Neuropsychological effects of subsequent exposure to phenylalanine in adolescents and young adults with early-treated phenylketonuria.

Severe mental handicap in phenylketonuria (PKU) can be prevented if dietary treatment is implemented at birth. Controversy remains about the optimum age for terminating treatment. A group of adolescents and young adults with PKU from the West of Scotland Register was identified which had received early treatment, been well-controlled on diet, ceased treatment at 10 years old and subsequently were hyperphenylalaninaemic for 3 years or more. They were given a battery of neuropsychological tests and their results were compared with those of on-diet subjects with PKU and normal controls. The findings generally supported the view that dietary cessation at age 10 is sufficient to prevent a substantial reduction of cognitive and motor ability, and that the central nervous system is probably mature enough to withstand the toxic effects of high blood phenylalanine by then. However, there were minor indications, in keeping with Welsh et al.'s hypothesis [M.C. Welsh, B.F. Pennington, S. Ozonoff, B. Rouse & E.R.B. McCabe (1990) Neuropsychology of early-treated phenylketonuria: specific executive function deficits.