Determining factors of the cognitive outcome in early treated PKU: A study of 39 pediatric patients

Phenylketonuria (PKU) is a disorder of phenylalanine metabolism, characterized by a neurotoxic phenylalanine (Phe) accumulation, and treatable with a life-long Phe-restricted diet. Though early and continuously treated PKU (ETPKU) patients exhibit normal IQ, their cognitive outcome remains suboptimal. In this longitudinal study, we aimed at assessing the determinants of IQ subscales and quality of metabolic control in ETPKU children. We collected blood Phe levels, numbers of blood samples for Phe determination, parents' socio-professional categories and school achievement data of 39 classical and moderate ETPKU patients who underwent two cognitive evaluations performed by the same neuropsychologist (at 6.5 and 10y of mean age). We then sought to evaluate the determinants of 1) the changes in their IQ between the two testings (delta IQ) and 2) the quality of metabolic control (evaluated by the median Phe levels during the year before the second test) with multivariate regression analysis. Though in the normal range, mean total IQ slightly decreased between the two evaluations, and we observed a better verbal than performance outcome. Modeling the determining factors of the delta IQ, we found a significant influence of the number of blood samples (β = 0.46, 95%CI = 0.13 to 0.79, p < 0.01) and the moderate type of PKU (β = 12.40, 95%CI = 3.69 to 21.11, p < 0.01) on verbal outcome. We failed to find any determining factors that would statistically influence metabolic control. In conclusion, ETPKU cognitive outcome is influenced by a network of metabolic and environmental factors, which is not reflected by the sole metabolic control.