Are VEP abnormalities in optic neuritis (ON) dependent on plaque size? A reappraisal of the physiopathology of ON based on improved MRI and multiple-lead recordings.

Twenty patients with optic neuritis (ON) described in the previous study [23] underwent serial VEP recordings (using multiple electrode arrays) for two years. The VEPs could be correlated with the lesions revealed by MRI, Visual Field tests and other clinical findings. On the basis of their scalp distribution, they were classified as "really delayed" VEPs and "pseudo-delayed" VEPs. Real delays could be recorded at the onset of ON or shortly afterwards, and their appearance indicated the recovery of visual function and a good prognosis. Pseudo-delays indicated an alteration in the visual field and, unless a breakthrough of normal or delayed components appeared in the first three months, following acute ON, indicate a poor prognosis for the recovery of visual function. The pseudo-delayed VEPs were mainly observed in patients with longer lesions revealed by means of LTE-STIR MRI [23]; there was no correlation between VEP latency and the length of plaques. Our findings contradict previous theories on the timing of conduction alterations in ON and multiple sclerosis.