Abstracts

Rationale: Gamma rhythms are found in the ictal EEG of epileptic spasms in patients with West syndrome, and they are indicated to have a close relationship with the generative mechanisms of spasms. Therefore, we tried to investigate the pathophysiology of symptomatic generalized epilepsy (SGE) including Lennox-Gastaut syndrome (LGS) in older patients by exploring for gamma rhythms in the ictal EEG of their tonic seizures. Methods: The subjects of the present study were 27 patients (16 males, 11 females; age range: 3 years 1 month to 29 years 3 months) who had at least one digitally recorded tonic seizure with minimal artifacts in the past four years. The diagnosis was LGS in 17 patients and other types of SGE in the remaining 10. We performed time-frequency analysis with the Gabor transform to investigate the ictal EEG activity in the four derivation pairs of F3-C3, F4-C4, P3-O1 and P4-O2. Results: A total of 71 seizures were analyzed. Spasms in clusters were excluded from the study. The ictal EEG of the tonic seizures showed diffuse slow waves or spike-waves at onset in 33 seizures, and these discharges were followed by beta, alpha or theta activity in 20 of these seizures. The ictal discharges started as low-amplitude beta activity without slow waves or spike-waves in 10 seizures, and as high-amplitude beta or alpha activity in 28. Gamma rhythms with frequencies ranging from 40 to 90 Hz were detected by spectral analysis in a total of 25 seizures from 13 patients. In 24 (96%) of these seizures, ictal gamma rhythms were observed at the seizure onset corresponding to the visually identified slow waves or spike-waves. In one seizure, gamma rhythms were found in association with a transient suppression of the ictal beta activity. In the remaining 46 seizures, the ictal activity lacked clear gamma peaks in the time-frequency spectra. Conclusions: The detection of gamma rhythms in the ictal EEG of tonic seizures indicated that some tonic seizures might have common generative mechanisms with epileptic spasms in clusters and that these mechanisms are possibly related to the slow waves or spike-waves at the seizure onset.