Abstract
Patients with cortical reflex myoclonus manifest typical neurophysiologic characteristics due to primary sensorimotor cortex (S1/M1) hyperexcitability, namely, contralateral giant somatosensory-evoked potentials/fields and a C-reflex (CR) in the stimulated arm. Some patients show a CR in both arms in response to unilateral stimulation, with about 10-ms delay in the non-stimulated compared with the stimulated arm. This bilateral C-reflex (BCR) may reflect strong involvement of bilateral S1/M1. However, the significance and exact pathophysiology of BCR within 50 ms are yet to be established because it is difficult to identify a true ipsilateral response in the presence of the giant component in the contralateral hemisphere. We hypothesized that in patients with BCR, bilateral S1/M1 activity will be detected using MEG source localization and interhemispheric connectivity will be stronger than in healthy controls (HCs) between S1/M1 cortices. We recruited five patients with cortical reflex myoclonus with BCR and 15 HCs. All patients had benign adult familial myoclonus epilepsy. The median nerve was electrically stimulated unilaterally. Ipsilateral activity was investigated in functional regions of interest that were determined by the N20m response to contralateral stimulation. Functional connectivity was investigated using weighted phase-lag index (wPLI) in the time-frequency window of 30–50 ms and 30–100 Hz. Among seven of the 10 arms of the patients who showed BCR, the average onset-to-onset delay between the stimulated and the non-stimulated arm was 8.4 ms. Ipsilateral S1/M1 activity was prominent in patients. The average time difference between bilateral cortical activities was 9.4 ms. The average wPLI was significantly higher in the patients compared with HCs in specific cortico-cortical connections. These connections included precentral-precentral, postcentral-precentral, inferior parietal (IP)-precentral, and IP-postcentral cortices interhemispherically (contralateral region-ipsilateral region), and precentral-IP and postcentral-IP intrahemispherically (contralateral region-contralateral region). The ipsilateral response in patients with BCR may be a pathologically enhanced motor response homologous to the giant component, which was too weak to be reliably detected in HCs. Bilateral representation of sensorimotor responses is associated with disinhibition of the transcallosal inhibitory pathway within homologous motor cortices, which is mediated by the IP. IP may play a role in suppressing the inappropriate movements seen in cortical myoclonus.
Highlights
Conventional neurophysiological studies have demonstrated that one type of myoclonus originates from the cerebral cortex [1,2,3]
The cardinal features of benign adult familial myoclonus epilepsy (BAFME) consisted of six items [18, 19]: [1] autosomal dominant inheritance; [2] cortical tremor, which consists of continuous, distal, fine twitches of the hands that resemble essential tremor; [3] infrequent generalized seizure; [4] features of cortical reflex myoclonus demonstrated in electrophysiological studies; [5] lack of cognitive decline or other neurological symptoms during the early stage of the clinical course; and [6] lack of clear progression, which impairs activities of daily living in the early stage of the clinical course
Cortical myoclonus in Celiac diseases and corticobasal degeneration shows no preceding positive spikes because of repetitive nature and high frequency of the myoclonus [23], jerk-locked back averaging (JLA) may sometimes show no activity in cortical tremor
Summary
Conventional neurophysiological studies have demonstrated that one type of myoclonus originates from the cerebral cortex [1,2,3]. CR, or long-loop reflex, is the EMG response associated with myoclonic jerks that is recorded from the stimulated hand at a latency of around 45 ms after stimulation of the median nerve in the wrist [5, 8, 9] These characteristics are thought to result from a release effect that causes increased excitability at central synapses along the pathway that begins from peripheral input to the spinal cord, the contralateral nucleus of thalamus, contralateral S1/M1, corticospinal tract, anterior horn cell, and to the stimulated hand muscle [9]
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