Are HFOs in the Intra-operative ECoG Related to Hippocampal Sclerosis, Volume and IQ?

Abstract

Temporal lobe epilepsy (TLE) is the most common form of refractory focal epilepsy and is often associated with hippocampal sclerosis (HS) and cognitive disturbances. Over the last decade, high frequency oscillations (HFOs) in the intraoperative electrocorticography (ioECoG) have been proposed to be biomarkers for the delineation of epileptic tissue but hippocampal ripples have also been associated with memory consolidation. Healthy hippocampi can show prolonged ripple activity in stereo- EEG. We aimed to identify how the HFO rates [ripples (80–250 Hz, fast ripples (250–500 Hz); prolonged ripples (80–250 Hz, 200–500 ms)] in the pre-resection ioECoG over subtemporal area (hippocampus) and lateral temporal neocortex relate to presence of hippocampal sclerosis, the hippocampal volume quantified on MRI and the severity of cognitive impairment in TLE patients. Volumetric measurement of hippocampal subregions was performed in 47 patients with TLE, who underwent ioECoG. Ripples, prolonged ripples, and fast ripples were visually marked and rates of HFOs were calculated. The intellectual quotient (IQ) before resection was determined. There was a trend toward higher rates of ripples and fast ripples in subtemporal electrodes vs. the lateral neocortex (ripples: 2.1 vs. 1.3/min; fast ripples: 0.9 vs. 0.2/min). Patients with HS showed higher rates of subtemporal fast ripples than other patients (Z = −2.51, p = 0.012). Prolonged ripples were only found in the lateral temporal neocortex. The normalized ratio (smallest/largest) of hippocampal volume was correlated to pre-resection IQ (r = 0.45, p = 0.015). There was no correlation between HFO rates and hippocampal volumes or HFO rates and IQ. To conclude, intra-operative fast ripples were a marker for HS, but ripples and fast ripples were not linearly correlated with either the amount of hippocampal atrophy, nor for pre-surgical IQ.

Nocturnal synchronization between hippocampal ripples and cortical delta power is a biomarker of hippocampal epileptogenicity

“…Previous studies demonstrated that epileptic ripples are biomarkers of epileptogenicity in patients with epilepsy (Agudelo Valencia et al, 2021; Akiyama et al, 2011; Jacobs et al, 2010; Jiruska et al, 2010; Zijlmans et al, 2012). A greater percentage of ripples was found in the seizure onset zone than in normal tissue (González Otárula et al, 2019); thus, the EP hippocampus can be inferred based on the ripple rate in unilateral mesial temporal lobe epilepsy (Řehulka et al, 2019).…”

“…In medial temporal lobe epilepsy, HFOs are linked to hippocampal sclerosis (Agudelo Valencia et al, 2021; Řehulka et al, 2019). More HFOs are detected via intraoperative EEG from the hippocampus in patients with hippocampal sclerosis than in patients with other epileptic conditions (Agudelo Valencia et al, 2021). Additionally, the non-epileptic hippocampi can be differentiated by a lower ripple rate than the epileptic hippocampi (Řehulka et al, 2019).…”

“…Inhibition of the SWR causes memory impairment, and hippocampal damage impairs recall and future planning (Girardeau et al, 2009;Kurczek et al, 2015;Lah and Miller, 2008;Scoville and Milner, 1957). A hypothesis suggests that the crosstalk between physiological and pathological activities in a similar frequency range may be associated with memory loss and cognitive impairment in long-term epilepsy (Agudelo Valencia et al, 2021;Řehulka et al, 2019). Several studies have demonstrated that hippocampal interictal epileptic discharges disrupt memory and cognition (Kleen et al, 2013(Kleen et al, , 2010.…”