Intellectual and school performance in adolescents with phenylketonuria according to their dietary compliance. The Czech-Slovak Collaborative Study.

The intellectual development and school performance of 81 adolescents with hyperphenylalaninaemia type I between the ages 15 and 19 years were evaluated in a retrospective study. All adolescents were born between 1975-1979. In spite of the continuous changes and improvement of dietary treatment, there were no differences in the dietary approach in all phenylketonuria centres involved. A significant negative correlation between IQ of adolescents with phenylketonuria and the onset of dietary treatment was observed (r = -0.37, P < 0.01). The best school performance was found in the group of adolescents with dietary therapy which was introduced in the first weeks of life. There was no correlation between IQ of the adolescents and their phenylalanine level in the neonatal period. There was a significant negative correlation between IQ of adolescents and the average level of blood phenylalanine in the first 5 years of life (r = -0.42, P < 0.01), but this correlation was less significant later on and was not present after the age of 14 years. Dietary compliance was much more difficult to achieve in adolescence than in childhood.