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

Abstract

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. 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. 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. 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.