PKU causes severe intellectual impairment. In classical PKU
developmental delay is apparent within the first year of life and
progresses to severe mental retardation (IQ < 50). Examination
shows limb spasticity, tremor and microcephaly. A seizure disorder is frequently present and EEG abnormalities are common.
Other findings may include hypopigmentation of the hair, skin
and iris due to reduced melanin synthesis. Parkinsonian features
and gait abnormalities are also often observed in the untreated
individual. Abnormalities of behaviour are very common
including hyperactivity, aggression, anxiety and social withdrawal. The natural phenotype is rarely seen now due to widespread newborn screening for this condition. However, PKU
should be considered as a possible diagnosis particularly in an
individual born