This exploratory observational study characterized high-frequency electromyographic (EMG) components embedded in scalp electroencephalography (EEG) recordings during episodes of focal-to-bilateral tonic–clonic seizures (FBTCS) and evaluated their potential for FBTCS detection and lateralization.

We retrospectively analyzed 21 FBTCS episodes with head version in 16 patients who underwent long-term video–EEG monitoring and simultaneous bilateral deltoid EMG. High-frequency components (64–256 Hz) were extracted from EEG and EMG signals. The mean integrated root mean square (iRMS) was calculated to identify EEG electrode pairs showing the highest sensitivity to EMG activity embedded in scalp EEG. Power spectral density (PSD) and cross-correlation analyses were used to assess signal similarity. Mean iRMS amplitude and RMS rise slope were compared between ipsiversive and contraversive (head-version side) electrode pairs during head version.

High-frequency EMG components were most prominent at T3/T4 electrodes under Cz and average references, and they showed strong correlations with deltoid EMG activity (median Pearson’s r > 0.90, p < 0.05) as revealed by PSD and cross-correlation analyses. During head version, mean iRMS was significantly higher at contraversive electrodes relative to the Cz, average and Laplacian references, while RMS rise slope was significantly higher relative to Cz and average references.

High-frequency EMG components embedded in the scalp EEG, particularly at T3/T4, showed strong similarity to deltoid EMG signals under Cz and average referencing and reflected greater contraversive EMG-related activity during ictal head version. These findings suggest that EEG-embedded EMG activity may provide supplementary electrophysiological markers relevant to the detection and lateralization of FBTCS.