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Phenylketonuria (PKU)

Approximately one of every 15,000 babies in the United States is born with PKU. One of the most common metabolic disorders in children is a genetic disorder known as phenylketonuria (fen-ul-ke-to-NU-re-uh) or PKU, in which the body lacks an enzyme needed to metabolize excess amounts of an amino acid called phenylalanine. This amino acid occurs in significant amounts in all proteins, including breast milk, standard baby formulas, eggs, meat, fish and beans. Left untreated, PKU can lead to a buildup of this amino acid in the blood stream and eventually to irreversible brain damage and marked mental retardation within the first few months of life.

However, mental retardation can be totally prevented — if these babies begin a carefully regulated low-phenylalanine diet just after birth. For that reason, early detection and diagnosis through newborn screening is critical.

Symptoms

Newborns rarely shown signs of PKU, although some infants with the disorder may be lethargic and have difficulty feeding. Infants with PKU may have lighter-colored hair, skin, and eyes than other family members, as well as a rash similar to infantile eczema. Older children may develop reflex problems, seizures, extreme hyperactivity, and psychoses. In addition, children with PKU may have a distinct body odor due to the elimination of excess phenylactic acid in urine and sweat.

Diagnosis

Almost all cases of PKU are found through routine newborn screening. All 50 states in the United States require that newborns be tested for PKU.

A day or two after your baby's birth, a small amount of blood is tested for metabolic disorders, including PKU. If the blood sample indicates high levels of phenylalanine, your doctor may recommend diet changes and order additional tests to confirm the diagnosis, including more blood tests and urine tests. You and your baby may also undergo genetic testing to identify mutations in the PAH gene.

If you have PKU or a family history of PKU, your doctor may also recommend pre-natal screening tests. You can have a simple blood to determine if you are a PKU carrier. Your baby can also be tested in utero using amniocentesis or chorionic villus sampling (CVS). During this procedure, the doctor inserts a needle through the mother's lower abdomen, or passes a catheter up through her cervix into the chorionic villi, which forms the lining of the placenta. A small sample of cells is then removed to be tested for genetic disorders, including PKU. Your doctor and a genetics counselor can help you decide if these tests are right for you.

Treatment

Children with PKU are treated with a rigorous, regimented, life-long diet that is low in protein, with very limited levels of phenylalanine. This special diet can prevent mental retardation if it's started just after birth. The amount of phenylalanine allowed in the PKU diet differs for each individual, depending on age, height, weight and a person's ability to metabolize this amino acid.

The major source of calories and nutrients in the PKU diet is actually a synthetic formula that is high in protein and essential nutrients but contains little or no phenylalanine. As the child grows, parents introduce solid foods with low levels of phenylalanine on the same schedule used for normal infants. But older children and adults still drink several glasses of formula each day, as directed by a doctor or dietitian. The formula for older kids and adults is not the same as the one used for infants, but it works on the same principle. It acts as a nutritional substitute for forbidden foods and is continued for life.

Prevention

Women with PKU can prevent their children from developing serious birth defects by sticking to — or returning to — a low-phenylalanine diet during pregnancy. Even women with mild cases of PKU may place their unborn children at risk by not following the special PKU diet. If you're a woman with a history of PKU, talk to your doctor before you start trying to conceive.

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