High-density Electric Source Imaging of interictal epileptic discharges: How many electrodes and which time point?

Bernd J Vorderwülbecke,Margherita Carboni,Sebastien Tourbier,Denis Brunet,Martin Seeber,Laurent Spinelli,Margitta Seeck,Serge Vulliemoz

doi:10.1016/j.clinph.2020.09.018

Bernd J Vorderwülbecke, Margherita Carboni + Show 6 more

Open Access

https://doi.org/10.1016/j.clinph.2020.09.018

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Abstract

ObjectiveTo assess the value of caudal EEG electrodes over cheeks and neck for high-density electric source imaging (ESI) in presurgical epilepsy evaluation, and to identify the best time point during averaged interictal epileptic discharges (IEDs) for optimal ESI accuracy. MethodsWe retrospectively examined presurgical 257-channel EEG recordings of 45 patients with pharmacoresistant focal epilepsy. By stepwise removal of cheek and neck electrodes, averaged IEDs were downsampled to 219, 204, and 156 EEG channels. Additionally, ESI at the IED’s half-rise was compared to other time points. The respective sources of maximum activity were compared to the resected brain area and postsurgical outcome. ResultsCaudal channels had disproportionately more artefacts. In 30 patients with favourable outcome, the 204-channel array yielded the most accurate results with ESI maxima < 10 mm from the resection in 67% and inside affected sublobes in 83%. Neither in temporal nor in extratemporal cases did the full 257-channel setup improve ESI accuracy. ESI was most accurate at 50% of the IED’s rising phase. ConclusionInformation from cheeks and neck electrodes did not improve high-density ESI accuracy, probably due to higher artefact load and suboptimal biophysical modelling. SignificanceVery caudal EEG electrodes should be used for ESI with caution.

Highlights

In patients with pharmacoresistant focal epilepsy, epilepsy surgery aims at eliminating the epileptogenic zone, i.e. the specific area of cerebral cortex which is indispensable for the generation of seizures
We retrospectively examined presurgical 257-channel EEG recordings of 45 patients with pharmacoresistant focal epilepsy
Until July 31st, 2019, the database of the EEG and Epilepsy Unit at the University Hospitals Geneva was retrospectively screened for patients meeting the following criteria: (a) a first resective brain surgery to treat pharmacoresistant focal epilepsy; (b) age older than 6 years at evaluation; (c) presurgical 257-channel EEG recording with a minimum of 3 focal interictal epileptic discharge (IED); (d) presurgical highresolution magnetic resonance imaging (MRI); (e) known 12month postsurgical outcome

Summary

Introduction

In patients with pharmacoresistant focal epilepsy, epilepsy surgery aims at eliminating the epileptogenic zone, i.e. the specific area of cerebral cortex which is indispensable for the generation of seizures. A valuable, non-invasive neurophysiological tool to approach the epileptogenic zone is electric source imaging (ESI). Based on the patient’s scalp electroencephalogram (EEG), ESI plots the sources of epileptic activity within a 3D model of the patient’s brain (Zijlmans et al, 2019; Foged et al, 2020).

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