Decreased amplitudes in multiple sclerosis patients with normal visual acuity: a VEP study

Primary demyelination with relative preservation of axons is considered to be one pathological hallmark of multiple sclerosis (MS), a chronic inflammatory disease of the central nervous system. However, imaging and pathomorphological studies have stimulated a recent re-emergence of interest in the axonal, neurodegenerative aspect of MS pathology. Axonal injury appears to be a key factor of disability and permanent neurological deficit in MS patients. In the present electrophysiological study, visual potentials evoked by pattern reversal (VEPs) were recorded in 25 MS patients with normal visual acuity and unimpaired visual functions. Compared to a control population, VEP amplitudes for two different spatial frequencies were significantly decreased. From this observation, we conclude that an underlying pathological process threatening axonal integrity may not be reliably reflected by clinical parameters due to the distinct ability of the visual system to compensate for axonal loss. Pattern VEP may thus serve as an objective tool to diagnose and to monitor axonal pathology in MS. Focal conduction block due to demyelination as a cause for reduced VEP amplitudes can not be fully excluded, but would appear less likely since latency prolongation in the MS group was moderate compared to controls.