S28-3 D-wave monitoring in brain tumor surgery

through: (1) Somatosensory evoked potential phase reversal technique; (2) DCS with a short train of 5 7 monopolar stimuli (0.5ms duration, ISI 4.1, intensity up to 20mA) at 1 2Hz. To monitor motor pathways during tumor removal, MEPs are recorded from controlateral limb muscles after either DCS and/or transcranial electrical stimulation. When approaching tumor borders, MEP monitoring is combined with periodical direct subcortical stimulation (DSS) to localize the corticospinal tract (CT) and guide resection.A more than 75% drop in transcranial MEP amplitude at the end of surgery, and a DSS thresholds lower than 3 4mA are associated to post-operative neurological worsening. However these two neurophysiological warning signs are not significantly related and do not necessarily identify the same patients, suggesting that continuous MEP monitoring and periodical subcortical mapping should be combined to minimize risks. Recently, thanks to the introduction of diffusion tensor imaging and neuronavigation, there has been an increasing interest for the correlation of fiber tractography with subcortical stimulation. This work is providing some degree of reliability in the assessment of subcortical current spreading. Preliminary data suggest that 1mA = 1mm may be an acceptable practical parameter to judge on the distance from the CT, when using subcortical mapping.