Phenylketonuria (PKU)

Phenylketonuria (fen-ul-ke-toe-NU-re-uh), also called PKU, is a rare inherited disorder that causes an amino acid called phenylalanine to build up in your body. PKU is caused by a defect in the gene that helps create the enzyme needed to break down phenylalanine.

Without the enzyme necessary to process phenylalanine, a dangerous buildup can develop when a person with PKU eats foods that are high in protein. This can eventually lead to serious health problems.

For the rest of their lives, people with PKU — babies, children and adults — need to follow a diet that limits phenylalanine, which is found mostly in foods that contain protein.

Babies in the United States and many other countries are screened for PKU soon after birth. Recognizing PKU right away can help prevent major health problems.

Symptoms Causes Risk factors Complications Prevention

Newborns with phenylketonuria initially don't have any symptoms. Without treatment, babies usually develop signs of PKU within a few months.

PKU symptoms can be mild or severe and may include:

  • Intellectual disability (formerly called mental retardation)
  • Delayed development
  • Behavioral, emotional and social problems
  • Psychiatric disorders
  • Neurological problems that may include seizures
  • Hyperactivity
  • Poor bone strength
  • Skin rashes (eczema)
  • A musty odor in the child's breath, skin or urine, caused by too much phenylalanine in the body
  • Fair skin and blue eyes, because phenylalanine cannot transform into melanin — the pigment responsible for hair and skin tone
  • Abnormally small head (microcephaly)

Severity varies

The most severe form of the disorder is called classic PKU. Children and adults with less severe forms of PKU, in which the faulty enzyme retains some function, have a smaller risk of significant brain damage. But most children with these forms of the disorder still require a special diet to prevent intellectual disability and other complications.

Pregnancy and PKU

A woman who has PKU and becomes pregnant is at risk of another form of the condition called maternal PKU. If a woman doesn't follow the special PKU diet before and during pregnancy, blood phenylalanine levels can become high and harm the developing fetus or cause a miscarriage. Even women with mild PKU may place their unborn children at risk by not following the PKU diet.

Babies born to mothers with high phenylalanine levels don't often inherit PKU. However, they can have serious consequences if the level of phenylalanine is high in the mother's blood. Complications at birth may include:

  • Intellectual disability
  • Low birth weight
  • Delayed development
  • Abnormally small head
  • Heart defects
  • Behavioral problems

When to see a doctor

Seek medical advice in these situations:

  • Newborns. If routine newborn screening tests show that your baby may have PKU, your child's doctor will want to start dietary treatment right away to prevent long-term problems.
  • Women of childbearing years. It's especially important for women with a history of PKU to see a doctor and maintain the PKU diet before becoming pregnant and during pregnancy to reduce the risk of high blood phenylalanine levels harming their unborn babies.
  • Adults. Adults with PKU continue to receive care across their life span. Older adults with PKU who may have stopped the PKU diet in their teens may benefit from a visit with their doctors. Returning to the diet may improve mental functioning and behavior and slow damage to the central nervous system in adults with high phenylalanine levels.

A genetic mutation causes phenylketonuria. In a person with PKU, this defective gene causes a lack of or deficiency of the enzyme that's needed to process an amino acid called phenylalanine. A dangerous buildup of phenylalanine can develop when a person with PKU eats foods that are high in protein, such as milk, cheese, nuts or meat.

For a child to inherit PKU, both the mother and father must have and pass on the defective gene. This pattern of inheritance is called autosomal recessive. It's possible for a parent to have the defective gene, but not have the disease. This is called being a carrier. If only one parent has the PKU gene, there's no risk of passing PKU to a child, but it is possible for the child to be a carrier.

Most often, PKU is passed to children by two parents who are carriers of the disorder, but don't know it.

Risk factors for inheriting phenylketonuria include:

  • Having two parents with a defective PKU gene. Both parents must pass along a copy of the mutated PKU gene for their child to develop the condition.
  • Being of certain ethnic decent. The gene defect varies by ethnic groups and is less common in African-Americans than in other ethnic groups.

Children of mothers with PKU who didn't follow the PKU diet during pregnancy are at risk of birth defects due to the high levels of phenylalanine in their mothers' blood.

Untreated phenylketonuria can lead to:

  • Irreversible brain damage and marked intellectual disability within the first few months of life
  • Neurological problems such as seizures and tremors
  • Abnormally small head (microcephaly)
  • Behavioral, emotional and social problems in older children and adults
  • Significant health and developmental problems

If you're a woman with phenylketonuria who's considering having a baby:

  • Follow a low-phenylalanine diet. Women with PKU can prevent birth defects by sticking to (or returning to) a low-phenylalanine diet before becoming pregnant. If you're a woman with PKU, talk to your doctor before you start trying to conceive.
  • Consider genetic counseling. If you have a history of PKU, a close relative with PKU or a child with PKU, you may also benefit from preconception genetic counseling. A doctor who specializes in medical genetics can help you better understand how PKU is passed through your family tree. He or she can also help you determine your risk of having a child with PKU and assist with family planning.
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