In this study, we aimed to examine and compare the surface-EMG (sEMG) characteristics of scalp-EEG negative clonic seizures with those of non-epileptic movements mimicking clonic seizures, e.g. tremors.

Clonic seizures are characterized by twitching movements at a frequency of 0.2-5 Hz. These are produced by brief synchronized contractions of agonist and antagonistic muscles, followed by synchronized silent periods (SP).

We conducted a retrospective analysis at our center of patients diagnosed with scalp-EEG negative clonic seizures and tremors. We included patients monitored simultaneously with video-EEG and sEMG electrodes. sEMG electrodes were placed on agonist and antagonistic muscles using a standardized protocol developed at our institution. One typical event was analyzed per patient, focusing on sEMG characteristics like burst duration, amplitude, frequency, relationship between corresponding agonist and antagonistic bursts with temporal evolution.

Six patients were included in the study. Based on semiology, three were classified as clonic seizures and the other three as non-epileptic tremors. The following sEMG characteristics were observed: i)The sEMG bursts and corresponding SPs were always synchronous between agonists and antagonists in clonic seizures, as opposed to tremors which demonstrated an alternating pattern; ii)clonic seizures had a near-linear increase in sEMG burst amplitude and duration and no such pattern was seen in tremors; iii)sEMG burst frequency decreased through the duration of clonic seizure, while it remained mostly unchanged in tremors; iv)the sEMG burst frequency decrement in clonic seizures was mainly due to prolonged sEMG burst and SP duration.

Our study describes unique sEMG signature of scalp-EEG negative clonic seizures, which is quite distinct from tremors. These neurophysiological characteristics can be used to classify, diagnose, and treat patients with abnormal twitching movements, especially in the absence of a clear associated change on scalp-EEG.