Abstracts

Rationale: Interictal high-frequency oscillation (HFO) at ≥80 Hz and the phase-amplitude coupling between HFO and slow waves are intracranial EEG (iEEG) biomarkers used for localizing the epileptogenic zone. Investigators have recommended considering the topographical variations and developmental changes of these biomarker values based on the assessment of non-epileptic sites in extraoperative iEEG recordings. To facilitate the possibility of localizing the epileptogenic zone during general anesthesia, this study aimed to generate atlases showing the normative values of these iEEG biomarkers across different lobes and patient ages.


Methods: The study included 72 children (mean age: 10.4 years; range: 1.0-19.8 years) with drug-resistant epilepsy who achieved seizure freedom following extraoperative iEEG recording and focal resection. We examined five minutes of intraoperative iEEG data recorded under Isoflurane anesthesia, with a mean level of 0.8% (range: 0.5-1.1%). We analyzed iEEG signals from non-epileptic sites, defined as those outside the resection, MRI lesions, and spike-generating zones based on visual assessment of extraoperative iEEG. We quantified the rate of HFO at 80-300 Hz and the modulation index (MI) quantifying the coupling between HFO at 80-300 Hz and slow waves at 3-4 Hz as dependent variables. We then employed mixed-model analysis incorporating age, sex, presence of an MRI lesion, number of anti-seizure medications, Isoflurane level, end-tidal carbon dioxide concentration, sampled hemisphere, and sampled lobe as fixed-effect variables, with patient intercepts as random effects. We also performed regression analysis incorporating √age as an independent variable against HFO rates or MI values as dependent variables. Three-dimensional atlases were developed to map the mean biomarker values across twenty neighboring non-epileptic sites among two age groups: young children (1-4 years) and older children (5-20 years).


Results: Mixed-model analysis revealed significant variations in HFO rates and MI values across different lobes, independent of included fixed effects (p< 0.01). Higher HFO rates were observed in the frontal (+0.772 per minute) and occipital (+0.964) lobes, with lower rates in the temporal (-0.886) and parietal (-0.513) lobes. MI values were similarly variable, with significant increases in the frontal (+0.007) and occipital (+0.032) lobes and decreases in the temporal lobe (-0.027). Regression analysis indicated a developmental diminution of HFO rates (p< 0.01) in the frontal (-0.530 per minute/√age), temporal (-0.548), and parietal (-0.592) lobes. Conversely, MI values showed a developmental increase (p< 0.01) in the frontal (+0.008/√age), temporal (+0.004), and occipital (+0.013) lobes. Figure 1 presents the topographical and developmental variations in both biomarkers during Isoflurane anesthesia.