Updated 19 February 2016


This is a genetic condition in which enzyme phenylalanine hydroxylase, is deficient.


Phenylketonuria is a genetic disorder in which a key enzyme which is responsible for metabolising the amino acid phenylalanine is deficient. The result is that phenylalanine accumulates, and is then converted into another molecule called a phenylketone. Phenylketones are detected in the urine. When the level of phenylketones become too high, brain development is affected, causing seizures and mental retardation.

Alternative names

In Norway, named after the doctor who discovered the condtion - Folling's Disease. Typically abbreviated to PKU.

What is phenylketonuria?

It is a genetic condition in which an enzyme called phenylalanine hydroxylase, is deficient. This enzyme is normally responsible for converting the amino acid phenylalanine (part of a protein) into tyrosine. When this happens, phenyalanine is converted into phenylketones, which cause impaired brain development and results in severe retardation. The name of the condition comes from the observation that levels of phenylketones are high enough that they are able to be detected in the urine.

What causes phenylketonuria?

Phenylketonuria is a genetic condition, inherited at birth. It is what is known as an autosomal recessive genetic disorder, meaning that an affected child must inherit the defective allele (an alternative form of a gene) from both parents in order to present with symptoms. If only one allele is inherited, the child is a carrier, but will not present with PKU symptoms.

What are the symptoms of phenylketonuria?

Infants with PKU will appear normal at birth, but may have fairer skin and blue eyes. Later, symptoms include vomiting, a musty odour to urine (as a result of the high levels of phenylketones), increased muscle tone, decreased body growth, and a small head. Severe symptoms which develop later include seizures and mental retardation.

How is phenylketonuria diagnosed?

It is possible to screen for PKU from birth. This involves a blood test for levels of phenylalanine, and subsequent tests to confirm PKU if this test is returned positive.

How is phenylketonuria treated?

Phenylketonuria is treated through the use of a controlled diet which is low in phenylalanine throughout the patient's life. This diet must be very strictly followed, and is typically very low in foods including milk, meats, chicken, cheese, fish and nuts, all of which contain high amounts of phenylalanine. The sweetener aspartame is also cut from the diet. Because this diet restricts not only phenylalanine, but other amino acids, a child is typically supplemented with infant formulas. A number of new treatments, including genetic treatment are currently being investigated. These are very preliminary and come at considerable cost at present.

What is the prognosis?

If the special diet is followed, prognosis is good and the patient will live a normal life, with only dietary restrictions. PKU is in fact one of the few genetic disorders that can be managed with controlled dietary change.

When to call your doctor

If your child displays any of the symptoms described above, a physician's opinion is recommended. However, the test for PKU is extremely accessible and easily performed, and so the prudent approach is to perform this test on all newborns.

How can phenylketonuria be prevented?

At present, the condition is not preventable, since it is genetically-inherited.


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