Neurocognitive function in mild hyperphenylalaninemia.

AIM The purpose of this review was to provide an update on cognitive function in individuals with mild hyperphenylalaninemia (mHPA), the most clinically and biochemically benign form of phenylketonuria. METHOD A review was conducted of the existing literature on mHPA. Individuals with mHPA, whose plasma phenylalanine concentration had always remained lower than 360 μmol/L without dietary restriction, were considered. RESULTS The review of the literature indicated that there is no consensus concerning the definition of mHPA. There are few studies regarding the cognitive functions of individuals with mHPA, results are contradictory, and samples are difficult to compare from one study to another. Most studies focus only on descriptions of IQ when assessing cognitive functions. The existing literature indicates that, in general, children with mHPA do not show significant cognitive impairments, but usually achieve scores between those of individuals with phenylketonuria and those of comparison groups with regard to the cognitive functions assessed. INTERPRETATION When assessing cognitive functions in individuals with hyperphenylalaninemia, it is not enough to measure only IQ, as deficits in executive functions can be present even when an individual's IQ is within a normal range. Further studies are needed of individuals with mHPA, using consistent selection criteria, in order to make it possible to exclude the presence of cognitive impairment and to establish a consensus regarding the level of phenylalanine that necessitates dietary treatment.