Phase reversal technique decreases cortical stimulation time during motor mapping

Neurophysiologic mapping of the primary motor cortex (PMC) is commonly used in supratentorial surgery. Electrical cortical stimulation is guided by anatomic landmarks towards the precentral gyrus, with recording of the triggered primary motor responses (TPMR) in the contralateral hemibody. Thus, factors such as distortion of the pericentral anatomy, small surgical fields, brain shifts and miscalibrated neuronavigational systems may lengthen the process and result in unnecessary stimulations, increasing the probability of triggering seizures. We hypothesized that central sulcus localization via the median somatosensory evoked potentials phase reversal technique (MSSEP PRT) accurately guides the surgeon, resulting in prompt identification of the PMC with minimal electrical stimulation. Multivariate Cox regression was used to study the impact of MSSEP PRT on time spent performing electrical cortical stimulation to TPMR. The analysis was adjusted for presence of increased cortical excitability, high motor thresholds, lesions close to PMC and fMRI data, in 100 consecutive standardized motor mapping procedures for brain tumor resection and epilepsy surgery. Phase reversal and change morphology of the recorded somatosensory evoked potentials quadrupled (hazard ratio [HR] 4.13, p < 0.0001) and doubled (HR 2.14, p = 0.02) the rate of obtaining TPMR, respectively. A 1 mA increase in motor threshold decreased the rate by 9% (HR 0.91, p = 0.0002). Afterdischarges triggered before TPMR and lesions in close proximity to PMC decreased the rate of TPMR by 76% (HR 0.23, p < 0.0001) and 48% (HR 0.52, p = 0.04), respectively. Informative PRT decreases stimulation time. Afterdischarges triggered before TPMR, high motor thresholds and lesions close to the PMC increase it.