Eight early treated phenylketonuric children, all over 3 years old, and their six siblings were evaluated with the Stanford-Binet Intelligence Scale and the Illinois Test of Psycholinguistic Abilities (ITPA). Intelligence quotient scores and ITPA total and subtest scores were analyzed with the Mann-Whitney "U" test. In contrast to some reports, these early treated children did not differ significantly from their siblings on either the Binet IQ or the ITPA total score. There was a significant difference found on one ITPA subtest (vocal encoding) which may be due to small sample size. The ITPA profiles from repeated testing in a small number of cases were not stable, and observed fluctuations did not appear related to any observable changes in intellectual or academic functioning.
O'Grady DJ, Berry HK, Sutherland BS. Cognitive Development in Early Treated Phenylketonuria. Am J Dis Child. 1971;121(1):20-23. doi:10.1001/archpedi.1971.02100120056005