[Electromyographic analysis of cortical myoclonus and focal dystonia]

We have done a few physiological studies on six patients with cortical reflex myoclonus and three patients with unilateral focal dystonia caused by a discrete cerebrovascular lesion in the basal ganglia. The silent period after magnetic cortical stimulation was normal or slightly prolonged in all the patients with cortical myoclonus. In contrast, in patients with focal dystonia, it was shortened in the muscles on the contralateral side to the lesion, whereas normal in duration on the ipsilateral side. Excessive inputs from the disinhibited supplementary motor cortex to the primary motor cortex due to a lesion of the basal ganglia must cause this shortening in the dystonic hand muscles. Excitability recovery of the motor cortex after hypersynchronous activation by magnetic stimulation should be prolonged in the myoclonic patients. Cortico-cortical inhibition of the motor cortex studied with paired magnetic stimulation technique was disturbed in both the patients with myoclonus and those with dystonia. This result suggested that the disinhibition of the motor cortex is present in both disorders. Based on these results, we conclude that the motor cortex is similarly disinhibited in both disorders, but recovery function from hypersynchronous activation is different between these two groups.