High‐frequency oscillations, extent of surgical resection, and surgical outcome in drug‐resistant focal epilepsy

Abstract

SUMMARYPurpose: Removal of areas generating high-frequency oscillations (HFOs) recorded from the intracerebral electroencephalography (iEEG) of patients with medically intractable epilepsy has been found to be correlated with improved surgical outcome. However, whether differences exist according to the type of epilepsy is largely unknown. We performed a comparative assessment of the impact of removing HFO-generating tissue on surgical outcome between temporal lobe epilepsy (TLE) and extratemporal lobe epilepsy (ETLE). We also assessed the relationship between the extent of surgical resection and surgical outcome. Methods: We studied 30 patients with drug-resistant focal epilepsy, 21 with TLE and 9 with ETLE. Two thirds of the patients were included in a previous report and for these, clinical and imaging data were updated and followup was extended. All patients underwent iEEG investigations (500 Hz high-pass filter and 2,000 Hz sampling rate), surgical resection, and postoperative magnetic resonance imaging (MRI). HFOs (ripples, 80-250 Hz; fast ripples, >250 Hz) were identified visually on a 5-10 min interictal iEEG sample. HFO rates inside versus outside the seizure-onset zone (SOZ), in resected versus nonresected tissue, and their association with surgical outcome (ILAE classification) were assessed in the entire cohort, and in the TLE and ETLE subgroups. We also tested the correlation of resected brain hippocampal and amygdala volumes (as measured on postoperative MRIs) with surgical outcome. Key Findings: HFO rates were significantly higher inside the SOZ than outside in the entire cohort and TLE subgroup, but not in the ETLE subgroup. In all groups, HFO rates did not differ significantly between resected and nonresected tissue. Surgical outcome was better when higher HFO rates were included in the surgical resection in the entire cohort and TLE subgroup, but not in the ETLE subgroup. Resected brain hippocampal and amygdala volumes were not correlated with surgical outcome. Significance: In TLE, removal of HFO-generating areas may lead to improved surgical outcome. Less consistent findings emerge from ETLE, but these may be related to sample size limitations of this study. Size of resection, a factor that was ignored and that could have affected results of earlier studies did not influence results.

The morphology of high frequency oscillations (HFO) does not improve delineating the epileptogenic zone

“…These two results agree with results obtained with the visual marking reported in the same dataset (Haegelen et al, 2013).…”

“…We also calculated the ratio between rates of HFOs of each type in resected (Res) and non-resected (nonRes) channels, using the following formula (Haegelen et al, 2013):…”

“…This dataset was used before for evaluating the impact of removing HFO-generating tissue on surgical outcome (Haegelen et al, 2013). Thirty patients with intractable epilepsy were implanted and recorded in the same way as Datasets 1 and 2.…”

High Frequency Neuromagnetic Signals: A New Biomarker for Localizing Epileptic Areas Running Title: High-Frequency Neuromagnetic Signals in Epilepsy

“…Although HFOs or ripple, fast ripples can record with intracranial recordings in epilepsy [28][29][30][31], it has significant limitation. Specifically, intracranial recordings are typically done with micro-or macroelectrode EEG placed within the scalp.…”

“…Specifically, intracranial recordings are typically done with micro-or macroelectrode EEG placed within the scalp. Those recordings are also called intracranial EEG (iEEG), or ECoG [28][29][30][31]. The placement of electrodes require risky surgery, it is very invasive.…”