Loss of intellectual function in children with phenylketonuria after relaxation of dietary phenylalanine restriction.

Fourteen patients with classic phenylketonuria (PKU) were treated with a phenylalanine restricted diet from early infancy. All had satisfactory dietary control, with serum phenylalanine concentrations ranging between 2 to 5 mg/dL. Dietary restriction was discontinued in all these children between ages 5 and 6 years, and a free diet allowed. Developmental testing was performed using the Cattell Infant Intelligence Scales (1 to 2 years), Stanford-Binet Intelligence Scale (2 to 4 years), Wechsler Intelligence Scale for Children (WISC) and the revised version (WISC-R) (less than 5 years). Mean IQ for the group (Stanford-Binet and WISC) at termination of dietary therapy was 104 +/- 13. Four to 7 years after discontinuation of dietary therapy, mean IQ for the group was 90 +/- 13. The severity correlated, to some degree, with duration of unrestricted diet, but not with initial serum phenylalanine concentrations, age at initiation of therapy, or IQ at time diet was discontinued. Several children are experiencing difficulties, both attentional and academic, in school. Two children have had a change in the EEG from normal to abnormal. Neurologic testing performed after 4 to 7 years off diet demonstrated deficits in visual-motor integration or cognitive problem-solving in most children. The mean developmental age for the group for perceptual-motor integration was 1.2 years below the mean chronologic age of the group. This deterioration in intellectual function suggests that discontinuation of the phenylalanine-restricted diet is hazardous for some children with classic phenylketonuria.