gms | German Medical Science

Objective: Microsurgery within motor cortex is a controversial approach due to the high-risk of permanent motor deficit. Few data exist showing the relationship between the mapping stimulation intensity required for eliciting a muscle motor evoked potential (MEP) and distance to the motor neurons; furthermore, the motor threshold at which no deficit occurs remains to be defined. Therefore the objective was to evaluate the safety of low threshold MEP mapping for tumor resection close to the primary motor cortex.

Methods: Fourteen patients undergoing surgery for tumors involving important parts of the precentral gyrus were included. Motor threshold was defined as the stimulation intensity that elicited MEPs from target muscles at a minimum amplitude > 0.030 mV under total intravenous anesthesia. Monopolar high-frequency motor mapping with train-of-five stimuli (HF-TOF) (pulse duration = 0.5 ms; interstimulus interval = 4.0 ms; frequency = 250 Hz) was used to determine motor response negative sites where incision and dissection could be performed. At sites negative to 3 mA HF-TOF stimulation, the tumor was resected.

Results: HF-TOF mapping localized the motor neurons within the precentral gyrus by using variable, low stimulation intensities. The lowest motor thresholds after final resection ranged from 3 mA–6 mA (mean 3.8 mA, SD 1.0 mA) indicating close proximity of motor neurons in all cases. Postoperatively, twelve patients had no new motor deficit, one patient had a minor new temporary deficit (M4+, NIHSS 1) and another patient, a minor new permanent deficit (M4+, NIHSS 2). Thirteen patients had complete or gross total resection shown in early postoperative MRI.

Conclusions: These preliminary data demonstrate that a monopolar HF-TOF threshold higher than 3 mA was not associated with a significant new motor deficit in our series.