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.
Highlights
Temporal lobe epilepsy (TLE) is the most common epileptic syndrome of focal refractory epilepsy and is subcategorized in neocortical and mesial temporal epilepsy (MTLE)
Sixty-two patients diagnosed with MTLE had surgical resection of the hippocampus with IQIntraoperative electrocorticography (ioECoG) with grid and strip electrodes recorded at 2,048 Hz between 2008 and 2017
We found no relation between High frequency oscillations (HFOs) rates and hippocampal volumes or intellectual quotients (IQ)
Summary
Temporal lobe epilepsy (TLE) is the most common epileptic syndrome of focal refractory epilepsy and is subcategorized in neocortical and mesial temporal epilepsy (MTLE). High frequency oscillations (HFOs) (ripples: 80–250 Hz, fast ripples: 250–500 Hz) are a new electrophysiological biomarker in the ioECoG [5]. Relatively long periods of high amplitude ripple activity occurring in healthy hippocampi were not associated with epilepsy and may relate to physiological brain functioning [7, 8]. For this reason, recently a new type of HFO has been proposed: prolonged ripples, described as a ripple event lasting more than 200 ms [9, 10]. The differentiation between physiological and pathological HFOs in the hippocampus remains challenging and the overall significance of this biomarker is unclear
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