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Information for Health Visitors on PKU

What is phenylketonuria (PKU)?

Phenylketonuria (PKU) is a rare, disorder which prevents the normal metabolism of protein foods. It is an autosomal recessively inherited condition. Both parents carry the same altered gene, giving a 1 in 4 risk of having an affected child at each pregnancy.

In the process of digestion and metabolism, dietary protein is broken down into its constituent amino acids, and then reassembled into tissue for growth and repair of the body. Phenylalanine is one of the amino acids which make up protein and normally any excess phenylalanine is converted into another amino acid, tyrosine. In PKU this conversion does not take place and phenylalanine accumulates in the blood. Without treatment, high levels of phenylalanine would affect normal growth and development of the child, particularly brain development.

The History of PKU

PKU was first described in 1934 by a Norwegian doctor named Asbjorn Folling. It was about 20 years later that Dr. Horst Bickel tried a low phenylalanine diet on a two year old girl in Birmingham Children’s Hospital in England. In the late 1960s the blood screening test for phenylketonuria introduced by Dr. Bob Guthrie from the USA, was first introduced into the United Kingdom. This test is now offered to all babies at around 6-10 days of age, and is usually taken by community midwives. In 1992 Dr. Savio Woo working in Houston, Texas found the gene for phenylalanine hydroxylase in phenylketonuria. The combination of the screening test and low phenylalanine diet has improved the outlook for the PKU child and family.

A Medical Research Council report* in 1993 recommended that diet should be continued for life and that blood phenylalanine concentrations be maintained at lower levels than had been previously advised. “Diet for life” is now recommended for all PKU patients.

What happens after the diagnosis of PKU has been made?

As soon as the diagnosis has been confirmed, the baby will be given a special low phenylalanine feed until the high levels of phenylalanine fall to within the treatment range. This range is higher than the normal reference range. Usually this will take only a few days. Once the levels of phenylalanine are within the recommended range a small amount of breast milk or formula baby milk will be introduced to provide the limited amount of phenylalanine essential for normal growth. The two types of milk, low phenylalanine and breast or formula milk, will be continued, until weaning commences at the usual age.

Dietary Management

It is important that dietary advice is consistent and therefore the child’s dietitian should be the only person giving advice.

PKU is successfully controlled by a low phenylalanine diet. The diet is complex and high protein foods, e.g. meat, fish, eggs, cheese, soya, nuts as well as ordinary bread, biscuits, cakes and pasta, are not permitted. Foods and medicines containing the sweetener Aspartame are not permitted.

The low phenylalanine diet consists of 3 parts:

1. A protein substitute which includes all other amino acids except phenylalanine. Some protein substitutes also contain appropriate vitamins and minerals, if not, they need to be taken in addition to the protein substitute.

2. An allowance of a small measured quantity of phenylalanine, in the form of exchanges from foods such as potatoes, baked beans and breakfast cereals. Many patients will only be allowed 4 to 6 phenylalanine exchanges per day. Examples of one exchange equivalents include 20g baked beans or 15g cornflakes.

3. Free Foods. There are only a few natural foods low enough in phenylalanine to be permitted in normal quantities in the diet. Examples include most fruit, most vegetables, sugar, butter, boiled sweets and some squashes. In addition there are many specially produced low protein products such as low protein flour, bread and pasta, which are suitable for PKU and are ACBS listed and therefore available on prescription.

Blood monitoring and hospital clinic attendance

Instructions will be given about how to take blood tests and the frequency required. Most families are able to take this test at home and post it to their local laboratory for analysis. The family will then be advised by their dietitian of any changes to the diet that are required.

Maternal PKU

Babies born to mothers with high phenylalanine levels may be mentally and physically damaged in utero due to the effect of these levels. This risk can almost certainly be prevented if the mother has a low phenylalanine diet (with appropriate supplements) before conception and throughout pregnancy. It is therefore essential that PKU females are aware of this problem and seek advice prior to conception.

The Role of the Health Visitor

All immunisations can proceed normally. If the child’s phenylalanine control is kept within current recommended guidelines, growth and development will not be affected and extra assessments should not be required. “Routine health visiting” will probably be sufficient for families with a PKU child. However, dependant upon the support available from the child’s Consultant and/or PKU Team, the health visitor may need to identify and respond to local issues such as playgroups, nurseries and schools. The family may also need extra support in coping with shock, anxiety or stress on a family when a child has a long-term condition.

If treatment for PKU begins before the age of about 21 days, and phenylalanine control is maintained within the current MRC guidelines, families can be reassured that their child will be able to achieve his or her maximum potential within the family.

*Archives of Disease in Childhood 1993; 68: 426-427

For further information, see our booklet The Management of PKU

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