Development of intelligence in early treated phenylketonuria

Designs, analyses and results of longitudinal studies of intelligence of pa tients treated early for phenylketonuria (PKU) are reviewed. All studies co nverge on the conclusion that after the age of 10 years, IQ development is stable for different degrees of dietary relaxation. On average, for each 30 0 mu mol/l increase in blood phenylalanine (Phe) levels pre-school, IQ decr eases by about half a standard deviation. Children with Phe levels below 40 0 mu mol/l in early and middle childhood had the best outcomes which were n ear normal. PKU seems to suppress the global level of IQ without impairment of domain-specific competencies. For historical reasons there is no resear ch on IQ development of early treated patients in middle or late adulthood, and it remains unclear whether older age groups might carry new risks. Conclusion It is argued that control group designs, meta-analysis, and inte rdisciplinary studies combining psychology, neurology and neuropathology co uld increase the understanding of phenylketonuria as well as the scientific basis of its treatment.