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Phenylketonuria (PKU): Symptoms, diagnosis, treatment and diet

A type of rare metabolic disorder, phenylketonuria (PKU) prevents the body being able to break down protein, and this can lead to brain damage. The condition is treated by following a restricted low-protein diet.

What is phenylketonuria?

The food and drink consumed in a normal diet supplies our body with protein, which is normally broken down into amino acids by enzymes released in our digestive system. In people with phenylketonuria, they lack one of the enzymes necessary to break down the protein in their diet. When this happens, a substance known as phenylalanine builds up in their blood and urine. Phenylalanine also builds up in the brain. If untreated, a baby with PKU can develop brain damage, learning difficulties and other complications.

About 1 in 10,000 babies across the UK are born with PKU. However, in Northern Ireland the rate is higher, at 1 in 4,500 births.

What causes phenylketonuria?

PKU is an inherited metabolic condition. It occurs when a baby inherits a faulty copy of the gene for PKU from each of its parents. Only a small number of people are carriers of this gene, meaning they don't have the condition, but they have a mutated gene that can be passed on to their offspring. If both parents are carriers of the mutated gene, each of their offspring has a 1 in 4 chance of having PKU. If you have a child with PKU, there is around a 1 in 100 chance they will also have a child with PKU.

What are the symptoms of phenylketonuria?

Babies with PKU do not have any symptoms at birth. However, symptoms gradually appear in the first few months in untreated babies as a build-up of phenylalanine damages their brain. If PKU remains untreated there will be permanent brain damage and learning difficulties. Symptoms include:

  • Frequent temper tantrums, self-harming, hyperactivity and other behavioural problems
  • Developmental delays and intellectual impairment
  • Eczema
  • A lack of melanin production (and therefore fair eyes, skin and hair in comparison to other family members)
  • Recurrent vomiting
  • Light sensitivity
  • Jerking arm and leg movements
  • Tremors
  • Epilepsy
  • Musty smelling breath, urine and skin.

It is possible to reverse some of these behavioural and physical symptoms if the person receives treatment, even if it has been delayed. If a child has been following a restricted low-protein diet under medical supervision, but stops, their motor skills and cognitive abilities can deteriorate.

Even with treatment, some children and adults with PKU can experience problems such as:

Is there a screening test for phenylketonuria?

All newborns are offered a 'heel prick' screening blood test in the first week after birth. This test screens for a number of inherited conditions including PKU.

If you already have a child with PKU and become pregnant, make sure your midwife and GP know there is a family history of PKU. You should be referred to a genetic counsellor or paediatrician, and their advice should be written down in your birth plan. It is important to make sure your newborn has a heel prick test 48–72 hours following birth.

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