VISUAL-EVOKED POTENTIALS IN PHENYLKETONURIA - ASSOCIATION WITH BRAIN MRI, DIETARY STATE, AND IQ

At separate institutions, pattern reversal visual evoked potentials (V EPs) were recorded in children and older patients with phenylketonuria and compared with MRI of the brain. In nine patients aged less than 1 4 years, who were still on a diet low in phenylalanine, VEPs were clea rly abnormal in only one and the abnormalities seen on MRI were mild. In 27 patients aged 14-31 years VEPs were abnormal in more than 80%, w ith significant reduction of amplitude and prolongation of latency des pite the general absence of visual symptoms and abnormalities on routi ne neuro-ophthalmological examination, Among the older patients there was no significant correlation between VEP measures and plasma phenyla lanine or tyrosine concentrations; neither was the incidence of VEP ab normalities dependent on whether or not the patients were still on a l ow phenylalanine diet. Some VEP amplitude measures were inversely corr elated with the MRI lesion score, perhaps reflecting the severity of w hite matter abnormalities in the parieto-occipital region. In the olde r patients the amplitude of VEPs to stimulation of the central 8 degre es of the visual held was significantly correlated with IQ. The study confirms the high incidence of subclinical visual pathway involvement in older children and adults with phenylketonuria, and suggests the po ssibility of a link between the abnormal appearance of subcortical whi te matter on MRI and a physiological index of function of the CMS. As there was no evidence of general intellectual decline, it is suggested that the correlation between central field VEP amplitude and IQ may r eflect abnormal development during infancy. Abnormalities on MRI, on t he other hand, seem to be more closely related to current dietary stat e and phenylalanine concentration.