Subcortical mapping and motor evoked potential monitoring in brain tumor surgery.

INTRODUCTION: During the removal of brain lesions involving motor areas it is critical to localize and preserve motor tracts subcortically. With this aim, either subcortical mapping using 60 Hz Penfield's technique or continuous motor evoked potential (MEP) monitoring using the monopolar short train technique has been reported, but their combined use remains anecdotal. We report our experience in 160 patients where both MEP monitoring and cortical/subcortical mapping were used to preserve motor function intraoperatively. METHODS: In the past 7 years we performed intraoperative neurophysiological monitoring in 160 patients operated on for brain tumors, the majority being high grade gliomas. After opening the dura, to identify cortical motor areas we routinely used: 1) the somatosensory evoked potential phase reversal technique; 2) direct cortical stimulation (DCS) with a short train of 5 to 7 monopolar square-wave stimuli of 0.5 ms duration, ISI 4.1, at 1-2 Hz and intensity up to 20 mA. Then, to monitor motor pathways during tumor removal, MEPs were recorded from controlateral limb muscles after either DCS from a strip electrode (105 cases) and/or transcranial electrical stimulation (145 cases). When approaching tumor borders, MEP monitoring was combined with periodical direct subcortical stimulation (DSS) (64 cases) to localize cortico-spinal tracts and guide resection. RESULTS: Two neurophysiological parameters were associated to some degree of post-operative neurological worsening: First, a significant drop (> 75%) in transcranial MEP amplitude at the end of surgery; second, a DSS thresholds lower than 3-4 mA. However these two warning signs were not significantly related. CONCLUSION: Our neurophysiological warning criteria are similar to those reported by other authors. However we proved that these criteria do not necessarily identify the same patients. Furthermore, a negative subcortical mapping may encourage tumor removal when MEPs are stable. Vice versa, since injury to the motor pathways may be vascular rather than mechanical, MEP monitoring may identify reversible vascular derangements not recognizable by mapping techniques only. We therefore suggest the combined use of continuous MEP monitoring and periodical subcortical mapping to minimize risks