Outcome of treatment in young adults with phenylketonuria detected by routine neonatal screening between 1964 and 1971.

Intellectual status at 18 years is reported in 192 patients with phenylketonuria born in the UK between 1964 and 1971, together with their school progress. Mean IQs expressed as standard deviation scores (IQ-SDS) were significantly below estimated population norms, and showed a small decrease from 14 to 18 years of age (mean IQ-SDS -1.06 +/- 1.3 at 14 years and -1.28 +/- 1.07 at 18 years, t = 5.7, p < 0.0001). At 18 years, 27% of subjects had IQs over 2 SDs below the estimated population mean. IQ-SDS at 18 years, although significantly related to average phenylalanine control both between birth and 14 years, and between 14 and 18 years, was not independently related to either variable after allowing for IQ-SDS at 14 years. We conclude that general ability in young adults with early treated phenylketonuria, although reduced in comparison with their peers and closely related to phenylalanine control in early childhood, is not directly influenced by phenylalanine control in the four years preceding the 18th birthday. The apparent fall in IQ-SDS between 14 and 18 years may be due to methodological problems in the analysis of longitudinal IQ data without a control group, rather than providing evidence of intellectual decline.