Two patients with action tremor that was thought to originate in the cerebral cortex showed fine shivering-like finger twitching provoked mainly by action and posture. Surface EMG showed relatively rhythmic discharge at a rate of about 9 Hz, which resembled essential tremor. However, electrophysiologic studies revealed giant somatosensory evoked potentials (SEPs) with enhanced long-loop reflex and premovement cortical spike by the jerk-locked averaging method. Treatment with beta-blocker showed no effect, but anticonvulsants such as clonazepam, valproate, and primidone were effective to suppress the tremor and the amplitude of SEPs. We call this involuntary movement "cortical tremor," which is in fact a variant of cortical reflex myoclonus.

Fifty-seven consecutive patients with myoclonus from various causes were studied by electrophysiological techniques. Giant somatosensory evoked potentials (SEPs) were observed almost exclusively in patients with progressive myoclonic epilepsy (PME) and diseases with similar clinical features that included lipidosis, neuronal ceroid lipofuscinosis and posthypoxic myoclonus. On the basis of combinations of the giant SEP and the myoclonus-related cortical spike demonstrated by jerk-locked averaging, myoclonus in these patients was classified into four types. In patients with 'cortical reflex' myoclonus (type I) who showed both the giant SEP and the myoclonus-related cortical spike, these two cortical activities were similar in terms of wave form, scalp topography, time relationship to either the long latency (C) reflex or myoclonus, the following cortical excitability, the effect of antimyoclonus drugs and alterations during slow wave sleep. It is therefore postulated that the giant SEP is generated, at least in part, by common physiological mechanisms to the myoclonus-related cortical spike, or that the latter may comprise a constituent of the former. In most patients with PME or allied diseases, both afferent and efferent components of the SEP are enhanced, but in some patients, only one of the two components seems to be predominantly enhanced.

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