Ictal high‐frequency oscillations at 80–200 Hz coupled with delta phase in epileptic spasms

Summary Previous studies of epileptic spasms reported that ictal events were associated with high‐frequency oscillations (HFOs) or delta waves involving widespread regions. We determined whether ictal HFOs at 80–200 Hz were coupled with a phase of slow‐wave, whether ictal slow‐waves were diffusely or locally synchronous signals, and whether the mode of coupling between HFOs and slow‐wave phases differed between ictal and interictal states. We studied 11 children who underwent extraoperative electrocorticography (ECoG) recording. The phases and amplitudes of slow‐waves were measured at the peak of ictal and interictal HFOs in the seizure‐onset sites. Ictal HFOs were locked tightly to the phase of slow‐wave at ≤1 Hz. Ictal slow‐waves propagated from the seizure‐onset site to other regions. In contrast, interictal HFOs in the seizure‐onset site were loosely locked to the phase of slow‐wave at ≤1 Hz but tightly to that of ≥3‐Hz. Ictal slow‐waves coupled with HFOs can be explained as near‐field and locally synchronized potentials generated by the neocortex rather than far‐field potentials generated by subcortical structures. Ictal slow‐waves in epileptic spasms may be generated by a mechanism different from what generates interictal HFOs–slow‐wave complexes.