Facial nerve monitoring in parotid gland surgery: Design and feasibility assessment of a potential standardized technique

Abstract

Background: Even though the use of nerve monitoring during parotid gland surgery is not the gold standard to prevent damage to the nerve, it surely offers some advantages over the traditional approach. Different from thyroid surgery, where a series of steps in intraoperative nerve monitoring have been described to confirm not only the integrity but-most importantly-the function of the recurrent laryngeal nerve, in parotid gland surgery, a formal guideline to follow while dissecting the facial nerve has yet to be described.

Methods: A five-year retrospective study was done reviewing the intraoperative records of patients who underwent parotid gland surgery under neural monitoring. The operative findings regarding the neuromonitoring process, particularly in regard to the amplitude of two main branches, were revised. A literature search was done to search for guidelines to follow when a facial nerve loss of signal is encountered.

Results: Fifty-five patients were operated on using the Nim 3 Nerve Monitoring System (Medtronic); 31 were female patients, and 47 patients had benign lesions. Minimum changes were observed in the amplitude records after a comparison was made between the first and the last stimulation. There were only three articles discussing the term loss of signal during parotid gland surgery.

Conclusion: Today, no sufficient attention has been given to the facial nerve monitoring process during parotidectomy. This study proposes a formal guideline to follow during this procedure as well as an instruction to consider when a loss of signal is observed to develop a uniform technique of facial nerve stimulation.

Keywords: amplitude; facial nerve; intraoperative neural monitoring; latency; loss of signal; nerve monitoring; parotid gland.

Figure 1

Right parotidectomy; the gland with the tumor—star—is about to be removed. Hemostat clamp and the nerve stimulator probe point to a superior branch that communicates with the inferior system depicted by the arrow. IB, inferior branch; MT, main trunk; SB, superior branch.

Figure 2

Main trunk responses upon stimulation. Predissection done at 1.48 mA, as there is tissue volume covering the different branches; post‐dissection is at 1.49 mA while all nerve branches have been dissected, preserved, and exposed. Even though the same electrical stimulus was done at the trunk for illustration‐comparison purposes, the postdissection check can definitively be done at a lower energy setting.

Figure 3

Inferior branch pre‐ and postoperative responses, Orbicularis Oris decreasing amplitude responses, with no change in latency. Since this is a selective inferior stimulation at 0.79 mA, the superior branches show hardly any response. Note: Images 1–3 were obtained for an illustrative example with consent from a recent patient who underwent a superficial parotidectomy due to a benign pleomorphic adenoma tumor. It does not form part of the series of cases described herein.