Focal subcortical reflex myoclonus. A clinical and neurophysiological study

Patients with progressive myoclonus epilepsy or progressive 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 classified it as a "cortical reflex myoclonus," with the assumption that it invariably arises from an abnormal corticifugal neuron discharge.

To identify the myoclonus source, using various neurophysiological techniques, in 5 patients with progressive myoclonus epilepsy/ataxia.

Extensive investigations were performed to ascertain the clinical diagnosis. Electrophysiologically, the main method was transcranial cortical stimulation and motor evoked potential measurement. The latency and amplitude of the spontaneous myoclonus and the premyoclonus cortical spike, the reflex myoclonus (C-reflex), and the giant somatosensory evoked potential were also analyzed. The behavior of giant somatosensory evoked potentials and C-reflexes were then studied on single, consecutive trials. Finally, the central motor pathway excitability and its changes attributable to a prior somatosensory input were determined.

The motor evoked potential studies showed that the expected corticomuscular conduction time (23 milliseconds) of the myoclonic electromyographic potential was longer than that previously 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 inputs.

The data pointed to a cortical myoclonus origin only in the patient whose C-reflex had the longest latency (44 milliseconds). In the remaining patients, a subcortical source was far more likely. In this group of patients, cortical stimulation disclosed a new myoclonus variety, for which the term focal subcortical reflex myoclonus is proposed; it mimics cortical reflex myoclonus but has a shorter latency.