Strong coupling between slow oscillations and wide fast ripples in children with epileptic spasms: Investigation of modulation index and occurrence rate

Objective: Epileptic spasms (ES) often become drug-resistant. To reveal the electrophysiological difference between children with ES (ES+) and without ES (ES−), we compared the occurrence rate (OR) of high-frequency oscillations (HFOs) and the modulation index (MI) of coupling between slow and fast oscillations. In ES+, we hypothesized that (1) pathological HFOs are more widely distributed and (2) slow oscillations show stronger coupling with pathological HFOs than in ES−. Methods: We retrospectively reviewed 24 children with drug-resistant multilobar onset epilepsy, who underwent intracranial video electroencephalography prior to multilobar resections. We measured the OR of HFOs and determined the electrodes with a high rate of HFOs by cluster analysis. We calculated MI, which reflects the degree of coupling between HFO (ripple/fast ripple [FR]) amplitude and 5 different frequency bands of delta and theta activities (0.5-1 Hz, 1-2 Hz, 2-3 Hz, 3-4 Hz, 4-8 Hz). Results: In ES+ (n = 10), the OR_{( FR s)}, the number of electrodes with high-rate FRs, and the MI_{( FR s & 3-4 Hz)} in all electrodes were significantly higher than in ES− (n = 14). In both the ES+ and ES− groups, MI_{(ripples/ FR s & 3-4 Hz)} was the highest among the 5 frequency bands. Within the good seizure outcome group, the OR_{( FR s)} and the MI_{( FR s & 3-4 Hz)} in the resected area in ES+ were significantly higher than in ES− (OR_{[FR s]}, P =.04; MI_{[ FR s & 3-4 Hz]}, P =.04). Significance: In ES+, the larger number of high-rate FR electrodes indicates more widespread epileptogenicity than in ES−. High values of OR_{( FR s)} and MI_{( FR s & 3-4 Hz)} in ES+ compared to ES− are a signature of the severity of epileptogenicity. We proved that ES+ children who achieved seizure freedom following multilobar resections exhibited strong coupling between slow oscillations and FRs.