Positive Intraoperative Changes of Muscle Motor Evoked Potentials in Patients with Intramedullary Cervicospinal Tumors
Introduction Intraoperative motor evoked potentials (MEPs) recorded as muscle motor evoked responses (MMEPs) and D wave are performed to avoid corticospinal tract (CST) lesions in spinal surgery.
Aim Not only CST but also the collaborative pathways play the significant role in conduction of evoked motor impulses. Using MMEPs, we can monitor the complete motor networking response.
Material and Methods Patients (17), 10 males and 7 females with significant motor deficit (tetraparesis or paraparesis), due to intramedullary spinal tumor located within the C2 to Th11 spinal levels, were operated on. In selected muscles, we measured muscle strength deficit preoperatively and postoperatively using Medical Research Council Scale (MRCS). During the neuromonitoring, we performed transcranial electrical stimulation (C3–C4/C4–C3) using short train stimuli technique while recording D wave, and MMEPs. For MMEPs recording, we used needle electrodes inserted preoperatively during patients' voluntary muscle contraction. We measured MMEP parameters and D wave, using unchanged low-threshold stimulation, at 6 points during the surgery.
Results During the middle part of tumor resection, we recorded the gradual rise in MMEP amplitude, duration, and number of phases, especially for hand and foot muscles, but unchanged D wave values. At the end of the tumor resection, we measured MMEP values again, and obtained the same results. Postoperatively, in 7 days' time, muscle strength of our patients improved, and was measured one point higher on the MRC-S, compared with the preoperative findings.
Conclusion The changes of MMEPs parameters during the tumor resection were good predictors of positive postoperative results. D wave remains reliable indicator of complete CST function. However, intraoperatively recorded improved MMEP values, with unchanged D wave, support the theory of alternative pathways connected with motoneurons. The influence of these pathways on evoked motor contraction seems to produce improved postoperative motor status later on.