Intelligence in mild atypical phenylketonuria.

In 82 children with mild phenylketonuria (PKU) (blood phenylalanine (Phe) concentrations consistently below 900 mumol/l throughout follow up) the relationship between intelligence at age 4 (IQ by Stanford-Binet) and average blood Phe concentrations from birth to 4 years was examined. Of the 82 children 24 had received no treatment. In the group as a whole, and in the 24 untreated subjects alone, mean IQs were significantly below population norms, with deficits of approximately 4.5 points and 9 points respectively. After allowing for social class IQ fell progressively by approximately 6 points for each 100 mumol/l rise in mean Phe concentrations in both the treated and untreated subjects. This relationship resembled that previously reported in early treated children with more severe forms of PKU, except that the scale of the relationship was even greater. We conclude that all children whose blood Phe concentrations reach 400 mumol/l or above should receive a low Phe diet, at least during the preschool years, and that the aim should be to control blood Phe levels below 400 mumol/l throughout early childhood in all forms of PKU.