Dynamics of hyperphenylalaninemia and intellectual outcome in teenagers with phenylketonuria

Insufficient treatment adherence after early childhood is frequently observed in patients with phenylketonuria. Assessment of these individuals' long-term metabolic control could enable early detection of the risk of intellectual deterioration resulting from high blood phenylalanine concentration. However, the predictive value of specific parameters related to individual dynamics of hyperphenylalaninemia is not clear. Here, we assessed the impact of blood phenylalanine fluctuations during the first 12 years of life on cognitive outcome in early and continuously treated teenagers with phenylketonuria. We have analyzed a total of 5141 results of blood phenylalanine measurements in 32 patients. The phenylalanine levels of these patients were usually acceptable during their early childhood, but the control of hyperphenylalaninemia worsened and the average treatment adherence dropped to 40% during the late primary school. Our analysis revealed a strong association between the Wechsler intelligence verbal scores and the mean of the yearly means of phenylalanine concentrations (r=-0.62). The correlations of IQ scores with median phenylalanine concentrations and the variability of blood phenylalanine levels gave weaker associations. The Wechsler verbal scores were also strongly correlated with the treatment adherence level during preschool and late primary school (r=0.61 and 0.72). The mean of the yearly means of blood phenylalanine concentrations appears to be a better predictor of cognitive outcome in children with phenylketonuria than other parameters related to phenylalanine fluctuations. The percentage of acceptable phenylalanine levels below 50-60% should be regarded as a "red flag" due to the risk of intellectual deterioration in patients.