Comparison between motor evoked potential recording and fiber tracking for estimating pyramidal tracts near brain tumors

Object. The utility of subcortical electrical stimulation and fiber tracking were compared to estimate the pyramidal tract near brain tumors. Methods. In 22 patients, the white matter at the bottom of a tumor was electrically stimulated near the fiber tracking of the pyramidal tract shown on a neuronavigation system. The distance between the center of the fiber tracking of these tracts and the stimulated region was measured and defined as the motor evoked potential (MEP) response. The MEP was consistently produced at distances less than 7 min (six patients), but was consistently absent at distances more than 13 mm (seven patients) from the fiber tracking of the pyramidal tracts. In the nine patients in whom the distance was between 8 and 12 mm, an MEP was elicited when stimulation was applied at the level of the corona radiata. Motor function was preserved or even improved with appropriate tumor resection in all patients. Conclusions. The anteroposteriorly running superior longitudinal fasciculus could cause complications in the fiber tracking of upper-extremity motor pathways at the level of the corona radiata. During resection of tumors located near the corona radiata, subcortical electrical stimulation should be applied at some distance from the pyramidal tract, as estimated by fiber tracking.