ABSTRACT
Objective
Preservation of the facial nerve is critical in surgery for facial nerve schwannoma. Intra‐operative monitoring systems typically assess facial nerve function by delivering intermittent electrical stimulation through electrodes; however, this approach cannot effectively achieve continuous stimulation and quantitative assessment. This study aimed to assess the efficacy of a quantitative intra‐operative facial nerve continuous monitoring system for preserving post‐operative facial nerve function.
Methods
Ten patients who underwent continuous nerve monitoring using this system during surgery for facial nerve schwannoma were included. Clinical data, electrode placement sites, and pre‐ and postoperative facial nerve function assessed using the House–Brackmann grading system were retrospectively analyzed. In patients with conductive hearing loss, postoperative hearing outcomes were also analyzed.
Results
Facial nerve function was preserved in all patients, except for one patient who experienced transient postoperative functional deterioration. In several cases, conductive hearing loss improved following schwannoma resection.
Conclusion
Continuous and quantitative monitoring of facial nerve function is a reliable intraoperative evaluation that may help prevent unexpected facial nerve injury. This system represents a valuable tool for application in facial nerve schwannoma surgeries.
Level of Evidence
Level 4
Keywords: facial nerve palsy, facial nerve schwannomafacial nerve, monitoring
We investigated the role of a continuous, quantitative intraoperative monitoring system in surgeries for facial nerve schwannomas, a rare tumor associated with a high risk of facial nerve dysfunction. By continuously stimulating the facial nerve and recording electromyographic responses in real time, the system enabled surgeons to detect subtle or sudden declines in nerve function and immediately adjust their surgical approach. The findings revealed that this strategy preserved postoperative facial nerve function in all patients, prevented permanent palsy even in high‐risk resections, and facilitated recovery from conductive hearing loss in selected cases by allowing for the safe removal of tumor‐related middle ear obstruction.
1. Introduction
Facial nerve schwannoma is a rare, benign tumor arising from the seventh cranial nerve. In a previous clinical series involving 600 temporal bones, the incidence of intratemporal facial schwannomas was 0.8%, and symptomatic cases are even rarer [1]. Facial nerve schwannomas are significantly rarer than vestibular schwannomas, another major schwannoma affecting the temporal bone. Facial nerve schwannomas can arise anywhere along the entire course of the facial nerve, from the cerebellopontine angle and internal auditory canal to the intraparotid region, although intratemporal facial nerve schwannomas are the most common [2, 3]. A previous single‐center analysis reported that 71% of cases involved the temporal bone, 49% were intracranial, and 23% were extratemporal [2].
Although classified as benign, facial nerve schwannomas can substantially impair patients' quality of life. The most common symptoms are facial nerve palsy and hearing loss [4, 5], both conductive and sensorineural [5, 6]. Conductive hearing loss arises from external auditory canal obstruction by the tumor, or tumor extension into the middle ear cavity [7, 8]; sensorineural hearing loss results from the deterioration of the cochlear nerve, which runs parallel to the facial nerve in the inner ear canal, due to tumor growth in this region or schwannoma‐induced ototoxicity [9]. Additional clinical manifestations associated with facial nerve schwannomas include facial twitching, spasms, dysgeusia, dry eyes, and vestibular symptoms [2].
Currently, three management options are available for facial nerve schwannomas: observation with serial imaging (“wait and scan”), surgical intervention, and radiotherapy [3, 10, 11]. Observation is often preferred when facial nerve function is preserved, as other interventions may compromise facial nerve function [12]. However, tumor growth during the observation period can lead to progressive deterioration of facial nerve function, and conductive hearing loss may worsen even if facial nerve function is unaffected.
Surgical intervention is a potential treatment for symptomatic cases or those with tumor growth [3]. Nevertheless, the criteria for surgical intervention in facial nerve schwannoma remain inadequately defined, often relying on patient preferences or institutional protocols. Approaches include complete resection with reanimation, facial nerve preservation, and decompression surgery [13, 14]. Complete resection with reanimation is typically reserved for tumors accompanied by moderate or severe facial nerve palsy, as full recovery of facial nerve function cannot be expected even after nerve reconstruction [15]. In contrast, facial nerve preservation and decompression surgeries can be performed for all facial nerve schwannomas. Decompression surgery has a relatively low risk of exacerbating facial nerve palsy; however, preoperative biopsy or small tumor resection is often required for pathological diagnosis, which in itself can cause facial nerve palsy. Facial nerve preservation surgery aims to preserve facial nerve function while removing as much of the tumor as possible to both prevent exacerbation of facial nerve palsy and create space for future tumor growth to avoid tumor compression. However, facial nerve‐preserving surgery carries a risk of further facial nerve functional impairment [16].
Radiotherapy can be used to prevent further tumor growth and preserve residual facial function [17]. While effective in controlling tumor volume, it does not enable pathological diagnosis and is generally ineffective for mitigating conductive hearing loss. Potential risks of radiotherapy include failure of tumor control, further deterioration of facial nerve function, and hearing loss [18, 19].
Managing facial nerve schwannomas involves a critical balance between tumor control and nerve function preservation. Deciding between conservative observation, surgery, or radiotherapy is particularly challenging in patients with slow‐growing tumors and well‐preserved facial nerve function. Although there is a trend toward adopting less aggressive management approaches [10, 13, 16], no consensus currently exists. Notably, surgical management is the only option for conductive hearing loss due to facial nerve schwannomas. Given these challenges, novel strategies for safer tumor resection are warranted. We recently developed a continuous intraoperative monitoring system to aid in preserving facial nerve function during surgery for temporal bone lesions [20]. The present study reports on a series of facial nerve schwannoma cases in which this continuous, quantitative monitoring system was used during surgical tumor resection.
2. Materials and Methods
2.1. Monitoring System
Facial nerve monitoring was performed using the CPAngle Master system (Nihon Kohden) (Figures 1 and 2), a device commonly used in vestibular schwannoma surgery and procedures for other internal auditory canal tumors [21, 22, 23]. This system has also demonstrated efficacy in reducing the risk of surgery‐related facial nerve palsy in temporal bone lesions [20].
FIGURE 1.
Continuous and quantitative intraoperative facial nerve monitoring system used in this study. In this setup, an electromyogram (EMG) was continuously recorded at 3‐s intervals during tumor removal. Electrical stimulation was delivered through an electrode placed proximally on the facial nerve, closer to the central portion than to the resection site (red box in A). Quantitative analysis was automatically performed by comparing each waveform with the baseline waveform obtained prior to tumor resection (arrow in A). When a sudden decrease in the EMG amplitude was detected, resection was immediately discontinued (B).
FIGURE 2.
Comparison between conventional non‐continuous monitoring and the continuous monitoring system used in this study. In conventional, non‐continuous monitoring systems, an electrode is applied to identify the location of the facial nerve. After the course of the nerve is determined, tumor resection proceeds without further stimulation (A). In contrast, the method employed in this study uses continuous stimulation of the facial nerve at a site proximal to the tumor. During resection, nerve activity is recorded continuously, providing real‐time electrophysiological data (B).
In typical surgery for facial nerve schwannomas, a continuous stimulating electrode is placed on the central portion of the facial nerve, rather than on the most high‐risk part of the intraoperative surgical invasion. During tumor resection, electrical stimulation was applied every 3 s, and needles were inserted into the orbicularis oculi and oris muscles for electromyography (EMG) recording.
The stimulation current was configured as follows: (1) The lowest saturated current level that achieved the maximum EMG amplitude was determined; any current level beyond this point results in saturation of the EMG amplitude. (2) The stimulation current was set approximately 0.2–0.5 mA higher than the lowest saturated current level; typically, this stimulation current does not exceed 3.0 mA. (3) In instances where the saturated EMG amplitude was not achieved, a stimulation current of 3.0 mA is utilized. The following parameters were employed for stimulation and recording: pulse width of 2 ms, square‐wave pulse shape, and filter settings with a high‐cut of 2 kHz and a low‐cut of 5 Hz.
The ratio of the myogenic potential amplitude recorded at each stimulation to that recorded at the initial stimulation was automatically computed as EMG%. The amplitude and its trend relative to the baseline were displayed on the system monitor. A decrease in amplitude during resection signaled the need for caution.
Surgical tumor removal followed the policy outlined as follows: (1) The primary objective was subtotal tumor resection while avoiding further postoperative deterioration of facial nerve function; (2) In patients with conductive hearing loss, maximal tumor excision within the middle ear cavity was attempted, followed by tympanoplasty when feasible; (3) The facial nerve canal bone was also decompressed; (4) If a sudden EMG amplitude drop occurred or if the EMG amplitude decreased to a level deemed hazardous by the surgeon, resection was immediately halted and no further removal was attempted.
2.2. Enrollment
This study included 10 patients with intratemporal facial nerve schwannomas who underwent surgery with intraoperative continuous monitoring at Keio University Hospital between August 2023 and July 2025. Due to the differing enrollment periods, the patients included in this study were not part of our previous cohort of patients with temporal bone lesions [20].
The primary surgical indications in our department are as follows: (1) Presence of a growing tumor with House–Brackmann (HB) grade I–V facial nerve paralysis; (2) Anticipated progression of facial nerve palsy based on the tumor size and location; (3) Other symptomatic cases, such as those with hearing loss or a sensation of ear fullness; and (4) Cases presenting with preoperative imaging findings suggestive of diagnoses other than facial schwannoma, requiring histological confirmation. Patients meeting the following criteria are rejected for surgery: (1) Those with large tumors accompanied by severe facial nerve palsy, typically extending much deeper into the facial canal, who are more appropriate candidates for complete resection and reanimation surgery; (2) Asymptomatic cases with no tumor growth or with small tumors; and (3) Patients who cannot tolerate general anesthesia or who opt against surgical intervention.
The primary outcome of this study was the preservation of facial nerve function. The following parameters, along with general patient information, were retrospectively analyzed: (1) preoperative and postoperative facial nerve function, assessed using the HB grading system [24] and preoperative electroneurography (ENoG, %); (2) the ratio of myogenic potential amplitude obtained at the final stimulation to that obtained at the initial stimulation (EMG%); and (3) preoperative and postoperative hearing levels in patients with conductive hearing loss. The extent of tumor resection was defined based on the surgical findings: > 90% resection indicated subtotal resection, and < 90% resection indicated partial resection (PR). Hearing loss was evaluated based on the average air‐conduction thresholds at 0.5, 1, 2, and 3 kHz. The evaluation of postoperative facial nerve function commenced immediately following the patient's recovery from general anesthesia. In instances where follow‐up magnetic resonance imaging (MRI) was conducted, tumor recurrence was assessed.
The study was approved by the Ethics Committee of Keio University School of Medicine (approval number: 20200033). All study procedures adhered to the principles outlined in the 2013 revision of the Declaration of Helsinki. The Ethics Committee waived the requirement for informed consent for the publication of anonymized data in this study.
3. Results
The clinical characteristics of the study cohort are summarized in Table 1. The cohort comprised six men and four women, with a mean age of 53.1 years. The median follow‐up duration was 14.5 months (range, 6–24 months). At each follow‐up, no tumor regrowth was observed in any of the patients. Hearing loss was the most common presenting symptom, followed by facial nerve palsy.
TABLE 1.
General characteristics of enrolled patients.
| Case | Age | Sex | Chief complaint | Tumor localization | Approach | Ossicular reconstruction | Electrode position | Resection rate | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| C | IAC | L | GG | H | 2G | V | ||||||||
| #1 | 78 | F | HL, otalgia | TM | IIIc | GG | PR | |||||||
| #2 | 45 | M | HL, FNP | TM | IVc | GG | STR | |||||||
| #3 | 46 | M | HL | RLA | — | REZ | STR | |||||||
| #4 | 56 | M | HL, FNP | TM | Wo | GG | PR | |||||||
| #5 | 51 | F | HL, tinnitus | TM | IIIo | GG | PR | |||||||
| #6 | 50 | M | FNP, HL, dizziness | RLA + TM | IIIc | REZ | STR | |||||||
| #7 | 30 | M | HL, FNP | MCF + TM | IVc | L | PR | |||||||
| #8 | 51 | F | Tinnitus, ear fullness | TM | IIIr | H | PR | |||||||
| #9 | 65 | F | FNP | TM | IIIr | H | PR | |||||||
| #10 | 59 | M | FNP | TM | IIIr | GG | PR | |||||||
Note: Gray scales represent the localization of the facial schwannoma within each segment of the facial nerve. 2G: 2nd genu.
Abbreviations: C: Cistern portion; F: Female; FNP: Facial nerve palsy; GG: Geniculate ganglion; H: Horizontal portion; HL: Hearing loss; IAC: Internal auditory canal; IIIc: Type III tympanoplasty with columella; IIIo: Type III tympanoplasty with stapes columella; IIIr: Type III tympanoplasty with repositioning of the incus; IVc: Type IV tympanoplasty with columella; L: Labyrinthine portion; M: Male; MCF: Middle cranial fossa approach; PR: Partial resection; REZ: Root exit zone of the facial nerve; RLA: Retrolabyrinthine approach; STR: Subtotal resection; TM: Transmastoid approach; V: Vertical portion; Wo: Tympanoplasty without ossicular chain reconstruction.
The tumor locations and surgical procedures performed for each patient are detailed in Table 1. The most frequent tumor site was the second genu, followed by the vertical portion of the facial nerve. Based on the specific lesion present in each patient and the chosen surgical procedure, a continuous stimulating electrode was strategically placed on the central portion of the facial nerve, as opposed to on the segment most susceptible to intraoperative surgical invasion. The electrode positions are listed in Table 1.
All of the patients, except for Case 3, underwent tumor removal via the transmastoid approach, combined with ossicular reconstruction. Subtotal resection was achieved in three cases, whereas the remaining seven cases achieved partial resection, preventing further exacerbation of facial nerve palsy.
Postoperatively, all of the patients, with the exception of Case 4, demonstrated stable facial nerve function (Table 2). In Case 4, no facial nerve palsy was observed immediately after surgery; however, HB grade III palsy developed on postoperative day 5. The patient underwent a 1‐week course of oral prednisolone, tapering from 30 mg. The palsy was transient, with recovery to HB grade II after 3 weeks.
TABLE 2.
Pre‐ and postoperative changes in facial nerve function.
| Case | HBG | |
|---|---|---|
| Preoperative | Postoperative | |
| #1 | I | I |
| #2 | V | V |
| #3 | I | I |
| #4 | II | II (Transiently III)* |
| #5 | I | I |
| #6 | IV | IV |
| #7 | II | II |
| #8 | I | I |
| #9 | V | IV |
| #10 | IV | IV |
Note: HBG: House–Brackmann grade.
Case 4 experienced transient (postoperative days 5–26) deterioration of facial nerve function.
In all cases, except for Case 7, follow‐up MRI was conducted, which revealed no tumor regrowth in any of the evaluated cases.
Conductive hearing loss was observed in five cases (Cases 1, 2, 4, 5, and 7). Among these, three (60%) cases experienced hearing recovery following surgical intervention, which involved tumor removal and successful ossicular reconstruction (Cases 1, 2, and 5) (Table 3).
TABLE 3.
Pre‐ and postoperative changes in hearing level of patients with significant preoperative conductive hearing loss.
| Case | Ossicular reconstruction | Hearing level (dBHL) | ||
|---|---|---|---|---|
| Preoperative | Postoperative | |||
| #1 | IIIc | 56.25 | 35 | ↑ |
| #2 | IVc | 82.5 | 57.5 | ↑ |
| #4 | Wo | 37.5 | 31.25 | → |
| #5 | IIIc | 77.5 | 37.5 | ↑ |
| #7 | IVc | 31.25 | 27.5 | → |
Abbreviations: IIIc: Type III tympanoplasty with columella; IVc: Type IV tympanoplasty with columella; Wo: Tympanoplasty without ossicular chain reconstruction.
Preoperative ENoG%, the final intraoperative amplitude of the electromyogram (EMG%), the product of the preoperative ENoG score and the final intraoperative EMG amplitude, and intraoperative EMG% were examined (Table 4). The mean final EMG% was 100.4% (range, 49%–194%) in the oculi and 106.8% (range, 75%–159%) in the oris muscles. The lowest final EMG% score was 49%, identified in Case 7. A product value < 40% was observed in six cases (Cases 2, 4, 6, 7, 9, and 10). Additionally, six (60%) patients experienced a sudden intraoperative decline in EMG%. Representative cases are illustrated in Figures 3 and 4.
TABLE 4.
Electrophysiological analysis of facial nerve monitoring in each case.
| Case | Pre‐operative ENoG% | Final EMG% | ENoG% × EMG% | Intraoperative sudden decline | EMG% at sudden decline | ||||
|---|---|---|---|---|---|---|---|---|---|
| Oculi | Oris | Oculi | Oris | Oculi | Oris | Oculi | Oris | ||
| #1 | 667 | 189 | 194 | 98 | 1293.98 | 185.22 | — | ||
| #2 | 0 | 8.24 | 87 | 107 | 0 | 8.8168 | + | 27 | 49 |
| #3 | 64.5 | 99.6 | 76 | 86 | 49.02 | 85.656 | — | ||
| #4 | 105 | 14 | 91 | 102 | 95.55 | 14.28 | — | ||
| #5 | 70.7 | 78.5 | 152 | 159 | 107.464 | 124.815 | + | 3 | 4 |
| #6 | 212.5 | 27 | 63 | 75 | 133.875 | 20.25 | — | ||
| #7 | 43.4 | 10.66 | 49 | 100 | 21.266 | 10.66 | + | 49 | 77 |
| #8 | 63.5 | 41.7 | 94 | 154 | 59.69 | 64.218 | + | 46 | 29 |
| #9 | 0 | 2.2 | ND | 101 | NA | 2.222 | + | ND | 21 |
| #10 | 0 | 0 | 98 | 86 | 0 | 0 | + | 64 | 10 |
Note: Gray scales denote cases where the ENoG% × EMG% is less than 40%.
Abbreviations: EMG: electromyography; ENoG: electroneurography.
FIGURE 3.
Representative case of facial nerve schwannoma (Case 2). A 45‐year‐old male presented with severe right‐sided facial nerve palsy and conductive hearing loss caused by tumor extension into the middle ear cavity (arrow in A; A–C). Surgical resection was performed via a transmastoid approach (D–F). The tumor occupied the middle ear cavity and mastoid sinus (arrow in D). A continuous stimulation electrode was positioned near the first geniculate ganglion (arrowhead in E). The tumor was excised under continuous intraoperative monitoring (F). Postoperative magnetic resonance imaging demonstrated a marked reduction in the tumor volume (G). Postoperatively, facial nerve function was preserved, and hearing improved following tumor reduction (H, I).
FIGURE 4.
Representative case of facial nerve schwannoma (Case 5). A 51‐year‐old female presented with a left‐sided facial nerve schwannoma (arrow in A). Although she exhibited no facial nerve palsy, she experienced left‐sided conductive hearing loss due to tumor extension into the external auditory canal (B, C). Surgical resection was performed via a transmastoid approach (D–F). The tumor was identified in both the mastoid cavity (arrows in D) and the external auditory canal. A continuous stimulation electrode was placed adjacent to the first geniculate ganglion (arrowhead in E). Tumor removal was performed under continuous intraoperative monitoring (F). Postoperative magnetic resonance imaging demonstrated a reduction in the tumor volume (H). Facial nerve function remained intact, and hearing levels improved following resection of the tumor in the external auditory canal and middle ear cavity (H, I).
4. Discussion
In this study, we implemented a quantitative, continuous monitoring system during facial nerve schwannoma surgery to help preserve postoperative facial nerve function. This system, previously applied in temporal lesion surgeries [20], maintained postoperative facial nerve function in all included cases, demonstrating its potential utility for functional preservation.
Currently, there is no consensus regarding the management of facial nerve schwannomas. However, there is a discernible trend toward adopting less aggressive treatment strategies because of the risk of postoperative facial nerve palsy [10, 13, 14, 16]. Consequently, a “wait‐and‐scan” approach is frequently adopted, resulting in fewer surgical interventions being performed. Patients with no or only mild facial nerve palsy often do not undergo surgery. Nevertheless, previous studies have demonstrated that schwannomas involving multiple facial nerve segments are associated with a higher risk of facial weakness [25]. This suggests that achieving a secure volume reduction of the facial nerve schwannoma may help prevent further deterioration of residual facial nerve function. Proactive resection performed under continuous monitoring of the facial nerves could therefore reduce the risk of facial nerve injury. Moreover, several patients with facial nerve schwannoma experience conductive hearing loss due to external ear canal obstruction or tumor extension into the middle ear. In such cases, safe resection of the tumor appears especially promising.
An accurate and reliable monitoring system may provide surgeons with a valuable intraoperative tool for assessing and preserving facial nerve function, potentially improving surgical outcomes. For instance, a previous case report highlighted the effectiveness of continuous monitoring during the resection of an intracranial facial nerve schwannoma [26]. By offering real‐time feedback on nerve activity, this system may help balance the goals of tumor removal and functional preservation, thereby reducing the risk of postoperative facial nerve palsy. This approach could also shift treatment paradigms, enabling more confident and precise surgical interventions when indicated.
Conventional monitoring systems, such as the nerve integrity monitor, are commonly employed to reduce the risk of facial nerve palsy during surgery [15]. In these systems, a stimulating electrode is used to identify the facial nerve (Figure 2). After determining the nerve's course, tumor removal is performed without further stimulation. Although free‐run EMG signals triggered by inadvertent nerve contact can aid the surgeon, such responses are inconsistent and may not occur during resection. By contrast, our system continuously stimulates the facial nerve at a site proximal to the tumor throughout resection and simultaneously records the resulting electrophysiological activity. This method provides continuous, real‐time information on nerve function, offering several advantages over conventional methods. Compared with conventional systems that do not allow for continuous monitoring, the continuous monitoring enabled by our system allows surgeons to make intraoperative adjustments, detect subtle changes in nerve integrity, and minimize the risk of injury, thereby improving patient outcomes and lowering the incidence of postoperative palsy.
Conductive hearing loss adversely affects the quality of life of patients with facial nerve schwannoma. Typically, such hearing loss remains untreated in patients with no or mild facial palsy because of the risk of exacerbating nerve functional impairments after surgery. In the present study, we demonstrated that tumor resection under continuous monitoring can facilitate the recovery of conductive hearing loss in selected patients. Furthermore, unlike facial nerve decompression surgery, which preserves nerve function but does not address hearing loss, this approach provides both functional preservation and hearing improvement. Thus, our results suggest the superiority of monitored tumor resection over decompression surgery for managing facial nerve schwannomas. However, given the limited number of cases evaluated in this study, larger‐scale studies are warranted in the future.
Preoperative ENoG, EMG%, and the product of the preoperative ENoG score and the final intraoperative EMG amplitude have been previously associated with the preservation of facial nerve function during vestibular schwannoma surgery [22]; accordingly, these indicators were assessed in the current study. Prior research has revealed significant facial nerve palsy in cases of vestibular schwannoma when the product of the preoperative ENoG score (%) and the final intraoperative EMG% was less than 40 [22]. However, in the present cohort, even when this product fell below 40%, postoperative deterioration of facial nerve function was not observed. This discrepancy may reflect microstructural differences between the facial nerves of the temporal bone and those of the internal auditory canal or cisternal segments; notably, the temporal bone portion of the facial nerve possesses an epineurium [27, 28], which provides mechanical protection and may enhance tolerance to surgically induced mechanical damage. Future studies are warranted to define the lower limit of intraoperative EMG% predictive of postoperative outcomes in facial nerve schwannoma surgery.
In the current study, intraoperative monitoring revealed that 60% of the patients exhibited a sudden decrease in EMG%. It is challenging to determine the magnitude of EMG% decrease that is considered dangerous; however, it is hypothesized that, in a manner analogous to vestibular schwannoma surgery [21], a decrease in EMG% below 65% would be hazardous. The ability to detect such an abrupt decline represents a key advantage of the continuous quantitative monitoring system used in this study, which is not achievable using conventional methods. Immediate cessation of tumor resection in response to a sudden EMG% decline prevented further facial nerve deterioration in the current study: all cases, except for orbicularis oculi in Case 7, demonstrated recovery following the cessation of tumor manipulation. In the instance of Case 7, no postoperative facial nerve palsy was observed, yet recovery did not occur. Consequently, it is challenging to draw conclusions regarding the tolerance of the sudden EMG% decline during surgery for facial nerve schwannoma based on the current findings. Accordingly, future large‐scale case studies are warranted.
To preserve facial nerve function, total resection was intentionally avoided. Subtotal resection was achieved in 30% of the cases, while partial resection was achieved in the remainder. In most cases, achieving partial resections, either a sudden EMG% decline—likely reflecting axonal involvement of the facial nerve within the schwannoma [13, 29]—or a gradual EMG% decrease during resection, prompted cessation of tumor removal, which is misaligned with the surgical plan. Consequently, no permanent facial nerve deterioration occurred. Nevertheless, previous research has demonstrated that the rate of tumor recurrence or regrowth is higher following partial resection compared to subtotal resection [3, 30]. For example, Li et al. reported that tumor recurrence occurred in 60% of cases achieving 70%–80% tumor resection, whereas a 10% recurrence rate was observed in cases achieving approximately 95% resection [30]. Therefore, long‐term postoperative outcomes, particularly following partial resection, require careful follow‐up.
This study had some limitations. First, it adopted a retrospective design and lacked a control group. Although randomized controlled trials are the gold standard for assessing novel methods, facial nerve schwannomas are rare, making treatment randomization challenging. Moreover, because our method incorporated conventional monitoring, withholding continuous quantitative monitoring from controls would raise ethical concerns, given the risk of severe postoperative facial nerve palsy. Second, the follow‐up period was relatively short, and long‐term prognostic data were unavailable. Future studies should incorporate extended follow‐up evaluation, particularly for cases achieving partial resection, to assess long‐term facial nerve function and tumor recurrence rates.
Furthermore, the novel method proposed herein may have limited applicability, as there may be patients for whom intraoperative continuous EMG responses cannot be adequately obtained; this is more likely in patients with severe facial nerve palsy and/or an ENoG of 0%. For instance, in Case 9, sufficient EMG data from the orbicularis oculi could not be obtained. Conversely, in Case 10, the preoperative ENoG% was 0% in both the orbicularis oculi and oris muscles, yet we were able to obtain sufficient EMG data for continuous monitoring. Therefore, it cannot be conclusively stated that patients with severe facial nerve palsy or an ENoG of 0% should be excluded from the application of this continuous monitoring system. Nonetheless, it would be advantageous to inform such patients of the potential limitations of this system before surgery.
5. Conclusion
The continuous facial nerve monitoring system presented herein demonstrated several advantages, including enabling quantitative intraoperative assessments and preventing unexpected nerve injury during facial nerve schwannoma resection. In all cases, the system enabled safe tumor manipulation while preserving facial nerve function, and no patient experienced permanent postoperative palsy. Additionally, several patients experienced recovery from preoperative conductive hearing loss after secure excision of the causative tumor. These findings suggest that continuous quantitative monitoring is a valuable adjunct in facial nerve schwannoma surgery.
Funding
The Japan Society for the Promotion of Science supported this work with the Grant‐in‐Aid for Scientific Research (C) (24K12728).
Conflicts of Interest
The authors declare no conflicts of interest.
Acknowledgments
The Japan Society for the Promotion of Science supported this work with the Grant‐in‐Aid for Scientific Research C 24K12728.
Data Availability Statement
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.