Direct motor cortex mapping and monitoring: intraoperative CMAP analysis and its prognostic value for the postoperative neurological outcome

Background: Tumor invasion in functional cortex areas, tumor-related mass displacements and functional cortical reorganization can greatly impede intraoperative orientation in eloquent areas of the brain, such as the primary motor cortex (Brodman Area 4). Intraoperative neurophysiological examination methods are nowadays thus indispensable for surgery in or near the motor cortex. Methods: This study analyzed the data obtained from a total of 255 cerebral interventions for lesions with direct contact to (121) or immediately adjacent to (134) Brodman Area 4 in order to optimize stimulation parameters and to search for direct correlation between intraoperative potential changes and specific surgical maneuvers when using monopolar cortex stimulation (MCS) for electrocortical mapping and continuous intraoperative neurophysiological monitoring. Results: Compound muscle action potentials (CMAPs) were recorded from the thenar muscles and forearm flexors in accordance with the large representational area of the hand and forearm in Brodman Area 4. By optimizing the stimulation parameters in two steps (step 1: stimulation frequency and step 2: train sequence) MCS was successful in 91% (232/255) of the cases. Statistical analysis of the parameters latency, potential width and amplitude showed spontaneous latency prolongations and abrupt amplitude reductions as a reliable warning signal for direct involvement of the motor cortex or motor pathways. Conclusions: MCS must be considered a stimulation technique that enables reliable qualitative analysis of the recorded potentials, which may thus be regarded as directly predictive. However, there is no statistical prove that MCS can be used to quantify or validate the grade of paresis. Having performed a detailed analysis of the 232/255 monitoring cases, the authors are of the opinion that a latency prolongation of >15% and/or an amplitude reduction of >80% should be established as significant potential changes requiring action. A variety of surgical and non-surgical influences can be reason for false positive or false negative measurements.