Cognitive function in untreated subjects with mild hyperphenylalaninemia: a systematic review.

It is suggested that Phe levels between 120 and 360 μmol/L are generally safe. Some studies showed that untreated patients with higher levels might show some degrees of cognitive impairment. In conclusion, current knowledge is insufficient to state that treatment is not required for HPA patients to preserve their cognitive status, especially in patients with Phe levels of 360-600 μmol/L. Further studies with a larger sample size and standardized cognitive function evaluation tools are needed.