Abstracts

Rationale: Accurate delineation of the epileptogenic zone (EZ) during presurgical workup of focal drug-resistant epilepsy patients can be challenging (Spencer et al., Lancet Neurol. 2008). Stereo-electroencephalography (SEEG) recordings, considered as the gold-standard for the localization of the EZ, might be the step towards mapping the seizure-onset zone (SOZ) and determining surgical candidacy (McGonigal et al., Brain 2007). However, a successful investigation requires a strong pre-implantation hypothesis on the localization of the EZ, which can be derived from non-invasive investigations such as EEG/magnetoencephalography (MEG) and EEG/functional MRI (EEG/fMRI) (Pellegrino et al., HBM. 2018; Heers et al., HBM. 2014; Pittau et al., Neurology 2012). Here, we aimed at: (1) proposing a truly objective and quantitative comparison of (EEG/MEG) source-imaging (EMSI) and EEG/fMRI responses for similar spikes to primary-irritative zone (most frequent spikes, i.e., PIZ) and SOZ defined by SEEG, (2) evaluating the value of EMSI and EEG/fMRI to predict postsurgical outcome.

Methods: Seventeen patients who underwent EEG/MEG, EEG/fMRI, and subsequent SEEG were included. We quantified concordance between all data, as spatial overlap and distances in the SEEG channel space: (i) we converted EMSI spike maps into SEEG electrical potentials (Grova et al., HBM. 2016) (EMSIe-SEEG), (ii) t-values from the fMRI most significant cluster (Khoo et al., Epilepsia. 2017; Khoo et al., Neurology 2018) were projected on the SEEG channels (fMRIp-SEEG). Then, the EMSIe-SEEG channel with the maximum amplitude and the fMRIp-SEEG channel with the highest t-value were compared to Spike-onset (Sp-onset), Spike-maximum (Sp-maximum) and the Seizure-core (Sz-core) reference channels (Figure1), for corresponding spatial overlap with PIZ and SOZ classified as good or poor.

Results: The spatial overlaps of EMSIe-SEEG, fMRIp-SEEG with PIZ and SOZ were not significantly different except for the EMSIe-SEEG overlap with PIZ which was significantly larger when compared to fMRIp-SEEG (p=0.01). Combination of EMSI and EEG/fMRI results did not add additional information. Good EMSIe-SEEG overlap with PIZ was associated with a smaller distance to Sp-max channel (p=0.04) when compared to poor overlap. However, a good spatial overlap with SOZ was not associated with a smaller distance to Sz-core channel (p=0.1). Conversely, good fMRIp-SEEG overlap with SOZ was associated with smaller distances to Sp-onset and Sz-core channels (both p=0.01) when compared to poor overlap. EEG/MEG correctly predicted postsurgical outcome in 12/15 patients and EEG/fMRI in 6/11 patients.

Conclusions: Using a unique quantitative approach consisting in estimating EMSI and fMRI results on the SEEG channel space, which allowed a truly objective and quantitative comparison, we found EEG/MEG was more sensitive to the main peak of epileptic discharges (Sp-max), whereas EEG/fMRI was more sensitive to Sp-onset and Sz-core, regions where an increased oxygen consumption can be expected at the time of spikes. Therefore, EMSI and EEG/fMRI provide complementary localization that can help SEEG implantation and selecting good candidates for surgery.

Funding: Please list any funding that was received in support of this abstract.: MOP-133619 and MOP-93614.