Simultaneous subdural and scalpEEGcorrelates of frontal lobe epileptic sources

Abstract

SUMMARYObjective: To assess the visibility and detectability in scalp electroencephalography (EEG) of cortical sources in frontal lobe epilepsy (FLE) as to their localization, and the extent and amplitude of activation. Methods: We analyzed the simultaneous subdural and scalp interictal EEG recordings of 14 patients with refractory frontal lobe epilepsy (FLE) associated with focal cortical dysplasia. Subdural spike types were identified and averaged for source localization and detection of their scalp EEG correlates. Both raw and averaged scalp EEG segments were reviewed for spikes, blinded to subdural segments. We further analyzed the correlation of spike-to-background amplitude ratios in subdural and scalp EEG. Results: We identified 36 spike types in subdural EEG, corresponding to 29 distinct sources. Four of 29 sources were visible by visual evaluation of scalp EEG and six additional sources were detectable after averaging: four in the medial frontal, two in the dorsolateral gyri, two in the depth of dorsolateral sulci, and two in the basal frontal region. Cortical sources generating scalp-detectable spikes presented a median of 6 cm 2 of activated cortical convexity surface and a subdural spike-to-backgroundamplitude ratio >8. These sources were associated with a higher number of activated subdural grid contacts and a higher subdural spike-to-background amplitude ratio than sources generating non-scalp-detectable spikes. Significance: Not only dorsolateral but also basal and medial sources can be detectable in FLE. This is the first in vivo demonstration derived from simultaneous subdural and scalp EEG recordings of the complementary significance of extensive source activation and higher subdural spike-to-background amplitude ratio in the detection of cortical sources in FLE.

Yield of MRI, high-density electric source imaging (HD-ESI), SPECT and PET in epilepsy surgery candidates

“…This is relatively high, given that intracranial EEG alone is probably an imperfect ''gold standard'' due to potential spatial undersampling and an interelectrode distance of already 10 mm. These results as well of studies with simultaneous intra-and extracranial EEG recordings also suggest that ESI is capable to detect deeper sources (Koessler et al, 2015;Ramantani et al, 2014).…”

Correlation of invasive EEG and scalp EEG

“…The last decade saw the advent of several novel technologies that derive from interictal EEG spikes, such as electrical source localization (ESL), magnetoencephalography (MEG), and functional magnetic resonance imaging (fMRI) [5,6,9,[18][19][20][21] rendering the correlation of cortical-to-scalp epileptic discharges even more crucial. At the same time, the advances of EEG technology permitted simultaneous scalp and invasive EEG recordings in the routine workup of epilepsy surgery patients, thus fuelling further research regarding scalp EEG spikes and their cortical substrates.…”

“…Whereas all previous in vivo studies, both historic and contemporary, have been conducted exclusively in temporal lobe epilepsy, these observations have only recently been extended to extratemporal epilepsy [5]. Extratemporal sources are equally prevalent as temporal sources in pediatric epilepsy surgery and constitute a major challenge in terms of electroclinical correlations and postsurgical outcomes [39][40][41].…”

“…The current methodology of invasive explorations in epilepsy patients has, however, inherent limitations, thus rendering multimodal comparisons particularly challenging [4]. Subdural recordings offer extensive cortical coverage, but are prone to sampling limitations for deep sources, such as sulcal sources [5][6][7]. Depth electrode recordings provide information for selected deep structures [7][8][9], but are plagued from sampling limitations due to incomplete and irregular cortical coverage.…”

“…shortcomings of both subdural and depth recordings and, most of all, to clarify their correlation with scalp EEG recordings [5,8,10,11].…”