Intraoperative neurophysiology of the motor cortex and corticospinal tracts: advantages, limits and future perspectives.

Background. Brain surgery in motor areas requires a balance between radical surgical resection and risk of postoperative motor deficits. Intraoperative neurophysiological monitoring, especially with motor evoked potentials (MEPs), provides a valuable help in such conditions; however, the correlation between MEP amplitude changes and clinical outcome is not always clear. A stronger neurophysiological predictor of outcome is therefore desirable. Objectives. The aims of this Thesis are: a. to analyze the limits of MEP monitoring during brain surgery in motor areas with a special attention to the confounding factors that may alter the interpretation of MEP changes during surgery; b. to verify and confirm the role of a strong neurophysiological predictor of outcome - the D-wave monitoring - during surgery for intramedullary spinal cord tumor; c. to apply the D-wave monitoring during brain surgery in motor areas. The Thesis is divided in three sections according to the aforementioned objectives. Materials and Methods. In the first section, a consecutive cohort of 157 patients submitted to surgical removal of a tumour adjacent to the motor areas and CST with simultaneous subcortical motor mapping and DCS MEP monitoring were analysed. Motor function was assessed the day after surgery, at discharge, and at further follow-up postoperatively. A post-hoc analysis was conducted in order to analyse possible pre- and postoperative confounding factors during MEP changes interpretation. In the second section, a consecutive cohort of 219 patients submitted to surgery for intramedullary spinal cord tumors (ISCTs) with simultaneous muscle MEP and D-wave monitoring were analysed. Motor function was assessed the day after surgery, at discharge, and at further follow-up postoperatively. A post-hoc analysis was performed in order to verify the reliability of D-wave monitoring as a strong outcome predictor. In the third section, we report the experience of 3 consecutive cases operated on for brain tumors in motor areas with the aid of D-wave monitoring. Results. Section I: the location of the tumour in the prefrontal cortex and along the CST are related with a higher rate of postoperative motor deficits (p=0.04 and p=0.008, respectively); for tumours located in the prefrontal cortex, 53% of patients showed new motor deficit with changes of MEP in 16% of them. Different muscles showed different capability to predict new motor deficits; furthermore, the higher is the number of muscles with MEP amplitude below the threshold, the higher is the probability of a new stable motor deficit. Section II: D-wave monitoring is a valuable help during surgery for ISCTs and show a sensitivity of 33.3%, a specificity of 99.2%; positive predictive value is 50% and negative predictive value is 98.4%. The accuracy calculated is 97.6%. Section III: we were able to record TES D-wave in patients 2 and 3; in patient 1 we obtained the D-wave only with TES of the hemisphere contralateral to the tumour. It was not possible to obtain a clear D-wave from DCS in all three patients. In patients 2 and 3 it was possible to obtain the D-wave through subcortical bipolar stimulation along CST. Conclusions. Intraoperative neurophysiology is a valuable help during surgery in motor areas. MEP monitoring provide useful and reliable information during surgery, but it is not always easy to analyse the relationship between intraoperative changes and clinical outcome. D-wave monitoring is a well-known technique and our results confirmed its role of strong outcome predictor. The application of this technique for brain surgery can help to overcome the limits of MEP monitoring alone.