Significance and usefulness of corticospinal motor evoked potential monitoring for lesions adjacent to primary motor cortex

: This study evaluated the usefulness of intraoperative corticospinal motor evoked potential (MEP) monitoring in preventing postoperative motor deficits, and whether this procedure contributed to surgery on intrinsic brain lesions in the vicinity of the motor area. The subjects were 45 patients with brain tumors located in and around the primary motor area. MEP was recorded through the cervical epidural electrodes in response to stimulation of the motor cortex. The amplitude of D-response of MEP was compared at the beginning and at the end of surgery. Then MEP changes were divided into five groups; "increase", "no change", "diminish", "decrease" and "disappear". We used the DeJong classification for qualitative analysis of motor function, and reviewed these findings in relation to the change in MEP. It was possible to record MEP when the preoperative motor weakness was DeJong 3 or better. There was no postoperative motor deficit when the MEP amplitude was preserved at better than 50% of a control amplitude. If the amplitude decreased to less than 50%, motor deficits were encoutered. When MEP amplitude increased during the surgery, preoperative motor weakness was improved after the surgery. It is concluded that there is little possibility of causing motor deficits even if tumor removal is aggressively pursued, as long as the amplitude of D-response remains at 50% or more of the baseline. This monitoring procedure is expected to improve the overall surgical results in patients with intrinsic brain tumors around the motor area.