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Phenylketonuria (PKU) in Children

What is phenylketonuria (PKU) in children?

Phenylketonuria (PKU) is a rare metabolic disorder. Children with PKU can’t process an amino acid called phenylalanine. Phenylalanine is in many common foods. It helps the body make protein. It's also important for brain growth. It's normally changed to tyrosine, which helps create all of the body's proteins. But with PKU, the amino acid can build up in the bloodstream and cause harm. PKU is found with a simple blood test. All newborn babies in the U.S. are screened for PKU.

What causes PKU in a child?

PKU is caused by a defect in a gene known as the PAH gene. This defect changes the way that phenylalanine is broken down by the body during digestion. PKU is passed on to children when each parent has 1 mutated gene. This means that neither parent has any symptoms of PKU, but both are carriers of the faulty gene. PKU is an autosomal recessive disease. This means that a child needs to inherit 1 faulty gene from each parent to show signs of the disorder.

Which children are at risk for PKU?

A child is at risk for PKU if their parents each have 1 faulty PAH gene. PKU affects 1 out of every 10,000 to 15,000 newborns born in the U.S.

What are the symptoms of PKU in a child?

If a baby is not tested and has undiagnosed PKU, they may show signs or symptoms at several months old. Some babies with PKU may seem more drowsy and listless than normal. They may have feeding problems. As they continue to take in protein and phenylalanine through their diets, they may have growth, mood, behavior, and thinking problems, seizures as well as other problems. Symptoms can range from mild to severe based on how much phenylalanine is in the blood.

The symptoms of PKU can be like other health conditions. Make sure your child sees their healthcare provider for a diagnosis.

How is PKU diagnosed in a child?

All newborn babies in the U.S. are screened for PKU with a blood test. If you adopt a child from another country, they may need to be screened for PKU and other genetic diseases.

How is PKU treated in a child?

PKU is treated with a special diet. Newborn babies who test positive for PKU are placed on phenylalanine-free formula right away.

As babies start to eat solid food, their diet will need to be restricted. This is because phenylalanine is found in many foods with protein. A child with PKU should not eat milk, fish, cheese, nuts, beans, or meat. A child with PKU can eat many foods low in protein, such as vegetables, fruits, and some cereals. Your child may also need to take mineral and vitamin supplements to make up for nutrients missing from the diet.

Children with PKU can’t tolerate the sugar substitute called aspartame. Aspartame contains phenylalanine. You will need to read all food labels to make sure your child avoids foods and drinks that contain this sweetener.

Babies born with a mild degree of PKU have partial enzyme activity. They'll only develop a mild increase in blood phenylalanine levels. These babies may not need any specialized diet or treatment.

What are possible complications of PKU in a child?

A baby born to a woman who has PKU that is not controlled with a special diet is at high risk for serious problems. The developing baby in the uterus can be exposed to very high levels of phenylalanine. This can cause low birth weight, slow growth, small head, behavior problems, and heart disorders. These mothers are also at risk for pregnancy loss.

If PKU in a child is not diagnosed and treated, phenylalanine will build up in the bloodstream until it reaches levels that can cause brain damage. It can also cause permanent damage to organs and tissues around the body. PKU can lead to:

  • Developmental delays
  • Heart defects, especially in infants born to mothers with PKU and uncontrolled phenylalanine in pregnancy
  • Autism
  • Intellectual disability
  • Very small head size (microcephaly)
  • Behavioral problems
  • Seizures
  • Eczema (a skin condition marked by an itchy red rash or blisters)
  • Pale hair and skin compared with other family members
  • Delayed physical growth
  • Musty body odor

How can I help prevent PKU in my child?

Women with PKU who are of childbearing age should be careful to follow a strict low-protein diet. Your healthcare provider may advise genetic counseling. You can discuss with a counselor the risk for PKU in a future pregnancy.

How can I help my child live with PKU?

Children born with PKU will need to follow a lifelong low-protein diet and stay away from aspartame. If they don't, they may have mood disorders, poor memory and problem-solving skills, depression, and uncontrollable shaking (tremors).

If your baby is born with PKU, you can do certain things to take care of them:

  • Keep all appointments with your baby’s healthcare provider.
  • To help with dietary restrictions and planning, your healthcare provider may refer you to a registered dietitian:
    • Natural sources of protein have too much phenylalanine for children with PKU. So your child can’t have meat, milk, or other common foods that contain protein. Instead, they must eat a variety of processed foods that are phenylalanine-free.
    • Your child can eat fruits, vegetables, and limited amounts of certain grain cereals.
    • Your child can have special nutritional products, including infant formula without phenylalanine.
  • Consider genetic testing and counseling to understand your risk of passing on the gene that causes PKU.
  • Tell others about your baby’s condition. Work with your child’s healthcare provider to develop a treatment plan.
  • Reach out for support from local community services. Being in touch with other parents who have a child with PKU can also be helpful.

When should I call my child’s healthcare provider?

Call the healthcare provider if your child has:

  • Symptoms that don’t get better, or get worse
  • New symptoms that concern you

Key points about PKU in children

  • Phenylketonuria (PKU) is a rare metabolic disorder. Children with PKU can’t process an amino acid called phenylalanine.
  • Phenylalanine is in many common foods. But it can build up in the bloodstream of children with PKU. This can cause growth, mood, behavior, and thinking problems, as well as other problems ranging from mild to severe.
  • If PKU is untreated, phenylalanine will build up in the bloodstream until it reaches levels that can cause brain damage and other serious problems.
  • With treatment and dietary restrictions, a child with PKU can grow and develop normally.
  • All newborn babies in the U.S. are screened for PKU with a blood test. If you adopt a child from another country, they may need to be screened for PKU and other genetic diseases.
  • PKU is treated with a special diet. Newborn babies who test positive for PKU are placed on phenylalanine-free formula right away.
  • If you are a woman with PKU, your healthcare provider may advise genetic counseling. You can discuss with a counselor the risk of PKU in a future pregnancy.
  • New medicines are available to adults with PKU that lower levels of phenylalanine in the blood.

Next steps

Tips to help you get the most from a visit to your child’s healthcare provider:

  • Know the reason for the visit and what you want to happen.
  • Before your visit, write down questions you want answered.
  • At the visit, write down the name of a new diagnosis, and any new medicines, treatments, or tests. Also write down any new instructions your provider gives you for your child.
  • Know why a new medicine or treatment is prescribed and how it will help your child. Also know what the side effects are.
  • Ask if your child’s condition can be treated in other ways.
  • Know why a test or procedure is recommended and what the results could mean.
  • Know what to expect if your child does not take the medicine or have the test or procedure.
  • If your child has a follow-up appointment, write down the date, time, and purpose for that visit.
  • Know how you can contact your child’s provider after office hours. This is important if your child becomes ill and you have questions or need advice.