Misrouting of the optic nerves in albinism: estimation of the extent with visual evoked potentials

In human albinism a part of the temporal retina projects abnormally to the contralateral hemisphere. An objective VEP procedure to quantify the extent of the abnormality was devised.

Monocular VEPs were recorded in 16 subjects with albinism and in 16 controls from occipital electrodes to pattern-onset stimulation in 1 of 10 adjacent rectangular apertures along the horizontal meridian covering a total of +/-27 degrees. For each eye interhemispheric difference potentials were calculated and correlated with each other to assess the lateralization of the responses: positive and negative correlations indicate lateralization on same or opposing hemispheres, respectively. Different stimulus conditions were compared to assess the sensitivity and specificity of the procedure for the detection of the misrouting of visual projections in albinism.

Locations that were affected by the projection abnormality were detected with a specificity of 100% and an average sensitivity of 97%. In the 16 subjects with albinism tested, the abnormal projection was confined to the central retina and varied in extent between subjects (2 degrees - 15 degrees; median, 8 degrees). The extent did not appear to be correlated with horizontal nystagmus amplitude or visual acuity.

Because of the great interindividual variability of the projection abnormality, studies of the contribution of the abnormally projecting retina to visual perception must be preceded by the localization of the abnormality. This VEP procedure allowed the authors to identify, with high sensitivity and specificity, visual field locations that are affected by the projection abnormality.