Intraoperative real-time continuous facial nerve monitoring in acoustic neuroma surgery

Abstract

Dear Editor: We were very pleased to read the letter to the editor from Acioly et al. They estimated the intraoperative continuous monitoring of evoked facial nerve electromyograms we reported [1] as "a new device for an old concept," and we agree with their assessment. Because the first step for starting continuous monitoring of the facial nerve is to identify the root exit zone (REZ), the authors expressed concern about a case in which a large acoustic neuroma obstructed the REZ of the facial nerve because of anatomical distortion of the brainstem. In Kohno's experience, there have been only four cases (0.7%) out of 550 surgically treated patients with acoustic neuromas, in which he could not identify the REZ of the facial nerve, a very low percentage despite the fact that large tumors (Koos 3 and 4) account for 84% of cases. For a large tumor, internal decompression of the tumor is necessary to find the REZ of the facial nerve. Until we start continuous monitoring, we can notice that the facial nerve is mechanically stimulated by the free-running spontaneous facial electromyography. Therefore, we do not consider the possibility of facial nerve impairment before starting continuous monitoring. An important part of developing new surgical methodologies is to keep them simple, effective and easy to perform. For a new method to become popular and widely used over time, however, certain techniques and tips are required for every operation in the neurosurgical field. In particular with acoustic neuroma surgery, all neurosurgeons know that individual technique and experience must be considered. The authors express concern about movement of the electrode for continuous stimulation; however, we actually replace it at most twice or three times in a surgery, with minimum adjustment within a minute. To be sure, there are tips for stabilizing the electrode to prevent movement. As we described in our paper, we use a small cotton pad to fix the electrode in place. The authors also refer to the necessity of placing recording needle electrodes on three muscles (frontalis, orbicularis oculi and oris). Actually, electromyograms from three muscles vary in initial voltages and intraoperative changes, and there is the possibility that problems with a needle electrode or an electrical cord might occur. Therefore, we believe that setting recording electrodes on all of these three muscles is effective and necessary. The monitoring by transcranial facial motor-evoked potentials (FMEP), which is recommended by Alcioly et al., is surely effective, particularly until the surgeon finds the REZ of the facial nerve. However, body movement at electrical stimulation is considered a problem. In FMEP, as they point out, it is necessary to stop microsurgical procedures to record electromyograms from the viewpoint of safety. We consider FMEP to be a different type of monitoring compared to our method, which allows us to assess the condition of the facial nerve continuously during the process of dissecting the tumor from the facial nerve in real-time without stopping microsurgical procedures. M. Kohno (*) Department of Neurosurgery & Stroke Center, Tokyo Metropolitan Police Hospital, 4-22-1, Nakano, Nakano-Ku, Tokyo 164-0001, Japan e-mail: mkouno-nsu@umin.ac.jp