a genetic disorder that causes multiple cysts to form in the kidneys.
Polycystic kidneys become very large, have a bumpy surface and contain many fluid-filled cysts. This can be associated with a number of conditions, including:
- High blood pressure
- Urinary and kidney infections
- Kidney stones
- Kidney failure
Kidney failure resulting from PKD is caused by a combination of:
- Pressure from expanding cysts, compressing normal kidney tissue
- Scarring in the normal support tissue in the kidney
Who is affected by PKD?
PKD can strike anyone at any time. About 5% of all people requiring dialysis or kidney transplantation have PKD.
PKD is an inherited disease. This means that it is passed on from parents to their children. Everyone has 23 different pairs of chromosomes, with one set coming from each parent. Chromosomes are scaffolds that carry individual hereditary units called genes. Genes are programmed to produce proteins. Proteins are the structural and functional building blocks that enable individual cells to work properly.
Forms of PKD
There are two forms of PKD: autosomal dominant PKD (ADPKD) and autosomal recessive PKD (ARPKD).
- Autosomal dominant PKD is the most common form of the disease. It affects about 1 in every 1,000 births and symptoms usually appear in midlife. ADPKD follows a dominant inheritance. This means that if either your mother or father has ADPKD, you will have a 50% chance of inheriting ADPKD. If the gene for ADPKD is not passed on, you will neither inherit the disease, nor be able to pass on the disease to future offspring.
ADPKD is not a single disease. There are at least two genes that are associated with ADPKD. The Type 1 variant occurs when you inherit a defective copy of the PKD1 gene on chromosome 16. This form of disease accounts for about 85% of all cases of ADPKD. The Type 2 variant occurs when you inherit a defective copy of the PKD2 gene on chromosome 4. This form of the disease accounts for most of the remaining cases. Generally, the symptoms of both of these subtypes overlap completely.
- Autosomal recessive PKD is a very rare form of PKD causing symptoms in infants and young children. You may develop ARPKD if each of your parents carries at least one defective copy of the disease gene; in this case, you have a 25% chance of having the disease. If only one parent carries the defective gene, you cannot inherit the disease.
PKD and kidney failure
Kidney failure occurs when you are left with less than 10-15% of total kidney function. When the kidneys fail, dialysis treatment or a kidney transplant is needed. Not everyone with PKD will go on to develop kidney failure. On average, half of the people with the Type 1 variant of ADPKD will require treatment for kidney failure by age 60. However, people affected with the Type 2 variant have a lower risk for developing kidney failure. There are treatments available that help to delay progression of ADPKD. Children born with ARPKD usually develop kidney failure within a few years.
Symptoms of ADPKD
ADPKD usually progresses very slowly. Early in the disease there are often no symptoms. The only way to detect ADPKD is to be tested for cysts in the kidneys. The most common symptoms of ADPKD include:
- Pain in the back and the sides (between the ribs and hips)
People with APKD may also experience the following:
- High blood pressure (hypertension)
- Urinary tract infections
- Blood in the urine (hematuria)
- Kidney stones
- Liver and pancreatic cysts
- Abnormal heart valves
- Bulges in the walls of blood vessels in the brain (aneurysms)
- Small sacs on the colon (diverticulosis)
How is PKD diagnosed?
The main method used to diagnose PKD is an ultrasound. This test uses sound waves to generate echo patterns to detect the presence of cysts. The ultrasound can show both the number and size of individual cysts in the kidney and liver. You might also be referred for a CT scan, a method that uses X-rays to detect very tiny cysts.
Another method of diagnosis is called gene linkage analysis. Blood tests from family members can be used to detect which family member carries the PKD gene. This method, however, requires availability of DNA from multiple affected members of the family, and is not routinely done for diagnostic purposes.
More recently, direct genetic testing for a mutation on the PKD1 and PKD2 genes is being used to diagnose autosomal dominant PKD. Using this testing, ADPKD can be detected before cysts start to develop.
Before you consider being tested for PKD, it is important to discuss with your doctor the benefits and risks involved. These can include determining the possibility of having a child with PKD, and what the test results might mean to career and insurance discrimination.
With acknowledgement to Dr. Michael Copland, Clinical Assistant Professor, Division of Nephrology, University of British Columbia Hospital (Vancouver Hospital and Health Sciences Centre), for his assistance in reviewing this information.