Patient education: Phenylketonuria (The Basics)

What is phenylketonuria? — Phenylketonuria, or "PKU," is a rare genetic disorder. It causes a chemical called "phenylalanine" to build up in the blood. Phenylalanine is mostly found in foods that contain protein. These include meat, poultry, seafood, eggs, dairy, nuts, seeds, legumes (like soy), and whole grains.

Normally, the body breaks down phenylalanine to use it. But when a person has PKU, their body cannot do this.

Without treatment, PKU can cause serious problems with the brain and development. For this reason, all newborn babies in the US are tested for PKU soon after birth. This way, treatment can start right away if needed.

What causes PKU? — PKU is caused by a change in a specific gene. This gene helps make the substance that normally breaks down phenylalanine from foods. If the gene is abnormal, the body cannot properly break down phenylalanine.

For a person to have PKU, they need to get the abnormal gene from both of their parents.

What are the symptoms of PKU? — A baby born with PKU does not have symptoms right away. If treatment is not started, symptoms will start after the baby starts getting phenylalanine through their diet. Phenylalanine is found in breast milk and baby formula.

Symptoms of untreated PKU can include:

●Intellectual disability – This can make it harder for a person to learn, communicate, and do certain tasks. Once these brain changes have happened, they cannot be reversed.

●Seizures

●A head that is smaller than normal

●Skin problems, like rash or color changes

●A musty smell of the body or urine

People who get early treatment for PKU can still have these symptoms. But they are usually much less severe.

Is there a test for PKU? — Yes. A blood test can check for PKU. This test is done on all babies born in the US and many other countries. It is part of routine "screening." This means that the test is done on all newborns, even if they appear healthy.

The screening test checks the level of phenylalanine in the blood. If it is high, the doctor can do other tests to confirm PKU. These might include blood and urine tests.

How is PKU treated? — The main treatment is making specific changes to the diet. This can treat many of the symptoms of PKU. It cannot reverse brain changes that have already happened, but it can help prevent more problems with brain development. The diet must be followed for life.

The goal is to restrict how much phenylalanine the person takes in, but still give them the nutrients they need:

●Babies need to drink a special formula that has protein but no phenylalanine. If your baby has PKU and you want to breastfeed, you might be able to do this, but you will need to work closely with a specialist. That's because all breast milk contains some phenylalanine. Your baby might be able to breastfeed some of the time, but they will also need to drink the special formula.

●Older babies, children, and adults with PKU need to follow a diet that is very low in protein (table 1). A doctor or dietitian (food expert) can help you understand which foods to eat and which to avoid. It's important to plan meals carefully to make sure that the person gets enough of the nutrients they need.

●People with PKU also need to take a protein substitute. This is called an "amino acid mixture." It is a liquid supplement that provides the protein the body needs without phenylalanine. Protein is important for growth and development.

●Some people need other vitamin supplements, too.

Some people with PKU are also treated with medicine. But in most cases, they still need to follow a special diet.

People with PKU need to see their medical team regularly. In addition to their primary care doctor, this includes a specialist in diseases like PKU (a genetic or "metabolic disease" specialist). They will likely also see a dietitian, a neurologist, and a mental health expert.

At each visit, the doctor will do a blood test to check the level of phenylalanine. They will also review the diet plan and what the person eats each day.

What if I want to get pregnant? — If you have PKU and want to get pregnant, talk to your doctor before you start trying. You will need to be extra careful about your diet before and during pregnancy. That's because if you have too much phenylalanine in your blood, it can harm your fetus (developing baby). This is true even if the baby does not have PKU.

You might also want to talk to your doctor about genetic testing. This can show if your partner is a "carrier." Being a carrier means that they have a copy of the abnormal gene. If you have PKU and your partner is a carrier, it is possible that your baby will have PKU. Genetic testing can also help if someone else in your family has PKU and you want to know if you are a carrier. Your doctor or a genetic counselor can help you understand this risk.

When should I call the doctor? — Call for advice if you or your child has:

●New or worsening problems with learning, communication, or behavior

●A new skin rash

●A seizure

●Trouble following the special diet

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