Regional Differences in the Sensitivity of MEG for Interictal Spikes in Epilepsy

Abstract

MEG interictal spikes as recorded in epilepsy patients are a reflection of intracranial interictal activity. This study investigates the relationship between the estimated sources of MEG spikes and the location, distribution and size of interictal spikes in the invasive ECoG of a group of 38 epilepsy patients that are monitored for pre-surgical evaluation. An amplitude/surface area measure is defined to quantify and rank ECoG spikes. It is found that all MEG spikes are associated with an ECoG spike that is among the three highest ranked in a patient. Among the different brain regions considered, the fronto-orbital, inter-hemispheric, tempero-lateral and central regions stand out. In an accompanying simulation study it is shown that for hypothesized extended sources of larger sizes, as suggested by the data, source location, orientation and curvature can partly explain the observed sensitivity of MEG for interictal spikes.

Magnetoencephalography in the presurgical evaluation of epilepsy

“…Magnetoencephalography shows good agreement with subsequently performed ECoG [30][31][32][33][34][35][36]. These studies indicate that all spikes have an ECoG correlate, and about 2/3rd of all ECoG spikes are detected by MEG [37]. Studies validating MEG using subsequent ECoG should be interpreted with care since epileptic discharges can vary in time and space within the irritative/epileptogenic zone [15].…”

“…Insular onset suspected Interhemispheric (especially frontal) onset suspected MEG can detect spikes from the interhemispheric area [10,37,50]. MEG can lateralize medial frontal spikes even when the EEG is nonlateralizing because of varying dipole orientation or secondary bisynchrony [50].…”

MEG Source Localization of Spatially Extended Generators of Epileptic Activity: Comparing Entropic and Hierarchical Bayesian Approaches

“…As mentioned by Huiskamp et al [11], inter-ictal spikes are spontaneous signals that can have relatively large amplitude (∼3 pT in MEG). This implies that epileptic MEG signals are likely to arise from large spatially extended regions of active cortex [12].…”

“…A study by Mikuni et al [13] suggested that MEG can detect epileptiform activity when a cortical area greater than 4 cm 2 is synchronously involved. Comparing MEG spikes with Electro-CorticoGraphy (ECoG) spikes, studies performed by Oishi et al [14] and Huiskamp et al [11] showed that MEG sensitivity varies for different regions in the brain. As a result, not only the size of the generators matters, but their location and orientation affect the detection of the MEG epileptic activity [1], [13].…”

Klinischer Stellenwert der Quellenanalyse epileptischer Aktivität

“…Im MEG ist das Signal-zu-Rausch-Verhältnis für Quellen im Frontallappen besser als im EEG, sodass es hier eine besonders hohe Sensitivität hat [15,23]. Ossenblok et al [46] untersuchten Patienten mit Frontallappenepilepsien mithilfe der MEG-und EEG-Quellenanalyse.…”