Purpose: Complete yet safe resection close to motor areas in medically intractable epilepsy requires functional information. New deficit may occur despite preservation of motor cortex, e.g., through vascular compromise. Here, we explore for the first time the feasibility, safety, and the clinical value of continuous motor-evoked potential (MEP) monitoring in focal epilepsy surgery.
Methods: High-frequency stimulation for MEP monitoring was performed during 100 consecutive lesionectomies critically related to motor areas and pathways. Extraoperative motor cortex mapping was performed in 27 of these cases via chronically implanted subdural grid electrodes. MEP monitoring results, postoperative motor outcome, and seizure control were correlated in a prospective observational design.
Results: Reliable MEP monitoring was achieved in 86 cases. Young age was the only discernible cause of unsuccessful recordings. Seizures from cortex stimulation did not occur. MEP changes (36%) predicted new motor deficit (17%) in all cases except purely cortical lesions. MEP changes predicted occurrence and permanence of new pareses. New deficit was significantly more frequent without (as compared with) successful monitoring (43% vs. 17%); permanently severe pareses from ischemia occurred only without MEPs (21% vs. 0%). Complete seizure control was significantly more frequent in successfully monitored cases (60% vs. 31%). Even with extraoperative motor mapping, severe paresis occurred only among cases with unsuccessful MEPs.
Conclusions: Continuous MEP monitoring in epilepsy surgery is feasible and safe. It reflects motor function complementarily to the localizing motor mapping results. Successful MEP monitoring correlates with unimpaired motor outcome and full seizure control.