FOCAL SUBCORTICAL REFLEX MYOCLONUS - A CLINICAL AND NEUROPHYSIOLOGICAL STUDY

Background: Patients with progressive myoclonus epilepsy or progressiv e myoclonus ataxia often show a focal myoclonus, both spontaneous and reflex to somatosensory stimuli. Myoclonus is time-locked to large ('' giant'') electroencephalographic potentials. Previous authors have cla ssified it as a ''cortical reflex myoclonus,'' with the assumption tha t it invariably arises from an abnormal corticifugal neuron discharge. Objective: To identify the myoclonus source, using various neurophysi ological techniques, in 5 patients with progressive myoclonus epilepsy /ataxia. Methods: Extensive investigations were performed to ascertain the clinical diagnosis. Electrophysiologically, the main method was t ranscranial cortical stimulation and motor evoked potential measuremen t. The latency and amplitude of the spontaneous myoclonus and the prem yoclonus cortical spike, the reflex myoclonus (C-reflex), and the gian t somatosensory evoked potential were also analyzed. The behavior of g iant somatosensory evoked potentials and C-reflexes were then studied on single, consecutive trials. Finally, the central motor pathmay exci tability and its changes attributable to a prior somatosensory input w ere determined. Results: The motor evoked potential studies showed tha t the expected corticomuscular conduction time (23 milliseconds) of th e myoclonic electromyographic potential was longer than that previousl y suspected. Considering this, the premyoclonus cortical spike and the giant somatosensory evoked potential were so close to the spontaneous /reflex jerks that they could not reflect a cortical myoclonus source. In 4 patients, the C-reflex latency (<41.6 milliseconds) was shorter than that often reported in previous studies. The giant somatosensory evoked potential and the C-reflex showed no simple cause-effect link. Motor pathways were hyperexcitable only in response to somatosensory i nputs. Conclusions: The data pointed to a cortical myoclonus origin on ly in the patient whose C-reflex had the longest latency (44 milliseco nds). In the remaining patients, a subcortical source was far more lik ely. In this group of patients, cortical stimulation disclosed a new m yoclonus variety, for which the term focal subcortical reflex myoclonu s is proposed; it mimics cortical reflex myoclonus but has a shorter l atency.