| Phenylketonuria (PKU) is a hereditary disease that, if left untreated, can cause mental retardation and seizures. Because early intervention with the use of a special formula and diet can prevent these problems, almost all states in the United States screen all newborns for this condition.
Phenylalanine is an essential amino acid that is obtained from proteins in the diet. When proteins containing phenylalanine are eaten, this amino acid is used to build other proteins. Whatever is taken in and not used is normally eliminated by the body. In persons with PKU, phenylalanine cannot be eliminated normally. In so-called classic PKU, the affected child is missing an enzyme known as phenylalanine hydroxylase. Other types of PKU involve deficiencies of other enzymes, in particular, tetrahydrobiopterin.
Classic PKU is an autosomal recessive disorder meaning the child must inherit the trait from both parents. The parents themselves do not have the disease. It occurs in about 1 out of 10,000 live births overall. It is more common in Caucasians than other races, being only about 1 in 50,000 live births among African-Americans. Deficiency of other enzymes is much less common, about 1 out of 100,000 live births. Although children with this disease appear normal at birth, the slow accumulation of excess phenylalanine in the blood will lead to mental retardation by the end of the first year of life if the disease is not diagnosed and treated. Other findings in untreated children are seizures and vomiting, and a paler skin, hair and eye color than unaffected siblings.
PKU disease is diagnosed through newborn screening in the United States by means of a blood test performed about 48 to 72 hours after birth. If the blood test shows an elevated level of phenylalanine, then a second test is performed to confirm it. If the confirmation test also shows an elevated phenylalanine level above 20 mg/dl, then the child will need a complete evaluation including a blood test to measure tyrosine (another amino acid) and cofactor tetrahydrobiopterin (an enzyme), and a urine test to measure certain breakdown products of phenylalanine. These tests will help distinguish classic PKU from other types. Because the newborn screening test is performed in the hospital sometimes before the child is 48 hours old, many states have a mandatory second screening at one to two weeks of age.
A variant of PKU is found in some persons who have a low amount of the enzyme needed to breakdown excess phenylalanine in the blood. These children will have elevated phenylalanine levels in the blood but they will not have levels about 20 mg/dl. This is known as benign hyperphenylalaninemia. A special diet is not usually necessary.
 |
| Once the diagnosis of classic PKU is made, the child should be started on a special formula that is low in phenylalanine. |
|
Once the diagnosis of classic PKU is made, the child should be started on a special formula that is low in phenylalanine. Because phenylalanine is an essential amino acid, the diet is designed to provide just enough of this amino acid for normal growth and development. Dietary control is absolutely necessary for normal brain development. Once the brain is fully developed, some specialists will also the child to relax the dietary restrictions after age 6 years. However, older patients who eat excessive amounts of phenylalanine will report decreased ability to concentrate and a decline in school performance if the diet is abandoned. For children with other types of PKU, dietary control alone is usually not successful and other therapies must be tried.
Regular consultation with a specialist in metabolic diseases (usually a physician who specializes in genetics) and a registered dietician is crucial to good control once the child starts solid foods. The geneticist can also counsel the parents on the risk of having another child with the disease as well. Development is tracked closely as are blood levels of phenylalanine.
Of special note is that pregnant women who have the disease must return to a low phenylalanine diet before and during pregnancy to prevent complications to the unborn child. If a woman with PKU does not control her diet beginning before conception, she has an increased risk of spontaneous abortion. Her unborn child is at increased risk for mental retardation, microcephaly (small head and brain), heart defects, and brain malformation (particularly absence of the corpus Collosum a structure in the brain that allows signals to be sent from the right side of the brain to the left and vice versa).
PKU Internet Resources:
- PKU News: news, diet related information, personal stories and support groups for children with phenylketonuria.
- The Tyler For Life Foundation - PKU: "a nonprofit voluntary organization whose mission is to improve the lives of babies by preventing mental retardation and death resulting from disorders detectable through newborn screening. A greater understanding and awareness of these tests and disorders will help to prevent unnecessary complications since most can be effectively treated when caught early." Includes comprehensive info and links to other sites that describe the illnesses included in most newborn screen programs.
|
