What is phenylketonuria? (PKU)
PKU is a rare genetic condition that affects the enzyme called phenylalanine hydroxylase (PAH).
PAH helps to break down protein found in food, turning it into energy needed for growth and repair.
PKU is diagnosed at birth and can be controlled with a low protein diet.
What causes phenylketonuria (PKU)?
PKU is caused when the PAH enzyme is either missing or doesn’t work properly, resulting in a build-up of phenylalanine in the blood and brain.
Phenylalanine is an amino acid that is released when the body digests protein. PAH helps to break it down further so that it can be used around the body. When PAH is not able to do the job it should, phenylalanine builds up affecting blood and brain development.
PKU is an inherited condition which requires an altered gene from both parents. Parents that are carriers of the gene will not usually show symptoms themselves.
What are the symptoms of phenylketonuria (PKU)?
Symptoms of PKU will vary depending on whether the PAH enzyme is missing, or only partly working:
- Classic PKU – the most severe form in which the enzyme is missing or doesn’t work at all. If high phenylalanine levels are not treated early, it can affect the baby’s brain development.
- Mild or moderate PKU – the enzyme still works a little so phenylalanine levels aren’t as high. This can usually be treated with a special low-protein diet.
- Mild hyperphenylalaninemia (HPA) – the mildest form. The enzyme is only slightly affected, keeping phenylalanine levels only a little above normal. Your baby may not need a special diet, but regular monitoring is still important.
Mild symptoms include:
- A musty smell in urine and the breath (caused by a build-up of acids)
- Eczema
- Loss of pigment in hair, eyes and skin, causing lighter features
More severe symptoms that develop with higher levels of PKU include:
- Slower growth
- Delayed development (difficulty reaching milestones)
- Learning difficulties
- Seizures
- Microcephaly (a small head).
With only around seven babies born each year in the UK with PKU, families affected can find it hard to get the right support.
PKU is part of a group of conditions known as metabolic disorders, which affect how the body breaks down food for energy.
Learning about related conditions such as maple syrup urine disease, tyrosinemia and mitochondrial disease can help you understand more about PKU and be reminded that you are not alone.
