Pathologic substrates of focal epilepsy influence the generation of high‐frequency oscillations

Abstract

SUMMARYObjective: Although a clear correlation has been observed between high-frequency oscillations (HFOs) and the seizure-onset zone in distinct lesions, the role of the underlying pathologic substrates in the generation of HFOs is not well established. We aimed to investigate HFO correlates of different pathologic substrates in patients with drug-resistant epilepsy, and to examine the relation of HFOs with the anatomic location of the dysplastic lesion and surrounding tissue in patients with focal cortical dysplasia (FCD). Methods: We studied consecutive patients with drug-resistant epilepsy who underwent intracranial electroencephalography (iEEG) investigations with depth electrodes at the Montreal Neurological Institute and Hospital, between November 2004 and May 2013. Inclusion criteria were the following: a focal lesion documented by magnetic resonance imaging (MRI); EEG recording at a 2,000 Hz sampling rate; and seizures starting from depth electrode contacts placed in lesion and perilesional tissue. Results: Thirty-seven patients (13 FCD, 12 mesial temporal sclerosis, five cortical atrophy, three polymicrogyria, three nodular heterotopia, and one tuberous sclerosis) were included; 18 were women (median age 34). Ripples and fast ripples were found in all lesion types, except tuberous sclerosis, which showed no fast ripples. There was a significant difference in rates of ripples and fast ripples across different lesions (p < 0.001), with higher rates in FCD, mesial temporal sclerosis, and nodular heterotopia than in atrophy, polymicrogyria, and tuberous sclerosis. Regarding patients with FCD, HFOs rates differed significantly across the three types of tissue (lesional, perilesional, and nonlesional; p < 0.001), being higher within the borders of the MRI-visible dysplastic lesion, followed by the surrounding area, and rare in the remote cortex. Significance: Our findings suggest that in patients who are all intractable, the HFO rates vary with different pathologies, and reflect different types of neuronal derangements. Our results also emphasize the potential usefulness of HFOs as an additional method to better define the extent of the epileptogenic dysplastic tissue in FCD.

High frequency oscillations and high frequency functional network characteristics in the intraoperative electrocorticogram in epilepsy

“…the low number of channels within those patients showing FRs in general implies that the spatial sampling of clinical grids is not sufficient to reveal the underlying micro-scale FR-generating subnetworks. FR rates vary with different pathologies (Ferrari-Marinho et al, 2015) and disease activity (Zijlmans et al, 2009), but are low in general (van Klink et al, 2014). We corrected for the inter-patient variability in event-rate by normalizing the number of events scored per channel by the total number of events per patient.…”

Advances of Intracranial Electroencephalography in Localizing the Epileptogenic Zone

“…reported much lower mean rates of ripples (*1.1/ min) and fast ripples (*0.25/min) in the SOZs in patients with neocortical epilepsy. The recorded HFO rates are influenced by the pathological types of epileptogenic lesions [28]. HFO rates are higher in mesial temporal sclerosis and focal cortical dysplasia than in other lesion types.…”

Progress and remaining challenges in the application of high frequency oscillations as biomarkers of epileptic brain

“…However, it is unclear if the HFOs rate alone is specific enough to distinguish epileptic from non-epileptic regions at the single electrode level that is required to guide epilepsy surgery. The HFOs rate varies in different brain structures [59], types of lesions [60], and also SOZ morphological patterns [61]. These findings should be considered when comparing HFOs rates recorded from different channels.…”