Newborns with phenylketonuria initially don't have any symptoms. Without treatment, though, babies usually develop signs of PKU within a few months. Phenylketonuria symptoms can be mild or severe and may include:

  • Mental retardation
  • Behavioral or social problems
  • Seizures, tremors or jerking movements in the arms and legs
  • Hyperactivity
  • Stunted growth
  • Skin rashes (eczema)
  • Small head size (microcephaly)
  • 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

Varying severity

The most severe form of the disorder is known as classic PKU. Children with untreated classic PKU usually develop obvious, permanent mental retardation.

Less severe forms of PKU — sometimes called mild or moderate PKU — have a smaller risk of significant brain damage, but most children with these forms of the disorder still require a special diet to prevent mental retardation 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. In the past, people with PKU may have been told it was OK to stop following a low-phenylalanine diet once they'd reached their teen years. But, doctors now know that if a woman doesn't follow the diet prior to and during pregnancy, blood phenylalanine levels can become out of range and harm the developing fetus. Because of this, and other reasons, doctors recommend that anyone with PKU follow the low-phenylalanine diet for life.

Although babies born to mothers with high phenylalanine levels may have complications at birth, most don't actually inherit PKU and won't need to follow a PKU diet after birth. However, these babies have a high risk of being born with:

  • Mental retardation
  • Abnormally small head (microcephaly)
  • Heart defects
  • Low birth weights
  • 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. Newborns with PKU are given a special formula. Your doctor may refer you to a specialist in genetics and a registered dietitian who can help you learn about the PKU diet.
  • Adults. Older adults with PKU who stopped the PKU diet in their teens also may benefit from a visit with their doctors. Although adolescents with PKU were allowed to go "off-diet" in the past, doctors now recommend that anyone with PKU continue the special diet for life. Returning to the diet may improve mental functioning and behavior and slow damage to the central nervous system in adults with high phenylalanine levels.
  • Women. It's especially important for women with a history of PKU to see a doctor and return to the PKU diet before becoming pregnant. Even mild cases of maternal PKU can pose a serious risk to unborn babies. Pregnant women with PKU who aren't following the special diet also have a higher risk of miscarriage.
Nov. 17, 2011

You Are ... The Campaign for Mayo Clinic

Mayo Clinic is a not-for-profit organization. Make a difference today.