Neurocognitive evidence for revision of treatment targets and guidelines for phenylketonuria
Rianne Jahja1, Stephan C J Huijbregts2, Leo M J de Sonneville2
1Division of Metabolic Diseases, University Medical Center Groningen, Beatrix Children's Hospital, University of Groningen, Groningen, The Netherlands.
Objectives:
To compare the neurocognitive outcomes of patients with phenylketonuria (PKU) to determine whether decreasing phenylalanine (Phe) levels to <240 is preferable to the use of 360 μmol/L as an upper-target Phe level. An additional aim was to establish the influence of biochemical indices other than Phe on neurocognitive outcomes.
Study Design:
Patients with PKU (n = 63; mean age 10.8 ± 2.3 years) and healthy controls (n = 73; mean age 10.9 ± 2.2 years) performed computerized tasks measuring neurocognitive functions (inhibitory control, cognitive flexibility, and motor control). Lifetime and concurrent blood Phe levels, Phe-to-tyrosine ratio (Phe:Tyr), and Phe variations were examined in relation to neurocognitive outcomes using nonparametric tests and regression analyses.
Results:
Patients with PKU with Phe levels ≤240 μmol/L and healthy controls performed equally well. Patients with Phe levels between 240 and 360 μmol/L and ≥360 μmol/L performed more poorly than did controls across tasks. Patients with Phe levels ≤240 μmol/L performed significantly better than patients with levels between 240 and 360 μmol/L on tasks measuring inhibitory control and cognitive flexibility. Absolute Phe levels and Phe variation were the best predictors of motor control, whereas Phe:Tyr were the best predictors of inhibitory control.
Conclusions:
The results of this study suggest that upper Phe targets should be lowered to optimize neurocognitive outcomes. Moreover, Phe variation and Phe:Tyr appear to be of additional value in treatment monitoring.
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