Authors :
Presenting Author: Odile Feys, MD – Université libre de Bruxelles
Chantal Depondt, MD, PhD – Université libre de Bruxelles; Estelle Rikir, MD, PhD – Université libre de Bruxelles; Nicolas Gaspard, MD, PhD – Université libre de Bruxelles; Alec Aeby, MD, PhD – Université libre de Bruxelles; Olivier Bodart, MD, PhD – Université de Liège; Maxime Ferez, PhD – Université libre de Bruxelles; Pierre Corvilain, PhD – Université libre de Bruxelles; Vincent Wens, PhD – Université libre de Bruxelles; Xavier De Tiège, MD, PhD – Université libre de Bruxelles
Rationale:
Cryogenic MEG has a higher added value in the presurgical evaluation of extra-temporal lobe epilepsy compared with temporal lobe epilepsy (TLE; Rampp et al, Brain, 2019). On-scalp magnetoencephalography based on optically pumped magnetometers (OPM-MEG) allows to detect and localize interictal epileptiform discharges (IEDs) with a similar or higher signal-to-noise ratio than cryogenic magnetoencephalography (MEG; Feys et al, Radiology, 2022). Thanks to its reduced brain-to-sensor distance and the flexible placement of OPMs on the scalp, OPM-MEG might improve the detection of pathological activities originating from deep temporal sources (Tierney et al, Neuroimage, 2021). Historically, sphenoidal electrodes were used to detect mesiotemporal IEDs. We designed a glasses-like structure aiming at investigating the same mesiotemporal areas as sphenoidal electrodes in a less invasive way with sensors placed under eyes.
This study aims to assess the ability of OPM-MEG to detect and localize IEDs originating from the temporal lobe.
Methods:
Patients with TLE and frequent IEDs on EEG were recruited and underwent a 1-h OPM-MEG recording. Sensors were placed on an EEG-like cap (preferentially over the temporal lobe, including inferior temporal sensors over F9-T9-P9 and F10-T10-P10 positions) and on a glasses-like structure (Figure 1).
Patients were seated in a magnetic shielded room and encouraged to fall asleep. Depending on availability of the cryogenic MEG and ability to fall asleep twice in a row, patients subsequently underwent a 1-h cryogenic MEG recording.
Source localization was performed using minimum norm estimate.
Results:
Five patients (2M/3F, mean age: 42y, range: 22-63y) underwent OPM-MEG. Of these, three also underwent cryogenic MEG recording.
IEDs were visually detected in the five patients (higher amplitude on glasses, n=1; lower amplitude on glasses, n=3; not detected on glasses, n=1). IEDs were localized within the temporal lobe (mesial, n=1; anterior, n=2; posterior, n=1; basal, n=1). See Figure 2. These IEDs localization was similar on previous clinical EEG in 5/5 patients and on subsequent cryogenic MEG in 3/3 patients.
Conclusions:
OPM-MEG is efficient to detect and localize IEDs originating in both neocortical and mesiotemporal parts of the temporal lobe.
Hippocampi generate magnetic fields with an extremum over the homolateral temporal lobe and the other extremum at the mouth (Tierney et al, Neuroimage, 2021). OPM-MEG using the glasses-like allows to record the mouth extremum of the magnetic field in a less invasive approach than sphenoidal EEG and more comfortable approach than mouth OPM-MEG. Moreover, this innovative approach allows to place sensors over bilateral temporal lobes (two per temporal lobe).
Thanks to this setup, OPM-MEG could help to better detect anterior temporal IEDs that can propagate from mesiotemporal lobe. OPM-MEG could therefore contribute to increase the yield of MEG in patients suffering from mesial TLE.
Funding: FRIA, Fonds Erasme, FNRS