Evaluation of Interhemispheric Time Difference by Magnetoencephalography Before and After Total Callosotomy-Two Case Reports-

Bilateral Representation of Sensorimotor Responses in Benign Adult Familial Myoclonus Epilepsy: An MEG Study

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.

Generalized 3-Hz spike-and-wave complexes emanating from focal epileptic activity in pediatric patients

We describe two pediatric patients with an uncommon electrophysiological seizure propagation pattern. Both had dialeptic seizures as the main or only symptom. Case 1 had a small mass in the left medial temporal structures; case 2 had no lesion on magnetic resonance imaging. In both, the electroencephalogram showed not only left temporal spikes, but also bilaterally synchronous 3-Hz spike-and-wave complexes (SWCs) from onset and unusual secondarily generalized 3-Hz SWC patterns arising from the left temporal region. Case 1 was seizure free following resection of the mass; focal or generalized epileptiform electroencephalographic abnormalities were no longer present. In case 2, magnetoencephalography localized the spikes to the left superior and midtemporal gyrus, which ictal single-photon-emission computed tomography suggested was the origin of onset. These cases illustrate the close relationship between the focal epileptic area and 3-Hz SWCs and suggest that the focal area can trigger 3-Hz SWCs. The therapeutic strategy may need to be altered in such patients.