P 93 TMS recruitment curve and cortical silent period analysis: a sensitive tool to detect imminent motor deficits in brain tumor patients

Abstract

Background One of the challenges in surgery of tumors in motor eloquent areas is the individual risk assessment for postoperative motor disorder. Previously, we developed a predictive model that allows for objective evaluation of this risk prior to surgery based on topographical and neurophysiologial data derived from investigation with nTMS (navigated Transcranial Magnetic Stimulation). The input variables of the model are: (1) presence of infiltration of the primary motor cortex by the tumor, (2) the minimal distance of the tumor to the corticalspinal tract and (3) the ratio of the bihemispheric resting motor threshold (RMT). The aim of this study is the improvement of the prognostic power of the aforementioned model by including additional neurophysiological parameters: Recruitment Curve (RC) and Cortical Silent Period (CSP). Methods nTMS was used to map the cortical motor representation area of the FDI muscle in 149 patients with malignant gliomas in presumed motor eloquent areas prior to surgery. In addition to RMT determination, the RC was measured by applying 70 stimuli of randomized intensity between 80% and 150% of the RMT over the FDI hot spot. CSP was determined by applying 10 stimuli of 130% RMT intensity during tonic muscle contraction and measuring the duration of the plateau phase from EMG recording. Each metric was measured bihemispherically. Finally, the ratio between left and right hemisphere is calculated. For definition of a physiologic range, a control group of 17 healthy volunteers was investigated with the same protocol. In order to quantify the motor function we evaluated the muscle strength according to the British Medical Research Council Scale preoperatively and postoperatively after seven days and three months. Results In respect to the postoperative motor outcome after seven days, the RMT Ratio’s predictive value was confirmed (p = 0,036). Yet, in respect to the postoperative motor outcome after three months no significant correlation could be found (p = 0,16). For the CSP Ratio no correlation to the postoperative motor status was observed. A tendency to an increased presurgical cortical GABAergic inhibition in patients with new postoperative deficits was recognizable, but is was significant for neither the seventh postoperative day (p = 0,19) nor after three months (p = 0,75). For the presurgical RC Ratio on the other hand, a significant correlation to the preoperative motor status (p = 0,029) as well as for postoperative deterioration of motor function after three months in patients with cortico-subcortical tumors was found (p = 0,004). Conclusions Addition of the RC Ratio significantly improved the value of the nTMS based predictive model, since the RC Ratio allows for prediction of new motor deficits 3 month after surgery. Addition of the CSP Ratio on the other hand did not, based on the current analysis, add predictive power to the model. The predictive power of the previously established correlation between presurgical RMT Ratio and motor status on day 7 was improved based on the larger study sample. The current results demonstrate that the RC Ratio and the RMT Ratio are sensitive markers to predict the motor outcome 7 days and 3 month after surgery of brain tumors in presumed motor eloquent location and can therefore supply valuable information during preoperative risk-benefit-balancing.