Patterns of muscle activation during generalized tonic and tonic-clonic epileptic seizures

Tonic seizures and the tonic phase of tonic-clonic epileptic seizures are defined as "sustained tonic" muscle contraction lasting a few seconds to minutes. Visual inspection of the surface electromyogram (EMG) during seizures contributed considerably to a better understanding and accurate diagnosis of several seizure types. However, quantitative analysis of the surface EMG during the epileptic seizures has received surprisingly little attention until now. The aim of our study was to elucidate the pathomechanism of the tonic muscle activation during epileptic seizures.

Surface EMG was recorded from the deltoid muscles, on both sides, during 63 seizures from 20 patients with epilepsy (10 with generalized tonic and 10 with tonic-clonic seizures). Twenty age- and gender-matched normal controls simulated 100 generalized tonic seizures. To characterize the signal properties we calculated the root mean square (RMS) of the amplitudes, the median frequency (MF), and the coherence. Based on the spectrograms of both epileptic and simulated seizures, we chose to determine the relative spectral power (RP) in the higher (100-500 Hz) frequency domain.

During the tonic seizures there was a significant shift toward higher frequencies, expressed by an increase in the MF and the RP (100-500 Hz). The amplitude characteristic of the signal (RMS) was significantly higher during the tonic phase of the tonic-clonic seizures as compared to the simulated ones, whereas the RMS of the tonic seizures was significantly lower than the simulated ones. The EMG-EMG coherence was significantly higher during the epileptic seizures (both types) as compared to the simulated ones.

Our results indicate that the mechanism of muscle activation during epileptic seizures is different from the physiologic one. Furthermore the sustained muscle activation during the tonic phase of tonic-clonic seizures is different from that during tonic seizures: The tonic phase of tonic-clonic seizures is characterized by increased amplitude of the signal, whereas tonic seizures are produced by a significant increase in the frequency of the signal.