Afferent mechanism of cortical myoclonus studied by proprioception-related SEPs.

Proprioception-related somatosensory evoked potentials (SEPs) to passive flexion movement of the middle finger at proximal interphalangeal joint were recorded in 7 patients with myoclonus of cortical origin who demonstrated enlarged electrical SEPs (giant SEPs). In 3 out of the 7 patients, the proprioception-related SEPs were also enlarged. The remaining 4 patients showed giant electrical SEPs without enhancement of proprioception-related SEPs. Long loop electromyographic response was recorded during the resting condition in all of the 3 patients with enlarged proprioception-related SEPs. We have previously reported that proprioception-related SEPs are mainly generated by muscle afferent inputs, though electrical SEPs are thought to reflect mostly cutaneous inputs with some contribution from muscle afferents. Therefore, it is concluded that hyperexcitability of the sensorimotor cortex in cortical myoclonus is modality-specific. Cortical excitability is exaggerated to both cutaneous and deep receptor inputs in some patients, but only to cutaneous input in others.