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Phenylketonuria Causes and Risks:
PKU is inherited as an autosomal recessive trait. PKU occurs about once out of 16,000 births in the United States and is distributed equally between the sexes. Because PKU can be easily detected by a simple blood test and it is a treatable disease, most states require a screening test for all newborns. The genetically determined abnormality in phenylketonuria is a missing enzyme , phenylalanine hydroxylase.
Phenylalanine is one of the eight essential amino acids found in protein-containing foods. In PKU, phenylalanine cannot be used in a normal fashion because of the missing enzyme. Subsequently, high levels of phenylalanine and two closely related phenylalanine derivatives develop. These compounds are toxic to the central nervous system and cause brain damage.
Damage to the brain causes marked mental retardation by the end of the first year of life. Older children may develop movement disorders ( athetosis ), rocking, and hyperactivity . Because phenylalanine is involved indirectly in the production of melanin , the pigment responsible for skin and hair color, children with phenylketonuria often have lighter complexions than their unaffected siblings (PKU children often have blond hair and blue eyed). These children may have an odor described as mouse-like that results from the accumulation of phenylacetic acid. This odor may be detected on their breath, skin, and urine.
Prevention:
Genetic counseling is recommended for prospective parents with a family history of phenylketonuria. The carrier state for PKU can be detected by enzyme assays and PKU can be diagnosed prenatally. Screening immediately after birth is done routinely in all U.S. hospitals.
It is imperative that mothers with PKU who becomes pregnant adhere closely to the special low-phenylalanine diet-since accumulation of phenylalanine will damage the unborn baby even if the baby has not inherited the abnormality.
Symptoms:
Signs and Tests:
Many children with PKU have a light complexion, blond hair, and blue eyes.
Tests include: - enzyme assay to detect the carrier state
- chorionic villus sample to detect fetal PKU
- PKU screening (a heel-stick blood sample from the infant to screen for PKU; mandatory in most states in the U.S.)
Treatment:
Treatment includes a diet that is very low in, or free of, phenylalanine ; particularly when the child is growing. Strict compliance to the diet is necessary to reduce or prevent mental retardation . This requires close supervision by a registered dietitian or physician, and cooperation of the parent(s) and child.
Phenylalanine occurs in significant amounts in milk, eggs, and other common foods. Nutrasweet (aspartame) also contains phenylalanine, and products containing aspartame should be avoided for children with this disorder. 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 .
Adult women who have PKU and who plan to become pregnant should also adhere to a strict low-phenylalanine diet both before becoming pregnant and throughout the pregnancy.
Prognosis:
The outcome is expected to be very good if dietary treatment is followed closely beginning immediately after the child's birth. If treatment is started after 3 years (or if there is no treatment), brain damage is inevitable.
Complications:
Severe mental retardation occurs if the disorder is untreated.
Call Your Healthcare 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.
Blood is routinely drawn from newborn infants for testing. Blood is obtained by "heel stick" and collected on a special blotter paper. Routine testing includes phenylketonuria and blood type. Many hospitals include other tests such as thyroid function, hemoglobin S (sickle cell disease), or may test for other blood disorders (hemoglobinopathies). Testing can be tailored to the local population, taking into account race and ethnic background in determining what routine testing should be done.
Blood is routinely drawn from newborn infants for testing. The three circles on the special blotter paper are smears of blood. Patient information and identification is included on the paper, and is sent to the state testing laboratory. Results are available within a few days.
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