[A case of clinically diagnosed corticobasal degeneration with unilateral cortical reflex myoclonus showing so-called giant SEP].

We report a 66-year-old woman clinically diagnosed as having a corticobasal degeneration (CBD), who showed electrophysiologically cortical reflex myoclonus. She developed a clumsiness and action myoclonus on the right extremities, and aphasia. The extrapyramidal signs such as dystonia and rigidity were also noted on the right side. Sequential MR images showed a progressive brain atrophy in the left frontoparietal area, where a blood perfusion was reduced on single photon emission computed tomography (SPECT). The median nerve stimulation on the affected right side, but not left side, elicited an enhanced long-loop reflex. The onset latency of the long-loop reflex (43.8msec) was similar to that of the reported cases of CBD (Thompson et al, 1994); but, significantly shorter than that reported in the patients with typical cortical reflex myoclonus. The right median nerve stimulation also elicited so-called giant somatosensory evoked potentials (SEPs). On the basis of the scalp topography of the giant SEPs, we found the high amplitude central P22-N30 components to reflect a radial dipole. We also recorded the myoclonus-related cortical spike by jerk-locked back averaging. Both the giant SEP and myoclonus-related cortical spike were recorded only on the left scalp. We therefore suggest that these two cortical activities are similar in terms of wave form, scalp topography and time relationship to either the long-loop reflex or myoclonus and may be located in the precentral area. This is the first report of a patient with CBD presenting both the giant SEP and myoclonus-related cortical spike.