Low-threshold monopolar motor mapping for resection of primary motor cortex tumors

Microsurgery within eloquent cortex is a controversial approach because of the high risk of permanent neurological deficit. Few data exist showing the relationship between the mapping stimulation intensity required for eliciting a muscle motor evoked potential and the distance to the motor neurons; furthermore, the motor threshold at which no deficit occurs remains to be defined.

To evaluate the safety of low threshold motor evoked potential mapping for tumor resection close to the primary motor cortex.

Fourteen patients undergoing tumor surgery were included. Motor threshold was defined as the stimulation intensity that elicited motor evoked potentials from target muscles (amplitude > 30 μV). Monopolar high-frequency motor mapping with train-of-5 stimuli (HF-TOF; pulse duration = 500 microseconds; interstimulus interval = 4.0 milliseconds; 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.

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 to 6 mA, indicating close proximity of motor neurons. Postoperatively, 12 patients had no new motor deficit, 1 patient had a minor new temporary deficit (M4+, National Institutes of Health Stroke Scale 1), and another patient had a minor new permanent deficit (M4+, National Institutes of Health Stroke Scale 2). Thirteen patients had complete or gross total resection.

These preliminary data demonstrate that a monopolar HF-TOF threshold > 3 mA was not associated with a significant new motor deficit.