Neurocognitive functioning in adults with phenylketonuria: Report of a 10-year follow-up

The long-term prognosis of early treated phenylketonuria (PKU) is still under discussion. Aim of this controlled long-term study was to assess the neurological and neuropsychological outcome in adult patients with early-treated PKU. We investigated 35 patients with early-treated classical PKU aged 29 to 51 years (mean age 41 years) and 18 healthy controls matched for age and socioeconomic status. Patients and controls were assessed for their intelligence quotient (IQ), 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 five-year and a ten-year follow-up. In the entire interval IQ, information processing and attention of patients and controls remained constant. At both follow-up assessment times the IQ scores were significantly lower in patients compared to controls. Older adult patients (> 42 years) showed poorer information processing and attention at both assessment times compared to young adult patients (< 42 years) and controls. IQ, information processing and attention showed no correlation to imaging results. IQ, however, was significantly correlated to blood phenylalanine (Phe) levels in patients´ childhood and adolescence, and Phe levels had been higher in the adolescent years of older adult patients. Cognitive performance in adult patients with early-treated PKU does not seem to deteriorate in a ten-year interval. Neuropsychological assessment in adults with PKU revealed neurocognitive impairment particularly in older adult patients. This seems to refer to an early relaxation of diet that was recommended when the older patients were adolescents. Results indicate a benefit of dietary control during adolescence in PKU.