Scalp high frequency oscillations (HFOs) in absence epilepsy: An independent component analysis (ICA) based approach

Abstract

Background: High frequency oscillations (HFOs) have provided a new insight in understanding ictogenesis and seizure localization. In absence seizures, HFOs were predominantly localized in the medial prefrontal cortex (MPFC) in MEG studies.

Methods: We studied HFOs (80-250 Hz) in scalp EEGs in patients with absence epilepsy, and evaluated their frequency bandwidth and spatial-temporal distribution. EEG of 9 patients with absence epilepsy (CAE:JAE-8:1; M:F=7:2; age: 8.1 ± 2.1 years; age at onset: 6.6 ± 1.3 years) were evaluated with scalp EEG (sampling rate: 2048 Hz). Finite impulse response filters on the unipolar or longitudinal bipolar montages were band-passed between 80 and 250 Hz using Cartool(®) and EEGLAB(®). Sensitivity and paper speed were modified accordingly to study the HFOs. Thousand four hundred and thirty eight artifact free 'spike-wave' epileptiform discharges were analyzed. Sleep ictal SW discharges, defined as runs of 3 Hz GSWDs lasting ≥ 4s in stages 1 and 2 sleep, were analyzed by independent component analysis and component time-frequency and channel time-frequency maps using cyclical tapering wavelet transform. A total of 926 HFOs were identified of 1438 GSWDS.

Results: The HFOs were associated with inter-ictal generalized spike-wave discharges (IiGSWDs-241/454), ictal GSWDs (IcGSWD-634/884), sporadic SWDs (sSWDs-51/100). The percentage of HFOs was higher in IcGSWD when compared to both IiGSWDs and sporadic SWDs together (χ(2)=52.864, d.o.f=1, p<0.0001). The mean frequency of HFOs was 96.4 ± 10.4 Hz. A channel wise analysis showed the maximum HFO band width in the right fronto-central region (F4=28 Hz, C4=24 Hz).

Conclusion: Narrow bandwidth interictal and ictal HFOs can be identified in scalp EEG of patients with absence epilepsy. Further characterization of the various properties of pHFOs will be helpful in opening up a domain of clinical evaluation and interpretation of the various epileptic disorders. To improve the insights into the onset and spread of absence seizures, and to study the network properties, one could analyze the HFOs in high density EEG arrays with multimodal integration using MEG or fMRI.

Keywords: Absence seizures; High frequency oscillations; Independent component analysis; Ripples.