Multimodal intraoperative neurophysiological monitoring with special emphasis on facial MEPs for facial nerve preservation in vestibular schwannoma surgeries: Surgical nuances and outcome predictability

Abstract

Objectives: The objective of this study was to evaluate the utility of facial motor-evoked potentials (FMEPs) as a significant part of multimodal intraoperative neurophysiological monitoring (IONM) for preserving facial nerve (FN) function in vestibular schwannoma surgery guiding the resection, and predicting outcome. Materials and Methods: This prospective observational study included 35 cases of vestibular schwannomas operated under multimodal IONM using free-running electromyography (EMG), triggered EMG (with both monopolar probe and suction stimulator), FMEPs, electroencephalography and its spectral derivatives and train-of-four testing. Direct stimulation using a monopolar probe helped in the identification of FN and guiding tumour resection. The suction stimulator probe was used for quasi-continuous stimulus delivery and FN mapping. FMEPs helped in the assessment of the integrity of FN. Results: In 14 cases, there was a single instance, and in 11 cases, there was more than one instance of a significant drop in FMEPs. Sixteen cases had >50% fall in FMEPs during surgery. In 28 cases, the proximal stimulation threshold for FN was ≤0.1 mA at the end of the tumour resection. At the end of the first week following surgery, only 42.9% of the cases had good functional preservation (House–Brackmann [HB] Grade I or II) of FN, which increased to 78.8% at the end of one year. The remaining 21.2% had HB Grade III weakness. Percentage drop in FMEP amplitude and final FMEP amplitude correlated significantly (P < 0.01) with the post-operative HB Grade at 1st week, 3, 6 and 12 months following surgery. Conclusion: FMEPs, as a significant component of multimodal IONM, provide a real-time assessment of FN function during surgery, facilitate safe maximal resection, predict immediate post-operative FN outcomes, and improve long-term FN function by minimising the cumulative insult inflicted on the FN during surgery.