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Alternative Names
PKU, phenylalanine hydroxylase deficiency

Phenylketonuria (PKU) is an inherited condition in which the body cannot process a substance called phenylalanine. PKU is an inborn error of metabolism that can lead to severe mental retardation if it is not treated.

What is going on in the body?
Phenylketonuria is caused by an enzyme defect in the liver. Normally, the liver produces an enzyme that breaks down phenylalanine, a compound found in many foods. The liver of a person with phenylketonuria does not produce this enzyme. When this happens, phenylalanine and its by-products build up in the body. This leads to mental retardation.

What are the signs and symptoms of the disease?
The major symptom of phenylketonuria is a very serious learning disability. Untreated persons may have a head that is smaller than average in size. Seizures are also common.

What are the causes and risks of the disease?
A problem in a gene causes the enzyme defect that leads to phenylketonuria. In order to have PKU, a person must inherit the abnormal gene from both parents. Phenylketonuria occurs in 1 of 15,000 live births. It is much more common in Caucasians than in other racial groups.

What can be done to prevent the disease?
The genetic defect cannot be prevented. If a person is born with this defect and it is detected early in life, he or she can follow a special diet. This will prevent the mental retardation and learning disabilities.

How is the disease diagnosed?
Phenylketonuria is diagnosed when high levels of phenylalanine are found in the blood. Genetic testing for PKU is generally done with a screening blood test within 48 hours of birth. Babies who test positive on this screening test will be tested again with a more specific test for PKU.

What are the long-term effects of the disease?
If treatment is started early in life, there are few long-term effects. Otherwise, affected persons may have serious learning disabilities. They may also die at a young age.

What are the risks to others?
A person with phenylketonuria will pass on an abnormal gene to his or her children. The children will not be affected, however, unless the other parent also has the abnormal gene. Parents who have a child with phenylketonuria are at risk for having other affected children. Genetic counselling can be helpful in this situation.

A woman with phenylketonuria must take special care during pregnancy. If she has been treated in the past and has resumed a normal diet, the enzyme defect may cause her unborn child to be severely affected. She will need to go back on the special diet and avoid phenylalanine during pregnancy.

What are the treatments for the disease?
Treatment consists of limiting phenylalanine in the diet. Some doctors are willing to relax the diet after several years of life. Others suggest limiting phenylalanine throughout life. The diet is difficult to follow because phenylalanine is present in many foods. Foods that replace phenylalanine with another substance often do not taste very good.

What are the side effects of the treatments?
There are no side effects to limiting phenylalanine in the diet. A person may find it challenging to avoid such foods, however.

What happens after treatment for the disease?
Pregnant women with this disorder need to avoid phenylalanine.

How is the disease monitored?
The disease is monitored by checking the level of phenylalanine in the blood and measuring its by-products in the urine.

Author: Ronald J. Jorgenson, DDS, PhD, FACMG
Reviewer: HealthAnswers Australia Medical Review Panel
Editor: Dr David Taylor, Chief Medical Officer HealthAnswers Australia
Last Updated: 1/10/2001
Potential conflict of interest information for reviewers available on request

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