Phenylketonuria

Prevalence and causes

Phenylketonuria is caused by an inborn error of amino acid metabolism which affects approximately 1 in 12 000 people. This condition is inherited in an autosomal recessive manner and often causes severe mental retardation if it remains untreated. Affected subjects have fair hair, fair skin, and blue eyes because of the lack of the skin pigment precursor tyrosine. This metabolic disorder is caused by a deficiency of the hepatic enzyme, phenylalanine hydroxylase, which causes hyperphenylalaninaemia and phenylketonuria. Phenotypes such as absent (PH0) or deficient (PH-) phenylalanine hydroxylase activity, absent (DHPR0) or deficient (DHPR-) pteridin reductase activity, and deficient (BH 2-) biopterine synthesis are known to be associated with the condition.(52) Phenylketonuria is known to show considerable genotypic heterogeneity. The human phenylalanine hydroxylase gene has been located in the long arm of chromosome 12. In the United Kingdom the condition may be detected by routine screening of neonates by using a simple blood test called the Guthrie test. A corrective diet containing low phenylalanine used during the developmental period prevents mental retardation and behavioural problems in later life. However, even diet-treated phenylketonuria subjects as a group show lower than average IQ.

Clinical features

Non-specific neurological signs such as stooped posture, rigid broad-based gait, muscular hypertonicity, hyper-reflexia, tremor, stereotyped motor movements, digital mannerisms such as flicking, twiddling, pill-rolling movements of fingers, and habitual manipulation of objects such as wheels of toys have been reported in phenylketonuric subjects. In the older literature behavioural problems such as hyperactivity, irritability, episodes of screaming, noisiness, and uncontrollable temper tantrums have been described in subjects with phenylketonuria. Other behaviours may include short attention span, distractibility, low tolerance of frustration, perseveration, echolalia, and abnormal locomotor activities. (53>

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