Neuropsychological outcome of experimental manipulation of phenylalanine intake in treated phenylketonuria

Blood phenylalanine concentrations were experimentally increased for 3 months by means of a phenylalanine‐complemented amino acid supplement in a group of 16 children aged 10‐16 years with classical phenylketonuria who had been treated early and who had remained on the restricted diet. Average concentrations achieved during challenge were between 1000 and 1300 µmol/L. Psychological outcome was measured by a neuropsychological battery consisting of tests of verbal and spatial memory, attention and fine motor coordination. A triple‐blind, repeated measures, randomized, crossover design was adopted to control for practice and expectancy effects. Subjects were assessed at baseline and at the end of the first and second phenylalanine manipulation periods. Significant interactions (ANOVA) emerged as predicted for phenylalanine concentrations, but similar crossover effects were not found for any of the neuropsychological tests. The results suggested that medium‐term hyperphenylalaninaemia in treated PKU is not harmful to psychological functioning in older children and adolescents who have been continuously treated up to and beyond age 10 years, though the susceptibility of executive functions needs to be further researched. The findings add some weight to the idea that by late childhood the vulnerability of the nervous system to the neurotoxic influence of phenylalanine may be much reduced.