Systematized Intraoperative Facial Nerve Monitoring in Middle Ear and Mastoid Surgery: ’Surgical Dehiscence’ and ’Electrical Dehiscence’

Abstract

Background and Objectives: The use of intraoperative facial nerve monitoring (IOFNM) improves facial nerve outcomes in acoustic neuroma surgeries, but the role of IOFNM in middle ear and mastoid surgeries is poorly defined. This study was performed to evaluate the role of IOFNM in middle ear and mastoid surgeries and to systemize IOFNM.

Subjects and Method: We carried out a prospective study of 83 patients who undertook middle ear and mastoid surgeries with IOFNM. We checked the facial nerve dehiscence and estimated its location and length using a surgical microscope (’surgical dehiscence’). We stimulated the facial nerve with constant current, unipolar stimulation using Nerve Integrity Monitor (NIM)-2(TM) (Xomed (TM), U.S.A.) and estimated the minimal threshold of electric current making the electromusculography of facial muscle changes.

Results: Thirty six (43.4%) of 83 cases showed ’surgical dehiscence’ and all responded to 0.7 mA or less of electrical stimulation. The most common site of ’surgical dehiscence’ was middle portion of the tympanic segment. We defined the response to electrical stimulation within 0.7 mA as ’electrical dehiscence.’ ’Electrical dehiscence’ was presented in 63 (75.9%) cases and 82.5% of these cases responded to stimulation of 0.4 mA or less. The mean threshold of minimal electrical stimulation was 0.28 mA for tympanic segment and 0.48 mA for mastoid segment.

Conclusion: "Electrical dehiscence" based on responses of electrical stimulation is safer than "surgical dehiscence," which is based on microscopic observation in middle ear and mastoid surgery. Based on this study, we recommend the electrical stimulation of 0.7 mA for first screening and 0.4 mA for second exploration in defining facial nerve using intraoperative NIM-2(TM) monitoring in middle ear and mastoid surgeries.