Abstract
Magnetoencephalography (MEG) in the field of epilepsy has multiple advantages; just like electroencephalography (EEG), MEG is able to measure the epilepsy specific information (i.e., the brain activity reflecting seizures and/or interictal epileptiform discharges) directly, non-invasively and with a very high temporal resolution (millisecond-range). In addition MEG has a unique sensitivity for tangential sources, resulting in a full picture of the brain activity when combined with EEG. It accurately allows to perform source imaging of focal epileptic activity and functional cortex and shows a specific high sensitivity for a source in the neocortex. In this paper the current evidence and practice for using magnetic source imaging of focal interictal and ictal epileptic activity during the presurgical evaluation of drug resistant patients is being reviewed.
Highlights
Specialty section: This article was submitted to Applied Neuroimaging, a section of the journal Frontiers in Neurology
With similar studies it was shown that in neocortical epilepsy MEG picks up interictal epileptiform discharge (IED) that extend no more than 3–4 cm2 of activated lateral frontal neocortex on the subdural electrodes, up to 6 cm2 for more basal frontal and temporal neocortex whereas other studies showed that scalp EEG only detects IED when >10 cm2 of the neocortex is activated [17, 18]
Based on the review of the available literature patients who definitely need to be referred for magnetic source imaging are patients in whom a frontal, intrasylvian or insular focus is suspected, because MEG might be superior than EEG in localizing the irritative zone
Summary
The most common feature measured with MEG is the interictal epileptiform discharge (IED) rather than a seizure. Studies on simultaneously recorded scalp EEG and MEG comparing IED’s, show a complementarity between both techniques This complementarity is a result of the difference in sensitivity of EEG and MEG for radial and tangential sources in the brain. With similar studies it was shown that in neocortical epilepsy MEG picks up IED that extend no more than 3–4 cm of activated lateral frontal neocortex on the subdural electrodes, up to 6 cm for more basal frontal and temporal neocortex whereas other studies showed that scalp EEG only detects IED when >10 cm of the neocortex is activated [17, 18]. Heers et al compared the spike yield in EEG, MEG and EEG/MEG following sleep deprivation and reported, respectively, 51, 60, and 71% IED detection [10] Due to this high sensitivity for the cortical convexity, MEG has recently been claimed complementary with SEEG and subdural invasive EEG recording. They showed that MEG was able to fill the gaps in-between the recorded brain activity from the depth electrodes and allowed a more tailored resection of only a small amount of brain tissue [20]
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