The Seven-Color TcMsEP Grading System: A Novel Alarm Method For Intraoperative Neurophysiological Monitoring Using Transcranial Electrical Stimulated Muscle Evoked Potentials (TcMsEPs) In Intramedullary Spinal Cord Tumor Surgeries

Introduction Surgeons need precise information about motor deterioration risk during surgery for intramedullary spinal cord tumors (IMSCTs). However, the conventional TcMsEP alarm criterion provides limited information with a less than or a more than single alarm criterion without any grades in between, resulting in false-negative and false-positive outcomes. Therefore, we developed a "seven-color TcMsEP grading system" for neuromonitoring to provide more graded information. This study investigates the system's efficacy. Methods This study included 60 patients that underwent resection surgeries for IMSCTs. TcMsEP outcomes were classified into seven grades: Grade "D-0 Green" includes a wave amplitude decrease of 0%-50% compared with the baseline amplitude. Grade "D-1 Lime" includes a 50%-70% decrease. Grade "D-2 Yellow" includes a 70%-90% decrease. Grade "D-3 Orange" includes a more than 90% decrease with a clearly visible waveform. Grade "D-4 Red" includes a minimal and abnormally shaped wave. The severest, grade "D-5 Black," includes a wave that has completely disappeared. The additional grade "D-X Gray" includes cases in which the baseline wave is undetectable. Postoperative motor deterioration was evaluated in the upper limbs (PUMD) and lower limbs (PLMD) individually. Results PLMD only occurred in cases with more than a 90% wave amplitude decrease (from D-3 to D-5) and with the undetectable baseline wave (D-X). The PLMD rate increased according to the severity of the amplitude decreases (29% in D-3, 67% in D-4, 80% in D-5). Most PUMD occurred in cases with more than a 90% decrease, but one case with grade D1 had PUMD. Conclusions The seven-color graded alarm criterion supports surgeons' decisions on how to treat the wave amplitude decrease during surgery. It provides motor deterioration risk in each grade without false negatives. Moreover, the corresponding colors enable quick comprehension of the risks.