Objective: To evaluate the intellectual development of patients with phenylketonuria (PKU) or hyperphenylalaninaemia (HPA).
Design: Longitudinal follow-up study.
Setting: University Children's Hospitals of Amsterdam, Groningen, Nijmegen, and Rotterdam.
Methods: In September 1974 a nationwide neonatal screening for PKU started in the Netherlands. We obtained data on the intellectual development of 116 patients (101 classical PKU, 15 HPA), all detected and treated in the first 15 years of this screening. The children were tested at 6 ages with different instruments for the assessment of intellectual functions, and the scores were compared with the test norms. Performance and verbal capacities were determined as well as possible sex-related differences.
Results: Our data show that the levels of intellectual functioning of PKU and HPA patients do not differ from test norms until the age of 8 yr 6 months. In the older PKU patients there appears to be a trend towards lower IQ scores. We found no sex differences in the mental functioning of PKU patients.
Conclusion: Continuation of this study is necessary in order to investigate possible negative effects of PKU on cognitive functioning, especially in the older age groups.