Significance of Facial Cortico-bulbar Motor Evoked Potential (FCoMEP) Monitoring in Vestibular Schwannoma Surgery for Prediction of Postoperative Facial Nerve Palsy

Introduction: Direct facial nerve stimulation/ mapping and A-train monitoring during vestibular schwannoma surgery (VSS) are routinely used to preserve facial nerve function. However, intraoperative prognostication of post-operative nerve function remains challenging. Cortico-bulbar transcranial motor evoked potentials (CoMEPs) are a relatively new monitoring modality which may deliver a more accurate estimation. A retrospective study was conducted to find out if intraoperative CoMEP amplitude reduction and threshold elevation correlates with postoperative severity and prognosis of facial palsy.

Methods: In total, 51 patients (median age 52yrs; 19-84) with unilateral VS (median size 20mm; 8-50) were retrospectively investigated. Intraoperative neuromonitoring consisted of auditory evoked potentials (AEPs), direct facial nerve stimulation, A-train monitoring and CoMEPs. An anodal stimulation via subdermal corkscrew electrodes on the position of C3 or C4 and a cathode near Cz with a train of 6 pulses, each with a duration of 500µs at a frequency of 500Hz and an interstimulus-interval of 2ms at 50-70 mA mean intensity was used.

Results: Preoperative useful hearing was present in 78% (40/51) of patients and postoperative serviceable hearing was preserved in 45% (18/40) patients, closely correlated to preservation of wave V of the AEPs. Additionally, MEPs were successfully monitored in 49/51 patient during surgery, in 2 patients technical problems occurred. In 14/51 (27.5%) patients severe facial palsies (HB 5-6) occurred postoperatively, 9 were reversible (17.6%) and 5 (9.8%) were long lasting (21 month FU). All severe palsies were predicted by a facial MEP amplitude reduction between 50-100% and more than 50% increased stimulation threshold. No false negative or false positive results were encountered.

Conclusion :: In addition to direct facial nerve stimulation mapping and A-train monitoring, continuous intraoperative FMEP monitoring during VSS seems is of significant value for intraoperative facial nerve preservation and prognostication of postoperative facial palsy.