Object: The goal of this study was to compare motor evoked potentials recorded from muscles (muscle MEPs or corticomuscular MEPs) with corticospinal MEPs recorded from the cervical epidural space (spinal MEPs or corticospinal MEPs) to assess their efficacy in the intraoperative monitoring of motor function.
Methods: Muscle and spinal MEPs were simultaneously recorded during surgery in 80 patients harboring brain tumors. Each case was assigned to one of four groups according to final changes in the MEPs: (1) Group A, in which there was an increased amplitude in the muscle MEP with an increased 13 wave amplitude (12 cases); (2) Group B, in which there was no significant change in the MEP (43 cases); (3) Group C, in which there was a decreased muscle MEP amplitude (< 35% of the control) with a decreased I wave amplitude but an unchanged D wave (15 cases); or (4) Group D, in which there was an absent muscle MEP with a decreased D wave amplitude (10 cases). In patients in Group A, the increase in the amplitude of the muscle MEP (range of increase 128-280%, mean increase 188.75 +/- 48.79%) was well correlated with the increase in the 13 wave in corticospinal MEPs. Most of these patterns were observed in patients harboring meningiomas (10 [83.3%] of 12 cases). Patients in Group B displayed no changes in muscle and corticospinal MEPs and no signs of postoperative neurological deterioration. Patients in Group C showed a substantial decrease in the amplitude of the muscle MEP (range of decrease 5.3-34.8% based on the control waveform, mean change 21.81 +/- 10.93%) without deterioration in the corticospinal D wave, and exhibited severe immediate postoperative motor dysfunction. This indicates dysfunction of the cortical gray matter, including the motor cortices, which are supposed to generate I waves. Patients in Group D exhibited decreases in the corticospinal D wave (range of decrease 21.5-55%, mean decrease 39.75 +/- 11.45%) and an immediate cessation of the muscle MEP as well as severe permanent motor paresis.
Conclusions: These results indicate that, during surgery, monitoring of corticomuscular MEPs (which are related to I waves) is a much more sensitive method for the detection of immediate motor cortical damage than monitoring of corticospinal MEPs (D wave).