Extraoperative electrical cortical stimulation: characteristics of motor responses and correlation with anatomical motor strip

Background: Extraoperative cortical stimulation (CS) of motor function is used to map the primary motor cortex in the setting of preoperative evaluation for epilepsy surgery. We aimed to characterize the properties and stimulation thresholds of extraoperative CSs with respect to the anatomical area stimulated. Furthermore, we studied the occurrence of afterdischarges (ADs) and the latency between cortical stimulation onset and occurrence of clinical signs in primary motor cortex compared to other cortical areas. Methods: Retrospective review of 1496 bipolar extraoperative CSs in 4 patients with frontal lobe epilepsy in whom intraoperative photographs of electrode placement and coregistration of electrode position to the patient's preoperative MRI were available. Characterization of the stimulation threshold of the anatomical area stimulated, afterdischarges and latency to the observation of a clinical sign upon review of video analysis. Results: In extraoperative CS mapping, stimulus thresholds to induce clinical motor responses are lower when compared to stimulus thresholds to induce non-motor responses (p<0.01). Thresholds vary depending on the anatomical area stimulated, with the precentral gyrus requiring lower stimulation intensities to produce a clinical motor response (p<0.001) and afterdischarges (p<0.001) compared to motor stimulations outside the precentral gyrus. Afterdischarges are more likely to occur with motor stimulations outside the precentral gyrus (χ 2 (1, n=123), p<0.05). Latencies to observe a distal clinical motor response were greater than to observe a proximal clinical motor response (p<0.001), with face motor responses showing the longest latencies to observe a clinical sign. Motor stimulations outside the precentral gyrus show longer latencies to observe an arm motor response on video analysis when compared to stimulations of motor responses from within the precentral gyrus (p<0.05). Within the precentral gyrus, stimulation of the hand knob was achieved with lower stimulation intensities compared to stimulation outside the hand knob (p<0.001). Conclusion: Differences in clinical response thresholds, afterdischarge properties and latencies to observe clinical signs in video analysis during extraoperative CS provide insight into the functional organization of the motor cortex. Findings might aid the optimization of stimulation procedures to achieve comprehensive extraoperative CS motor mapping in a clinical setting.