Frequency content of EEG and EMG at seizure onset: possibility of removal of EMG artefact by digital filtering.

EEG recordings of epileptic seizures from scalp and sphenoidal electrodes are frequently obscured by EMG activity from contracting scalp muscles. We have examined the possibility of selectively filtering the EMG artefact in order to make apparent the activity of cerebral origin. It has been shown that, during voluntary contractions of scalp muscles, most of the energy of EMG activity is above 15-20 Hz. We have shown, in recordings from 50 patients, that rhythmic activity at the onset of seizures uncontaminated by artefact had almost always a fundamental frequency lower than 25 Hz. From these two observations, we concluded that total elimination of activity above 25 Hz would eliminate most of the EMG activity, with a minimal risk of eliminating rhythmic cerebral activity. Thirty-one seizure contaminated by EMG activity were analyzed. Spectral analysis was used to assess the presence or absence of rhythmic activity at seizure onset, in the presence of obscuring EMG artefact. The spatial and temporal distribution of such a rhythmic activity was then revealed by a sharp digital filter which did not introduce phase distortions. Seizures recorded on computer tape were played back on paper following filtering. In 16 of the 31 cases it was possible to clarify the originally obscured recordings. In 7 cases, it was found that no rhythmic cerebral activity was hidden by the EMG, a finding which was also important. In the last 8 cases, it was not possible to determine with certainty the origin, cerebral or muscular, of fast rhythmic activity present at seizure onset.