Bilateral synchronous gamma range firing of bulbar motor units during tonic seizures

We present a patient with tonic seizures of the bulbar muscles. Video-EEG recording during the multiple attacks showed no ictal EEG abnormalities. The tonic seizures were clearly visible as EMG artefacts on the EEG recording and consisted of 3-6 symmetric tonic bursts of EMG activity of 2-4 seconds and an interval of 2-6 seconds. Spectral analysis showed a very distinct, bilateral synchronous, dominant gamma peak on the EMG spectra during the seizures with a cross correlation of 0.67 between left and right without a time lag. We discuss whether this peak originates from the firing frequencies of the bulbar motor units, and whether this is consistent with the Piper rhythm described during maximal voluntary contraction in controls and related to firing in the gamma range of the cortical motor neurons. We conclude that the EMG artefacts recorded on EEG during seizures can be informative with regards to the type and pathophysiology of epilepsy.