A.D.A.M. Medical Encyclopedia.
Last reviewed: June 17, 2011.
Phenylketonuria (PKU) is a rare condition in which a baby is born without the ability to properly break down an amino acid called phenylalanine.
Causes, incidence, and risk factors
Phenylketonuria (PKU) is inherited, which means it is passed down through families. Both parents must pass on the defective gene in order for a baby to have the condition. This is called an autosomal recessive trait.
Babies with PKU are missing an enzyme called phenylalanine hydroxylase, which is needed to break down an essential amino acid called phenylalanine. The substance is found in foods that contain protein.
Without the enzyme, levels of phenylalanine and two closely-related substances build up in the body. These substances are harmful to the central nervous system and cause brain damage.
Phenylalanine plays a role in the body’s production of melanin, the pigment responsible for skin and hair color. Therefore, infants with the condition often have lighter skin, hair, and eyes than brothers or sisters without the disease.
Other symptoms may include:
- Delayed mental and social skills
- Head size significantly below normal
- Jerking movements of the arms or legs
- Intellectual disability
- Skin rashes
- Unusual positioning of hands
If the condition is untreated or foods containing phenylalanine are not avoided, a “mousy” or “musty” odor may be detected on the breath and skin and in urine. The unusual odor is due to a build up of phenylalanine substances in the body.
Signs and tests
PKU can be easily detected with a simple blood test. All states in the US require a PKU screening test for all newborns as part of the newborn screening panel. The test is generally done by taking a few drops of blood from the baby before the baby leaves the hospital.
If the initial screening test is positive, further blood and urine tests are required to confirm the diagnosis.
PKU is a treatable disease. Treatment involves a diet that is extremely low in phenylalanine, particularly when the child is growing. The diet must be strictly followed. This requires close supervision by a registered dietitian or doctor, and cooperation of the parent and child. Those who continue the diet into adulthood have better physical and mental health. “Diet for life” has become the standard recommended by most experts. This is especially important before conception and throughout pregnancy.
Phenylalanine occurs in significant amounts in milk, eggs, and other common foods. The artificial sweetener NutraSweet (aspartame) also contains phenylalanine. Any products containing aspartame should be avoided.
A special infant formula called Lofenalac is made for infants with PKU. It can be used throughout life as a protein source that is extremely low in phenylalanine and balanced for the remaining essential amino acids.
Taking supplements such as fish oil to replace the long chain fatty acids missing from a standard phenylalanine-free diet may help improve neurologic development, including fine motor coordination. Other specific supplements, such as iron or carnitine, may be needed.
The outcome is expected to be very good if the diet is closely followed, starting shortly after the child’s birth. If treatment is delayed or the condition remains untreated, brain damage will occur. School functioning may be mildly impaired.
If proteins containing phenylalanine are not avoided, PKU can lead to intellectual disability by the end of the first year of life.
Severe intellectual disability occurs if the disorder is untreated. ADHD (attention-deficit hyperactivity disorder) appears to be the most common problem seen in those who do not stick to a very low-phenylalanine diet.
Calling your health care provider
Call your health care provider if your infant has not been tested for PKU. This is particularly important if anyone in your family has the disorder.
An enzyme assay can determine if parents carry the gene for PKU. Chorionic villus sampling can be done during pregnancy to screen the unborn baby for PKU.
It is very important that women with PKU closely follow a strict low-phenylalanine diet both before becoming pregnant and throughout the pregnancy, since build-up of this substance will damage the developing baby even if the child has not inherited the defective gene.
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- Review Date: 6/17/2011.
Reviewed by: Kimberly G Lee, MD, MSc, IBCLC, Associate Professor of Pediatrics, Division of Neonatology, Medical University of South Carolina, Charleston, SC. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.