Multimodal Intraoperative Neurophysiological Monitoring in Total Endoscopic Ear Surgery: A Safe Protocol For Surgery with A Non-Negligible Risk of Permanent Facial Nerve Injury

Total Endoscopic Ear Surgery (TEES) has rapidly gained traction as a preferred method for managing middle ear pathology due to its minimally invasive nature. In the past few years, it has become involved in most otology operations due to its significant benefits [1]. Despite its increasing popularity, the close proximity of critical neural structures, especially the facial nerve, presents unique risks. In particular, early exposure of the tympanic segment increases vulnerability to mechanical, thermal, or ischemic injuries. This application has undergone a significant revolution in the past few years, becoming integral to tympanoplasty and cholesteatoma surgery. TEES is a relatively newer approach to managing middle ear disease. Both tympanomastoid surgery and TEES have their advantages and risks. In microscopic surgery, the mastoid segment of the facial nerve is identified in a later stage of surgery after mastoid bony drilling, and subsequently, the tympanic segment is traced once the middle ear is entered. However, in TEES, the tympanic segment of the facial nerve is determined early in the surgery as the middle ear is approached transcanally [2, 3].

One of the difficulties in middle ear surgery is the nature of the disease that causes erosion of the facial canal, thus exposing the nerve directly. Not only that, but often the polypoidal mucosa encases the exposed facial nerve, making it difficult to differentiate between normal and diseased structures. Concerns about higher complication rates are understandable, but as with the learning curve for a new approach, one has to be ready for reports of related procedural eventualities. However, endoscopic surgeries for cholesteatomas and other etiologies demonstrate risk advantages and favorable surgical outcomes. Numerous advantages are recognized for minimally invasive techniques, but neurological complications arising from this type of surgical procedure have been reported in several studies [2-4]. The neuroanatomical proximity of the facial nerve during access with drill justifies the risk of secondary neurological complications when using this technique [2-3]. Delayed facial nerve paresis (DFNP) following TEES surgery was observed in 1.71% 3 to 3.4% [1]. Another common facial nerve injury was chorda tympani nerve (CTN) transection (7.8% in 18 years of study) [4].

One of the possible causes for the high frequency of neurological lesions is the variability of facial nerve topography. Another possibility may be dehiscence and an overhanging facial nerve. The facial nerve is more likely to be injured by the intensity of the light source during microscopic work, which may heat the labyrinthine fluid and nerve structures in the middle ear, rather than by the microscopic work itself. This thermogenesis results in nervous tissue damage, which can manifest as DFNP [2].

IONMm, by means of its multiple stages of mapping and monitoring, should yield precise information to the surgeon about the functioning of the peripheral and central nervous systems involved with the surgical access pathway. The neurophysiological feedback allows surgeons to correlate their actions over a given surgical time, based on the IONMm findings, enabling them to interrupt maneuvers, retreat, and thereby prevent neurological lesions. This type of information is supplied to the surgical team throughout all stages of the procedure, including the multiple drilling phases (via the performance of corticobulbar transcranial evoked potentials (CoMEP), as well established in other skull-base surgeries) [5, 6].

Moreover, the safety profile, efficacy, and cost-effectiveness of IONM must be taken into consideration. The most used modalities in IONM during skull-base surgeries include Somatosensory Evoked Potentials (SEP), CoMEP, and Electromyography (EMG), which can be free scan (EMGfs) or free running (EMGfr), and stimulated (EMGst) or triggered (EMGtg) [5].

However, the EMGst technique, known solely as unimodal intraoperative neurophysiological monitoring (IONMu), only detects the approximation of the surgeon’s instruments, like the drill, to neural structures [5-7]. Moreover, it determines, through the depolarization threshold found, the distance at which these instruments, including the drill, are from the facial nerve (and its possible fascicular subdivisions due to anatomical variation) and offers feedback on whether this distance can be considered safe. However, we emphasize that EMGst is not a monitoring technique, but rather only a mapping tool. This means that EMG detects the proximity to neural structures but does not allow for the monitoring of occurring lesions. For this reason, unlike the use of CoMEP, EMGst is restricted to only a few phases of the surgery.

Therefore, as partly explained previously, unimodal or incomplete intraoperative neurophysiological monitoring (IONMu) — as the name we use suggests — is an incomplete technique when used in some services during TEES or mastoidectomies, since it only applies EMGfs and EMGst. A study by Marchioni, et al. [8] showed a low rate of complications in TEES. Perhaps the exposed facial nerve induces an inflammatory reaction in the nerve and a longstanding pressure effect from the cholesteatoma [9-10]. As such, one needs to be sure of the anatomy not just in a normal state but also in a diseased middle ear, which often sends chills to someone starting endoscopic ear surgery work. Therefore, delineation of facial nerve integrity is one of the most important steps in TEES, especially with an abnormal course of the facial nerve [3, 10]. This way, the IONMu, particularly EMGst, has significant limitations. While it can detect neural proximity during mapping, it fails to monitor dynamic functional loss. This often leads to false negatives, as thermal or ischemic injuries may progress undetected. Several reports highlight that IONMu lacks predictive sensitivity, especially in stages following drilling or dissection. This exposes patients to preventable complications and renders EMGst insufficient as a standalone method. Furthermore, CoMEPs are the only tools that confirm ongoing corticobulbar tract integrity, especially when facial nerve anomalies are suspected.

Given these facts, we highlight that IONMu is incomplete. To diagnose most lesions that occur in this pathway, it is essential to monitor the use of the aforementioned CoMEP.

The IONM offers comprehensive feedback by integrating somatosensory evoked potentials (SEP), CoMEP, and various EMG modalities. This allows for real-time detection of nerve integrity, especially when multiple anatomical variants or disease related distortions are present. Unlike EMG stimulation, which merely maps neural proximity, CoMEPs assess functional integrity, enabling early identification of lesions caused by traction, ischemia, or heat. Furthermore, this feedback supports safer surgical navigation and real-time correction of maneuvers [6-7].

Conduct CoMEP of the muscles innervated by facial nerves in at least 4 branches using previously established facial nerve injury alarm criteria5-6. As previously mentioned, lesions are not visualized with EMGst and EMGfs in IONMu. This is just one element in the mapping/location and cannot assess neurological lesions due to rupture, ischemia, or thermal injury6. Therefore, if a lesion occurs during the introduction of any instrument, EMGfs will show signs of mechanical depolarization; however, to observe whether the lesion is partial or complete, evoked potentials are necessary.

Recent publications support the superiority of IONMm over IONMu in surgeries involving the cerebellopontine angle, skull base, and middle ear. For example, Fernández-Conejero et al. (2022) [5] and Izzo et al. (2024) [6] describe how improved outcomes and reduced facial nerve morbidity are achieved in cases where CoMEPs are utilized. Other reports, such as those from Kamalden, et al. [2] and Moneir, et al. [1] confirm that TEES poses distinct risks of thermal and mechanical nerve injury — injuries that often go undetected in EMG-only strategies. The paradigm is clear: mapping alone is not monitoring. Only CoMEP-based systems provide a complete picture of neural function intraoperatively.

Given the inherent risks in TEES, especially thermal and mechanical insults to the facial nerve, implementing a protocol that integrates both mapping and monitoring is no longer optional—it is essential. CoMEPs, used alongside EMGfs and EMGst, offer layered information at each surgical stage. This ensures early warning signs are detected, allowing timely intervention. Additionally, long-term outcomes improve with multimodal strategies, as nerve damage correlates directly with postoperative morbidity and quality of life.

We suggest that IONMm be conducted in TEES. Thus, the CoMEP modalities should be used routinely to reduce the risk of reversible and irreversible facial nerve damage in the short, medium, and long term. Therefore, we suggest a paradigm shift to TEES that has not yet been proposed in the current literature. This shift is necessary for a more adequate neurophysiological multimodal protocol, leveraging previous experience in multimodal montages for skull base tumors with the aim of reducing the occurrence of possible false results when EMGst is used alone.

Anesthetic management in endoscopic mastoid surgery with intraoperative multimodal neuromonitoring requires a tailored approach that prioritizes total intravenous anesthesia, minimizes neuromuscular blockade, and integrates depth of anesthesia and nociception monitoring. Close interdisciplinary collaboration is essential to maximize the benefits of neuromonitoring while ensuring patient safety and optimal surgical outcomes.

The choice of anesthetic agents significantly impacts the quality and reliability of intraoperative neuromonitoring signals. Volatile anesthetics and neuromuscular blocking agents can suppress or alter evoked potentials and electromyographic activity, potentially reducing the sensitivity of facial nerve monitoring and other neurophysiological modalities. Total intravenous anesthesia (TIVA), typically using propofol and shortacting opioids, is generally preferred to optimize monitoring conditions, as it minimizes interference with neurophysiological signals. However, propofol may prolong emergence, which must be balanced against the need for high-fidelity monitoring [11-13].

Maintaining an appropriate depth of anesthesia is essential to prevent patient movement and awareness, while also avoiding excessive anesthetic dosing that could suppress neuromonitoring signals. Processed EEG monitors (e.g., BIS, entropy) can assist in titrating anesthetic depth, but their use must be integrated with neuromonitoring requirements. Nociception monitoring, though less developed, may further help optimize analgesia and reduce intraoperative opioid use, which can also affect neuromonitoring quality [12, 13].

The adoption of multimodal analgesia aims to reduce opioid requirements and their side effects, but the impact of adjunctive agents (e.g., ketamine, dexmedetomidine, lidocaine) on neuromonitoring remains incompletely characterized. Some agents may alter neurophysiological signals or interact with anesthetic depth, necessitating careful selection and dosing [13, 14].

The integration of multimodal neuromonitoring (e.g., facial nerve EMG, EEG, cerebral oximetry) requires meticulous setup and ongoing communication among the surgical, anesthesia, and neuromonitoring teams. Artifacts from electrocautery, irrigation, or endoscopic equipment can confound signal interpretation. Furthermore, the need for a bloodless field and controlled ventilation may further complicate anesthetic management [15].

In summary, the reliability of neuromonitoring is contingent upon optimal anesthetic management and technical expertise [16].

From a legal perspective, we emphasize that, in our field, IONMm and IONMu must be conducted by a physician with adequate training to perform such a procedure. As established by Resolution N° 2383 of the Brazilian Federal Council of Medicine in 2024, if there is no physician available in the operating room to conduct IONM and the procedure is carried out by a technician alone, this situation constitutes the illegal practice of medicine. This can lead to penal sanctions for the surgical team and hospital for complicity in this illegal practice. This resolution also states that the surgeon, their assistants, and the anesthetist cannot be responsible for actions in the room other than their primary roles. In summary, in Brazil, delegating this responsibility to non-medical personnel constitutes illegal practice and places the entire surgical team at risk of legal sanction. Surgeons must be aware that failure to use comprehensive monitoring, particularly in high-risk cases involving procedures like Transcranial Electrical Evoked Potentials (TEES), may constitute negligence under medico-legal scrutiny. This resolution reinforces that safety cannot be compromised by logistical convenience or outdated protocols.

Reinforcing this point, the unsupervised use or delegation of IONM to nonmedical personnel constitutes the unauthorized practice of medicine and may result in both civil and criminal liability. This legal framework emphasizes the importance of IONMm not only as a clinical safeguard but also as a medicolegal necessity. Furthermore, the surgeon cannot assume dual roles—surgical and neurophysiological—during the procedure. Therefore, a dedicated physician, under the responsibility of a neurophysiologist, must be present to interpret data and provide alerts due to the complexity of information acquired through multimodal neurophysiological techniques, which necessarily include CoMEP in addition to other described neurophysiological techniques.

We propose as a consistent and secure IONMm protocol transliterating other protocols for surgeries involving the assessment of the integrity of a cranial nerve [5, 6]:
1. Multimodal Anesthesia with a Focus on the Total Intravenous Anesthetic Protocol (TIVA);
2. Baseline and intraoperative CoMEPs for orbicularis oris, oculi, mentalis, and frontalis muscles (four branches of collection, due to the high anatomical variability of the facial nerve);
3. Continuous EMG free-running (EMGfr) for all relevant branches;
4. Triggered EMG (EMGst) for mapping proximity using the techniques described above in skull base protocols;
5. Optional SEP monitoring in more complex skull base or revision cases;
6. If possible, use intraoperative reflexology techniques on the facial nerve.

This IONMm strategy ensures the mapping, reflexology, and monitoring of nerve integrity using CoMEP (the most relevant neurophysiological tool), with alarm thresholds tailored to facial nerve dynamics.

TEES is a valuable advancement in otologic surgery, but its benefits must not be offset by preventable neurological injuries. While informative, unimodal EMGst-based strategies fall short in providing the dynamic, functional feedback essential for nerve preservation. IONMu is an incomplete and potentially hazardous approach during TEES. To maximize facial nerve preservation, multimodal neurophysiological strategies — particularly those incorporating CoMEPs — must become the standard of care. We advocate for the universal adoption of these protocols in all complex otologic surgeries, reinforcing patient safety and medicolegal compliance. As evidence mounts, the burden shifts to surgical teams and institutions to update their standards in alignment with contemporary neuroprotective practices.

In conclusion, the incomplete postoperative facial nerve deficit rate in TEES and mastoidectomies cannot be effectively avoided when using unimodal or incomplete monitoring in isolation. We suggest that, in a multidisciplinary approach, the use of IONMm in addition to EMGst, EMGsf, and routine CoMEP is the most adequate protocol to monitor facial neural integrity. Unfortunately, these are not widely used during TEES and other mastoid surgeries. We recommend the use of IONMm in all complex surgeries that require intraoperative neurophysiological monitoring.

Finally, the objective of IONMm is to guarantee and assure the surgical team a reduced rate of partial or permanent facial nerve damage, which is not perceptible using only IONMu with EMGst in isolation.

Multimodal IONM, including CoMEPs, likely represents the gold standard in neurophysiological surveillance, enabling safer surgeries and better long-term outcomes. Furthermore, compliance with legal standards and ethical responsibilities mandates the presence of trained physicians during monitoring. We call on the otologic surgical community to universally adopt multimodal strategies—not as an innovation, but as an obligation to patient safety.

None.

No conflict of interest.

1. Moneir W, El-Ekiaby R, Elkahwagi M (2025) Thermal injury in endoscopic ear surgery between reality and fiction. Eur Arch Otorhinolaryngol.
2. Kamalden TMIT, Yusof ANM, Misron K (2021) A Review of Delayed Facial Nerve Paresis as Complication Following Total Endoscopic Ear Surgery. J Int Adv Otol 17(6): 570-573.
3. Li KH, Chan LP, Chen CK, Kuo SH, Wang LF, et al. (2021) Comparative Study of Endoscopic and Microscopic Type I Tympanoplasty in Terms of Delayed Facial Palsy. Otolaryngol Head Neck Surg 164(3): 645-651.
4. Takihata S, Kurioka T, Mizutari K, Shiotani A (2022) Factors affecting the incidence of chorda tympani nerve transection in middle ear surgery. Laryngoscope Investig Otolaryngol 7(6): 2088-2094.
5. Fernández Conejero I, Ulkatan S, Deletis V (2022) Monitoring cerebellopontine angle and skull base surgeries. Handb Clin Neurol 186: 163-176.
6. Izzo MGA, Rossi Sebastiano D, Catanzaro V, Melillo Y, Togni R, et al. (2024) Three montages for Transcranial electric stimulation in predicting the early post-surgery outcome of the facial nerve functioning. Clin Neurophysiol 167: 282-293.
7. Dillon FX (2010) Electromyographic (EMG) neuromonitoring in otolaryngology-head and neck surgery. Anesthesiol Clin 28(3): 423-442.
8. Marchioni D, Rubini A, Gazzini L, Alicandri Ciufelli M, Molinari G, et al. (2018) Complications in Endoscopic Ear Surgery. Otol Neurotol 39(8): 1012-1017.
9. Nilssen EL, Wormald PJ (1997) Facial nerve palsy in mastoid surgery. J Laryngol Otol 111(2): 113-116.
10. Ryu NG, Kim J (2016) How to Avoid Facial Nerve Injury in Mastoidectomy? J Audiol Otol 20(2): 68-72.
11. Ma K, Bebawy JF, Hemmer LB (2023) Multimodal Analgesia and Intraoperative Neuromonitoring. J Neurosurg Anesthesiol 35(2): 172-176.
12. Nunes RR, Bersot CDA, Garritano JG (2018) Intraoperative Neurophysiological Monitoring in Neuroanesthesia. Curr Opin in Anaesthesiol 31(5): 532-538.
13. Gunter A, Ruskin KJ (2016) Intraoperative Neurophysiologic Monitoring: Utility and Anesthetic Implications. Curr Opin in Anaesthesiol 29(5): 539-543.
14. Casano K, Giangrosso G, Mankekar G, et al. (2020) Additional Benefits of Facial Nerve Monitoring During Otologic Surgery. Otolaryngol Head Neck Surg 163(3): 572-576.
15. Laferrière Langlois P, Morisson L, Jeffries S, et al. (2024) Depth of Anesthesia and Nociception Monitoring: Current State and Vision for 2050. Anesthesia and Analgesia 138(2): 295-307.
16. Bonatti G, Iannuzzi F, Amodio S, et al. (2021) Neuromonitoring During General Anesthesia in Non-Neurologic Surgery.
17. Best Pract Res Clin Anaesthesiol 35(2): 255-266.