Surface-electromyography characteristics of clonic seizures with no scalp-EEG correlate: A comparative analysis with tremors.

Introduction: Clonic seizures are characterized by twitching movements at a frequency of 0.2-5 Hz. The clonic "twitch" is produced by a brief synchronized contraction of agonist and antagonist muscles, followed by a synchronized silent period. In this study, we aimed to compare the surface-electromyography (sEMG) characteristics of scalp-EEG negative clonic seizures with those of nonepileptic movements like tremors that can resemble clonic seizures.

Methods: We retrospectively identified patients who were diagnosed with scalp-EEG negative clonic seizures or tremors. We only included patients (n = 6) who were monitored simultaneously with video-EEG and sEMG electrodes. sEMG was placed on agonist and antagonist muscles of the affected extremity using a standardized placement system developed at our institution. We analyzed the following characteristics of sEMG bursts: the relationship between agonist and antagonist muscles and the temporal evolution of burst duration, burst amplitude, and burst frequency.

Results: The following sEMG characteristics were observed: (i) sEMG bursts and corresponding silent periods were synchronous between agonist and antagonist muscles in clonic seizures. In tremors, an alternating pattern was seen. (ii) sEMG burst amplitude increased during the first 10 s of clonic seizures. There was no significant change in tremors. (iii) sEMG burst duration increased from the beginning to end of clonic seizures. There was no significant change in tremors. (iv) sEMG burst frequency decreased from the beginning to end of clonic seizures due to increased burst and silent period duration. There was no consistent change in burst frequency in tremors. (v) sEMG burst duration of ≥250 ms was indicative of a clonic seizure with a >90% positive predictive value.

Conclusions: Our study describes characteristic sEMG features of clonic seizures without scalp-EEG correlates, which can be used as an objective biomarker in distinguishing these from nonepileptic movements such as tremors.