Abstracts

Rationale: Previous studies of epileptic spasms using scalp EEG reported that ictal events were associated with high-frequency oscillations (HFOs) or delta waves involving widespread regions. While ictal HFOs are of cortical origin in epileptic spasms, the origin of ictal delta waves is still debated. Some speculated that such ictal delta waves may be far-field potentials generated by subcortical structures such as brainstem, while others suggested that these waves may be near-field potentials generated by neocortex. We determined whether ictal HFOs were coupled with a delta phase, whether ictal delta waves were diffusely- or locally-synchronous signals, and whether the mode of coupling between HFOs and delta phase differed between ictal and interictal states.Methods: We studied 636 epileptic spasms and 546 interictal epileptiform HFOs seen in 11 children who underwent extraoperative electrocorticography recording. The phase and amplitude of delta waves were measured at the peak of ictal and interictal HFOs in the seizure onset sites.Results: Ictal delta waves were propagated from the seizure onset site to other regions. Ictal HFOs in the seizure onset site were tightly locked to the phase of slow wave at ? 1 Hz, especially at 0.3 Hz and around (p<0.001 on the Rayleigh s test of non-uniformity). In contrast, interictal HFOs in the seizure onset site were not tightly locked to the phase of slow wave at ? 1 Hz but to that of ? 3-Hz. The degree of cross-frequency coupling with 0.3 Hz slow-wave phase was larger on ictal HFOs (mean z-value: 18.7 vs 4.0; p=0.001 on the Wilcoxon Signed-Rank Test), while that with 5 Hz theta phase was larger on interictal HFOs (mean z-value: 1.9 vs 15.7; p=0.005).Conclusions: Ictal HFOs in epileptic spasms are tightly coupled with a delta-phase at ? 1 Hz. Ictal delta waves observed in the seizure onset site can be explained as near-field potentials locally generated by the epileptic cortex rather than far-field potentials generated by subcortical structures. Ictal delta waves in epileptic spasms may be generated by a mechanism different from that generating interictal HFOs-slow wave complexes.