Practitioner's Manual Phenylketonuria (PKU) Phenylketonuria, or PKU, is caused by a recessively inherited enzyme defect in which the body cannot properly use the amino acid phenylalanine. All other metabolic processes are intact but phenylalanine, which comes from all dietary protein, accumulates in the blood. Excess phenylalanine cannot be converted to tyrosine due to a lack of phenylalanine hydroxylase, the enzyme that catalyzes the conversion of phenylalanine to tyrosine. Phenylalanine accumulates in the body and causes damage. Overall, PKU occurs in about 1 in 10,000 to 1:25,000 U.S. live births. Clinical Features With appropriate treatment, the risk of any of these conditions is substantially reduced. - Classical phenylketonuria is a disorder in which the blood phenylalanine, or phe, rises above 20 mg/dL on a normal diet (normal blood phe is less than 2.0 mg/dL). Without treatment, nearly all affected individuals develop severe mental retardation. Other symptoms include severe mental deficiency, microcephaly, eczematous or oily skin, cerebral palsy, convulsions, dysphasia, hyperactivity with purposeless movements, autistic-like behavior, and an abnormal EEG. In infants, vomiting may mimic pyloric stenosis. The skin and hair are usually fair, the eyes may be blue and a "mousey" odor of the baby's urine is frequent. The smell arises from phenylacetic acid. Hyperphenylalaninemia refers to any consistent elevation of phe levels, including classical PKU. If cases of classical PKU are excluded, this includes blood phe levels less than 20 mg/dL. These may be caused by liver damage, transient tyrosinemia of prematurity, mutation of the phenylalanine hydroxylase gene, disorders of cofactor synthesis or regeneration, or maternal PKU. In these cases, mental retardation may or may not be present. Blood levels may remain elevated throughout life or may gradually fall towards normal. In infancy, these patients can mimic the severe PKU condition, and even in mild cases there seems to be an increased risk of the maternal PKU syndrome, as described below.
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