Neurocognitive functioning remained unchanged in adults with phenylketonuria: Report of a 15-year follow-up.

Aim: The long-term prognosis of treated phenylketonuria (PKU) is a controversial issue. A controlled long-term study was performed to assess the neuropsychological outcome in adult patients with early treated PKU.

Methods: We investigated 27 patients aged 34-55 years (median age: 47 years) and 19 healthy controls, matched for age and socio-economic status. Patients and controls were assessed for their full-scale IQ (FSIQ), attention and information-processing abilities. Magnetic resonance imaging (MRI) of the brain was performed in all patients. Neuropsychological assessments and MRI were repeated at a 15-year follow-up.

Results: In the entire interval of 15 years, cognitive performance of patients and controls remained unchanged. Older adult patients showed poorer cognitive performance at all assessment times compared to young adult patients and controls. Cognitive performance showed no correlation to MRI results. FSIQ, however, was significantly correlated with blood phenylalanine levels in the adolescent years of adult patients older than 47 years.

Conclusion: Cognitive performance in adult patients with early treated PKU does not seem to deteriorate in a 15 years interval. Neuropsychological assessment revealed constant modest neurocognitive impairment in patients older than 47 years. This seems to refer to an early relaxation of diet that was recommended when the older patients were adolescents.