The optimal and safe intensity for facial nerve stimulation during intraoperative neuromonitoring in middle ear surgery

Hye Min Han,Euyhyun Park,Hyunjung Kim,In Hak Choi,Oded Cohen,Hak Hyun Jung,Gi Jung Im

doi:10.1371/journal.pone.0221748

Abstract

ObjectiveThis study aimed to investigate the optimal and safe intensity for facial nerve stimulation during middle ear surgery.MethodsThirty-seven patients who had their facial nerve exposed prior to surgery were prospectively enrolled in this study, and electromyography (EMG) recordings were obtained from the orbicularis oculi and orbicularis oris muscles. Four pigs were also enrolled in an animal study, and continuous stimulation was performed on the facial nerves of the pigs for 10 minutes. The EMG responses were measured and the pathologic outcomes of the facial nerve after stimulation were determined.ResultsIn the human study, the mean intensity of the minimal electrical stimulation threshold was 0.21 mA (range: 0.1–0.3 mA). A linear correlation was observed between stimulus intensity and response amplitude for intensities below 0.4 mA. Response amplitudes reached a plateau between 0.4 mA and 1.0 mA. The minimal stimulus intensity that could generate a maximal response was 0.4 mA in the orbicularis oculi (244 μV) and orbicularis oris (545 μV). In the animal study, there were no observed changes in EMG or nerve damage incidence after the continuous stimulation of 3.0 mA.Conclusions0.4 mA is considered to be the optimal intensity of facial nerve stimulation during middle ear surgery, and it was estimated through the animal study that a stimulation of 3.0 mA is safe from facial nerve damage.

Highlights

Facial nerve damage during middle ear surgery is a serious complication for patients and surgeons
The reported incidence of facial nerve damage during middle ear surgery is between 0.6% and 3.6% [2]
intraoperative neuromonitoring (IONM) has become frequently used in middle ear surgery

Summary

Introduction

Facial nerve damage during middle ear surgery is a serious complication for patients and surgeons. The reported incidence of facial nerve damage during middle ear surgery is between 0.6% and 3.6% [2]. To reduce this outcome, there have been numerous attempts to develop methods that reliably identify the facial nerve during surgery. In 1965, Shedd and Durham [3] reported the first technique to identify nerves, which was intraoperative neuromonitoring (IONM) This method provides real-time identification and a functional assessment of nerves during surgery [4]. Since middle ear surgery is most frequently performed in patients with inflammatory disease, it is highly likely that the facial nerve will be surrounded by inflammatory soft tissue, such as granulation, and the possibility of facial nerve damage during surgery increases. It is very important to distinguish the facial nerve from inflammatory soft tissues in the middle ear to avoid facial nerve damage

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