A visual evoke potential is an electrical impulse that can be detected by placing electrodes over the visual cortext of the brain and by giving a visual stimulus. Multifocal ERG testing measures the function of areas of the retina, while multifocal VEP testing measures the function of areas of the optic nerve and visual cortex. The retinal nerve fibre layer contains the neurons of the optic nerve and if the are damaged this can lead to retinal ganglion cell loss in the retinal and retinal nerve fibre layer thinning.
MS was associated with demyelination and conduction block and nerve loss. And changes in amplitude was associated with retinal nerve loss within a few months after optic neuritis. This takes a bit of time to show because the retina swells masking the true thickness
of the retinal fibre layer. When one eye is affected the other eye may be affected too especially in those developing MS,