Pattern evoked potentials in phenylketonuria.

Humans with untreated phenylketonuria (PKU) are candidates for having misrouted optic projections related to their hypopigmentation. Six tractable PKUs were tested using scalp-recorded evoked potentials. Both pattern reversal and pattern onset/offset (appearance/disappearance) stimuli were used under conditions of binocular and monocular stimulation. Evoked potentials were of normal form and hemispheric symmetry following both monocular and binocular stimulation in 5 of the 6 PKUs. The exception demonstrated a small hemispheric asymmetry upon stimulation of the left eye, but not following stimulation of the right eye. The asymmetry observed in this subject was probably idiosyncratic and not representative of PKUs in general. Four of the 6 PKUs produced reversal. This may be related to incomplete postnatal myelination. There appears to be no detectable optic misrouting in PKU. In spite of total absence of fetal phenylalanine hydroxylase activity, availability of tyrosine and melanin pigment is evidently sufficient during early development of the visual system to permit normal decussation of optic fibers at the chiasm.