S122. Intraoperative electrical stimulation to identify the corticospinal tract and dorsal column during spinal cord tumor surgery

Introduction Anatomical identification of the corticospinal tract (CT) and the dorsal column (DC) of the exposed spinal cord is difficult when anatomical landmarks are distorted by tumour growth. Intraoperative neurophysiological identification is complicated by the fact that direct stimulation of the DC may result in muscle motor responses due to the centrally activated H-reflex. This study aimed to provide a technique for intraoperative neurophysiological differentiation between CT and DC in the surgical exposed spinal cord. Methods Recordings were obtained from 32 consecutive patients undergoing intramedullary or intradural-extramedullary spinal cord tumour surgery from 07/2015 to 03/2017. A double train stimulation paradigm with an intertrain interval of 60 ms was devised by a hand held probe with simultaneous recording of responses from limb muscles. Results In non spastic patients (55% of cohort) an identical second response was noted following the first response after CT stimulation, but the second response was absent after DC stimulation. In patients with preexisting spasticity (45%), CT stimulation again resulted in two identical responses, whereas DC stimulation generated a second response that differed substantially from the first one. The recovery times of interneurons in the spinal cord grey matter for the CT were much shorter than those for the DC. Therefore, when a second stimulus train was applied 60 ms after the first, the CT-fibre interneurons had already recovered ready to generate a second response, whereas the DC interneurons were still in the refractory period. Conclusion Intraoperative mapping of the surgical exposed spinal cord using double-train stimulation paradigm allows neurophysiological distinction of CT from DC pathways during spinal cord surgery in patients with and without preexisting spasticity.