Generator mechanisms of giant somatosensory evoked potentials in cortical reflex myoclonus

In order to clarify the generator mechanisms for giant cortical evoked potentials, scalp topographies of somatosensory evoked potentials (SEPs) following various types of stimulation, including electrical nerve trunk and finger stimulation and mechanical stimulation, were investigated in 5 patients with cortical reflex myoclonus. For SEPs evoked by median nerve stimulation, not only the P25 and N34 components in central and parietal regions but also N25 in the frontal region were markedly enlarged in each patient. Their scalp distributions were not significantly different from those of normal subjects. P25 and N25, but not N34, were considerably attenuated by interfering tactile stimulation applied to the hand. The components corresponding to P25, N25 and N34 following electrical stimulation of the digital nerves as well as mechanical stimulation of the finger were also remarkably large and showed scalp distributions similar to those for median nerve SEPs. It is therefore concluded that the giant cortical SEPs of cortical reflex myoclonus are generated in those areas of the primary sensory cortex which generate normal SEPs, in response to an input, at least in part, from cutaneous afferents on the basis of an extremely enhanced cortical excitability.