Assessment of cortical visual field representations with multifocal VEPs in control subjects, patients with albinism, and female carriers of ocular albinism.

PURPOSE In human albinism, part of the temporal retina projects abnormally to the contralateral hemisphere. This study was undertaken to test whether this abnormality can be identified with multifocal visual evoked potentials (mfVEPs) and whether it is evident in carriers of ocular albinism (OA1). METHODS In 12 control subjects, 11 patients with albinism, and 5 female carriers of OA1 monocular pattern-reversal mfVEPs were recorded for 60 locations comprising a visual field of 44 degrees diameter (VERIS ver. 4.8; EDI, San Mateo, CA). For each eye and each stimulus location 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 opposite hemispheres, respectively. Misrouted optic nerves are expected to yield negative interocular correlations. Visual field locations without recordable responses were excluded from the analysis using a signal-to-noise threshold. The analysis also allowed assessment of the sensitivity and specificity of the detection of projection abnormalities. RESULTS Sizable mfVEPs were obtained in all control subjects, carriers, and the three patients with albinism who had negligible nystagmus and visual acuity>0.25. Ninety-seven percent and 99% of the visual field locations were identified as normal in control subjects and carriers, respectively. While this indicates a specificity of the procedure of 97%, the sensitivity was estimated as 75%. Finally, in albinism, 55% percent of the responses were abnormally represented. CONCLUSIONS In the absence of nystagmus mfVEPs are a powerful tool to identify, in a spatially resolved manner, abnormal visual field representations. No local representation abnormalities were evident in the female carriers of OA1.