Brain MRI changes in phenylketonuria. Associations with dietary status.

Following the introduction 30 years ago of neonatal screening and early dietary treatment for phenylketonuria there has been a dramatic decrease in the severity of neurological dysfunction associated with this disorder. However, there is evidence that subtle neurological impairment remains common in early treated subjects and in the last 3 years there have been a number of reports of overt neurological impairment with white matter abnormalities on MRI. The frequency of white matter changes in phenylketonuria, and the relation of these changes to dietary management, have remained unclear. The present study examines MRI findings in 34 subjects aged 8-33 years. Twenty-five subjects had been detected by routine neonatal screening and nine had been missed in the screening programme. At the time of the investigation 16 of the early treated and two of the late treated subjects were still receiving a diet low in phenylalanine. All but two of the 34 subjects showed abnormalities on MRI. In the early diagnosed group it could be shown that the severity of MRI changes (graded 1-5) was significantly and independently associated with phenylalanine concentrations at the time of investigation and the time since dietary treatment had been withdrawn. These data are consistent with studies in animals showing that hyperphenylalaninaemia increases myelin turnover in a dose dependent manner. It is suggested that the effects of phenylalanine on myelin pose a lifelong hazard to the nervous system.