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Phenylketonuria

An inherited chemical defect that can cause brain damage

  • Present from birth
  • Due to an abnormal gene inherited from both parents
  • Gender and lifestyle are not significant factors

Children with phenylketonuria lack the enzyme that is responsible for breaking down phenylalanine, which occurs naturally in most food containing protein. As a result, phenylalanine is converted into harmful substances that build up in the blood and may damage the developing brain. Although phenylketonuria is rare in the UK, affecting about 1 in 10,000 infants, all newborn babies are screened for the disorder because of the risk of serious brain damage. Like most inborn errors of metabolism, phenylketonuria is due to an abnormal gene that is inherited in an autosomal recessive manner (see Gene disorders). If both parents carry the abnormal gene, there is a 1 in 4 chance that their baby will be affected.

What are the symptoms?

At birth, some babies with phenylketonuria have a red, itchy rash similar to eczema, but most affected infants appear healthy. The symptoms of the disorder usually develop gradually over 6–12 months and may include:

  • Vomiting.

  • Restlessness and sometimes seizures.

  • Stale, unpleasant skin odour.

  • Delay in development (see Developmental delay).

If it is not treated, phenylketonuria may lead to serious brain damage, resulting in severe learning problems (see Generalized learning disabilities).

What might be done?

In the UK, a blood test for phenylketonuria is given to all newborn babies (see Blood spot screening tests). Early screening is important because prompt diagnosis and treatment are vital. If your baby is diagnosed as having phenylketonuria, he or she will probably be prescribed a special formula or milk substitute that is rich in protein but contains little phenylalanine. Your child should continue with a diet low in phenylalanine, at least throughout his or her childhood until the brain has stopped growing. Most doctors recommend that an affected person should follow a low-phenylalanine diet for life. Women with the condition who are planning a pregnancy are advised to follow a low phenylalanine diet before conception and throughout pregnancy.

With early diagnosis and treatment, children with phenylketonuria develop normally, attend mainstream schools, and do not have reduced life expectancy.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

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