Variation of visual evoked potential delay to stimulation of central, nasal, and temporal regions of the macula in optic neuritis

Objectives-To compare the degree of visual evoked potential (VEP) delay to stimulation of central, nasal, and temporal regions of the macula in optic neuritis, to determine whether the differential involvement of parvocellular and magnocellular fibre types suggested by other studies is governed by retinotopic factors. Methods-VEPs were recorded to reversal of 40' checks in the central (4 degrees radius) and the left and right surrounding regions of the visual field (as far as 10 degrees vertical and 14 degrees horizontal) in 30 patients recently recovered from the acute stage of optic neuritis, and in 17 age matched controls. Results-In the control group, VEP latencies were similar to stimulation of the central and temporal regions of the macula, marginally shorter from the nasal region. In the patients with optic neuritis, VEPs were significantly more delayed from the central region, on average by about twice as much as from the nasal and temporal regions. Delays seen in some of the VEPs from the patients' fellow eyes tended to be more uniformly distributed. Conclusions-Although the central region of the macula is where the density of parvocellular innervation is greatest, there is no reason to suppose that the VEPs to stimulation of the nasal and temporal regions (almost all P100 activity arising from within the central 10 degrees) are mediated by fibres of another type. Consequently it is suggested that the central fibres mere most affected by demyelination, not on account of their belonging to the parvocellular type but because of their particular situation in the optic nerve. Centrally located fibres may experience greater exposure to factors causing demyelination, or fibres located closer to the edge of the plaque may undergo more effective remyelination in the first few weeks after the acute episode.