- •Both clonic and tonic responses can be elicited from the primary motor cortex.
- •Type I clonic responses (<20 Hz stimulation) consist of simple EMG bursts.
- •Type II clonic responses (≥20 Hz stimulation) consist of complex EMG bursts.
- •Increasing the intensity and/or frequency of stimulation generates tonic responses.
- •These motor responses, including GTC seizures, induced by cortical stimulation rely on corticospinal tract.
To study the neurophysiology of motor responses elicited by electrical stimulation of the primary motor cortex.
We studied motor responses in four patients undergoing invasive epilepsy monitoring and functional cortical mapping via electrical cortical stimulation using surface EMG electrodes. In addition, polygraphic analysis of intracranial EEG and EMG during bilateral tonic-clonic seizures, induced by cortical stimulation, was performed in two patients.
(a) Electrical cortical stimulation: The motor responses were classified as clonic, jittery, and tonic. The clonic responses were characterized by synchronous EMG bursts of agonist and antagonistic muscles, alternating with silent periods. At stimulation frequencies of <20 Hz, EMG bursts were of ≤50 ms duration (Type I clonic). At stimulation frequencies of 20–50 Hz, EMG bursts were of >50 ms duration and had a complex morphology (Type II clonic). Increasing the current intensity at a constant frequency converted clonic responses into jittery and tonic contractions.
(b) Bilateral tonic-clonic seizures: The intracranial EEG showed continuous fast spiking activity during the tonic phase along with interference pattern on surface EMG. The clonic phase was characterized by a polyspike-and-slow wave pattern. The polyspikes were time-locked with the synchronous EMG bursts of agonists and antagonists and the slow waves were time-locked with silent periods.
These results suggest that epileptic activity involving the primary motor cortex can produce a continuum of motor responses ranging from type I clonic, type II clonic, and tonic responses to bilateral tonic-clonic seizures. This continuum is related to the frequency and intensity of the epileptiform discharges with tonic seizures representing the highest end of the spectrum.
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Published online: March 24, 2023
Accepted: March 14, 2023
Received in revised form: March 13, 2023
Received: January 30, 2023
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