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Phenylketonuria in Children

Medically reviewed by Last updated on Oct 3, 2022.

What is phenylketonuria (PKU)?

PKU is a condition that prevents your child's body from breaking down phenylalanine. Phenylalanine is a substance the body uses to make other proteins that are needed for normal growth. Phenylalanine is found in many foods, such as meat, poultry, fish, eggs, milk, cheese, beans, nuts, and seeds. When phenylalanine is not broken down properly, it builds up in the body. This can cause brain damage and lead to serious growth and learning problems.

What causes PKU?

PKU is caused by a defect in the gene that makes the enzyme needed to break down phenylalanine. The enzyme that breaks down phenylalanine may be present only in small amounts, or it may be absent. A child inherits PKU when both parents have the defective gene.

What are the signs and symptoms of PKU?

A child with PKU may look normal and completely healthy for the first few months of life. Signs and symptoms may appear between 3 to 6 months of age. Your child may be less active and develop more slowly than other children. He or she may lose interest in the things around him or her. Your child may also have any of the following:

  • Learning, speech, or behavior problems
  • More irritable, fussy, or restless than normal
  • Musty odor of his or her breath, hair, skin, or urine
  • Light or pale skin
  • Short length or small head
  • Skin may be dry or have rashes
  • Vomiting, muscle stiffness, or seizures

How is PKU diagnosed?

  • Blood tests are usually done during your child's first days of life. A sample of your child's blood is taken and sent to a lab. This helps your child's pediatrician learn if your child is at risk for PKU or other genetic disorders. Other blood tests may be needed if the PKU screening is positive.
  • Urine tests help diagnose PKU and other disorders your child may have.
  • Genetic tests may be needed to check your child's genes. This test may also help your child's pediatrician decide on a treatment plan.

How is PKU treated?

  • A special diet is needed to keep the amount of phenylalanine in your child's body low. It is started as early as the first few days of life or a few weeks after birth. Your baby will need to drink formulas that have little or no phenylalanine. These formulas have the right amino acids, calories, vitamins, and minerals your child needs. Ask your child's healthcare provider for more information about these formulas.
  • Your child will need to eat foods low in phenylalanine for the rest of his or her life. Phenylalanine is found mostly in foods that contain protein. The amount of protein your child can have depends on your child's phenylalanine levels, age, weight, and other factors. Your child's dietitian will plan and adjust the amount of protein your child is allowed to eat.

Treatment options

The following list of medications are in some way related to or used in the treatment of this condition.

Call your local emergency number (911 in the US) if:

  • Your child has a seizure.

When should I call my child's pediatrician?

  • Your child cannot eat or drink.
  • Your child is vomiting everything he or she eats or drinks.
  • Your child becomes more irritable and fussy than usual.
  • You have problems feeding your child.
  • You have questions or concerns about your child's condition or care.

Care Agreement

You have the right to help plan your child's care. Learn about your child's health condition and how it may be treated. Discuss treatment options with your child's healthcare providers to decide what care you want for your child. The above information is an educational aid only. It is not intended as medical advice for individual conditions or treatments. Talk to your doctor, nurse or pharmacist before following any medical regimen to see if it is safe and effective for you.

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