Correlation of age-specific phenylalanine levels with intellectual outcome in patients with phenylketonuria

Patients with treated phenylketonuria (PKU) can have subtle deficits in intellect, academic skills, and executive functioning. This study evaluates the relationship between intellectual outcome and concentration/variation in blood phenylalanine (Phe) during specific developmental periods (0–6 years, 7–12 years, >12 years) in our patients with PKU. Verbal comprehension, perceptual reasoning, and processing speed were used as measures of intelligence. Data were collected from 55 patients receiving treatment at the University of Utah Metabolic Clinic. Yearly median Phe levels increased and mean number of blood Phe samples decreased as patients aged. Yearly median blood Phe from 0–6 and 7–12 years were inversely associated with perceptual reasoning abilities using linear regression. Additionally, increased blood Phe concentration negatively impacted specific areas of verbal comprehension abilities for those 0–6 years of age (p = 0.001). Variation of Phe levels around the mean (assessed as standard deviation) in each patient was associated with diagnostic (highest pretreatment) Phe levels and yearly median Phe levels (p < 0.001 for both), but did not significantly impact intelligence in our group of patients. Frequent blood Phe monitoring from 7–12 years significantly reduced the probability of yearly median Phe exceeding 360 μM (p = 0.005). Our data show that compliance with treatment in patients with PKU affects both the concentration and variation of blood Phe levels, and may have a greater impact on verbal comprehension and perceptual reasoning skills during the first 12 years of life when compared the influence beyond 12 years.