Phenylketonuria (PKU)

What is phenylketonuria?

Phenylketonuria (PKU) is an inherited metabolic disorder which causes serious brain damage and other abnormalities if it is not diagnosed soon after birth, and the necessary diet followed.

The condition affects 60 - 70 new-born babies each year. It appears to be more common in Scotland and Northern Ireland. Boys and girls are affected in equal numbers.

PKU is thought to be an autosomal recessive disorder. This means that not only must both parents be carriers of the defective gene, but that the gene must also be passed onto the child from both parents to cause the disorder.

The parents are usually unaffected. Each of their children has a 25% chance of inheriting PKU.

What causes PKU?

Phenylketonuria (PKU)Phenylketonuria (PKU)PKU occurs when there is a defect in the activity of a metabolic enzyme (substance which brings about chemical change) called phenylalanine hydroxylase. This is the enzyme that facilitates the conversion of one of the amino acids (the building blocks of protein), called phenylalanine, into tyrosine, another amino acid.

In PKU, phenylalanine cannot be broken down normally. If it is not excluded from the diet, it builds up in the spinal fluid and bloodstream. An abnormally high level of phenylalanine prevents normal development of the brain and nervous system. If unchecked, this can lead to severe mental impairment.

How is PKU diagnosed and treated?

In order to detect PKU early, all newborn babies are routinely given a blood test called the Guthrie test. In the UK, this is usually done between 6-14 days after birth. It measures the amount of phenylalanine in the blood. If it is higher than normal, other specialized tests may then be performed to confirm a diagnosis of PKU.

PKU is effectively treated by restricting the intake of phenylalanine in the diet. Once diagnosis is confirmed, babies will be prescribed a special milk substitute or feed. The baby is carefully monitored by the doctors to ensure the level of pheny­lalanine is dropping.

After weaning, babies are given a very low protein, mainly vegetarian, diet. This is done to exclude phenylalanine, as it is a natural constituent of most protein containing foods, such as meat, fish, cheese and eggs.

Ordinary bread, flour, cakes and biscuits are also generally not allowed. Foods such as cereals and milk contain some pheny­lalanine and can be given only in limited amounts.

There are a number of special food products available for PKU sufferers, and your doctor, dietitian or health visitor will be able to give advice.

In addition to strict dietary control of protein intake, phenylalanine-free amino acid supplements may be taken to avoid deficiency of these vital nutrients, which are necessary for normal growth.

In general, a normal diet is introduced when the child is about 10-13 years old, apart from some restriction of protein intake.

Blood tests are carried out regularly to monitor the level of phenylalanine in the blood.

When should I see my doctor?

You should see your doctor if you notice that your child is not develop­ing properly. Your doctor will be able to coniirm whether or not the Guthrie test was carried out and, if it was, will inform you of the result.

A child with PKU will be monitored regularly by the doctor to ensure that the blood levels of phenylalanine do not get dangerously high. This is especially important when the child starts to eat a more normal diet.

If you, your partner, or a close member of either family has PKU, you should consult your doctor before attempting to have a child (see box above).

What can I do myself?

If you or your child has PKU, you should closely follow your doctor's dietary instructions.

Is PKU dangerous?

Yes. If left untreated, PKU can cause severe mental impairment, physical abnormalities, and neurological disorders.

However, when it is effectively treated from a very early age, there is little risk of damage.

Can a person with PKU have healthy children?

A woman with PKU can have a healthy baby as long as she goes back onto the phenylalanine-free diet before pregnancy. This is necessary because a baby is at a high risk of congenital heart defects, brain damage and other abnormalities if exposed to high concentrations of phenylalanine before birth.

The child of a person with PKU can only inherit the disorder if the other parent is a PKU carrier.

If you decide to have a family and you or your partner (or a close member of either family) has PKU, ask the doctor to refer you for genetic counselling. The counsellor will be able to help in making an informed decision, based on the risks involved.

What will the doctor do?

If your baby is diagnosed as having PKU from the Guthrie test, the family doctor will work closely with the hospital dietitian to advise on the proper diet for your child to ensure no long-term damage occurs.



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