Overview

What is phenylketonuria?

Phenylketonuria (PKU) is a rare inherited metabolic disorder that is characterised by high levels of the amino acid phenylalanine in the body. Our bodies metabolise proteins in food by breaking them down into amino acids, which are then used to build more protein or be removed from the body if no longer useful or needed. Phenylketonuria is caused by a discrepancy in the gene phenylalanine hydroxylase, the gene responsible for creating the enzyme needed to break down phenylalanine into tyrosine. Without the phenylalanine hydroxylase, a dangerous build-up of phenylalanine can occur when a person eats any food that contains proteins or aspartame (the artificial sweetener)and this can lead to serious health problems, brain abnormalities and cognitive impairment.¹

Children and adults with PKU must follow a rigorous diet that restricts or eliminates phenylalanine, which is found in most proteins and aspartame.

Symptoms of phenylketonuria

Newborns with PKU do not show symptoms; but, if left untreated, symptoms of PKU can present themselves within a few months and can include the following:²

• Phenylketonuria can damage the brain and nervous system, leading to learning and intellectual disabilities, delayed development, behavioural problems and nervous system issues such as seizures and epilepsy
• Small head size (microcephaly)
• Lighter skin, hair and eye colour compared to siblings (phenylalanine is involved in the production of melanin, a pigment responsible for skin, hair, and eye colour)
• Eczema
• Jerking movements in arms and legs
• High amounts of phenylalanine in the body produce a musty odour in the breath, skin, and urine.
• Repeatedly getting sick

Which children are at risk of developing PKU

The genetic mutation responsible for PKU is passed down by parents, who are carriers and do not exhibit symptoms of the disease. Phenylketonuria is an autosomal recessive disorder. This means a child needs to inherit 2 faulty genes, one from each parent, to develop the disease and show signs of PKU. If the child inherits only one faulty gene, they will only be a carrier of PKU.³

Diagnosis

A phenylalanine screening test can be carried out to detect the level of phenylalanine in blood. Newborns receive this test 24 to 72 hours after being born as part of newborn screening. If diagnosis is confirmed, your child will need regular monitoring to detect phenylalanine levels, and to ensure they are responding well to treatment. If you have PKU or a family history of it, your healthcare provider may recommend doing a screening test before pregnancy and birth.

Pregnancy and phenylketonuria

Pregnant women are at risk of developing another form of phenylketonuria called maternal PKU. Special PKU diet must be followed to prevent high levels of phenylalanine that can harm the growth and development of the baby. Newborns of mothers with maternal PKU don’t necessarily inherit the condition, but they can have serious health problems like heart problems, microcephaly (babies head is far smaller than normal) and low birth weight. Maternal PKU can also cause the child to have developmental and behavioural problems and intellectual disability.⁴

Treatment and management of PKU in children

Starting treatment early and following a life-long treatment plan can prevent the health problems associated with phenylketonuria. Treatments for PKU include the following: ⁵

• Following a life-time diet that contains low levels of phenylalanine but rich in nutrients. A safe amount of phenylalanine is different for each case, generally the goal is to consume only the needed amount for growth and development, this can be determined through regular blood tests for phenylalanine levels, reviewing growth charts, and carrying out tests to assess general health.
• Taking a PKU supplemental formula, to make sure your child is receiving essential proteins (but not phenylalanine) that are essential for growth and development.
• Taking vitamins, minerals and supplements.
• Adding a medication called sapropterin dihydrochloride to break down phenylalanine in your body.
• Medications that replace the enzyme responsible for breaking down phenylalanine like pegvaliase are being approved by the FDA. It allows people to eat an unrestricted diet without having to take sapropterin dihydrochloride.
• If your child is diagnosed with PKU, they need to limit eating food that contains high amounts of phenylalanine like: milk, eggs, cheese, nuts, fish, chicken, beef, beans and artificial sweetener (aspartame).
• Some medications and supplements may contain aspartame or amino acids, so always consult with your local pharmacist before taking any new medications.
• Your healthcare provider may refer you to a registered nutritionist or dietitian who can teach you how to adjust to the best PKU diet and to overcome any challenges regarding your diet.

Prevention  

There is no way to prevent phenylketonuria. Partners planning to become pregnant who want to understand the risk of having a child with a genetic disorder are advised to undergo genetic counselling.

Living and coping when you have a child with phenylketonuria

Phenylketonuria is a life-long condition, and it can be challenging to care for a child with the condition. Here are some helpful strategies that can help you cope better:

• Stay up to date with your healthcare provider and perform regular blood tests to monitor phenylalanine levels.
• Stay informed and know all the facts about PKU. Ask questions and discuss everything with your child’s paediatrician.
• Connect with local or online support groups for families who have children with PKU. Reaching out to people who are going through the same situation can be helpful.
• Seek advice from a dietician about meal plannings, and how to cook delicious food for special occasions and holidays.
• Let children manage their diet as early as possible.
• Call your local restaurants when you are planning to eat out, and ask them if they have PKU friendly foods.
• Talk to teachers and other staff at your child’s school about the importance of a PKU diet.
• Ask your healthcare providers if there are financial aid programs that cover the high cost of formula and PKU diet.

FAQs

Can children with PKU survive?

If early treatment and management was carried out, and the child can follow a strict diet and supplement regimen, they should be able to live a full active life.

At what age does phenylketonuria start?

Babies who are born with PKU don’t show symptoms in the first few months of age. Symptoms of PKU start at about 6 months of age.

What is the quality of life of someone with PKU?

Children with phenylketonuria can experience lower physical, social, and psychological health. But studies show that the quality of life of patients who were diagnosed and treated in early stages of life is almost equal to that of normal healthy people.⁶

Summary

Phenylketonuria (PKU) is a genetic disorder characterised by the inability to properly metabolise the amino acid phenylalanine, leading to its accumulation in the body and potentially causing intellectual disability and other neurological problems if left untreated. Managing PKU in children involves adherence to a phenylalanine-restricted diet. Regular monitoring of blood phenylalanine levels is crucial to ensure that levels remain within safe limits, and to ensure treatment provided is effective. In addition to dietary management, children with PKU may also require medical supervision to address potential nutritional deficiencies and to provide support for their overall health and development. Early diagnosis and intervention are essential to prevent the long-term complications associated with PKU and to optimise the child's quality of life.