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Electromyographic monitoring of facial nerve under different levels of neuromuscular blockade during middle ear microsurgery

CAI, Yi-rong; XU, Jing; CHEN, Lian-hua; CHI, Fang-lu

Section Editor(s): GUO, Li-shao

doi: 10.3760/cma.j.issn.0366-6999.2009.03.014
Original article

Background The evoked electromyography (EMG) is frequently used to identify facial nerve in order to prevent its damage during surgeries. Partial neuromuscular blockade (NMB) has been suggested to favor EMG activity and insure patients’ safety. The aim of this study was to determine an adequate level of NMB correspondent to sensible facial nerve identification by evaluating the relationship between facial EMG responses and peripheral NMB levels during the middle ear surgeries.

Methods Facial nerve evoked EMG and NMB monitoring were performed simultaneously in 40 patients who underwent tympanoplasty. Facial electromyographic responses were recorded by insertion of needle electrodes into the orbicularis oris and orbicularis oculi muscles after electrical stimulation on facial nerve. The NMB was observed objectively with the hypothenar muscle's twitching after electrical stimulation of ulnar nerve, and the intensity of blockade was adjusted at levels of 0, 25%, 50%, 75%, 90%, and 100% respectively with increased intravenous infusion of Rocuronium (muscle relaxant).

Results All of the patients had detectable EMG responses at the levels of NMB ≤50%. Four out of forty patients had no EMG response at the levels of NMB ≥75%. A significant linear positive correlation was present between stimulation thresholds and NMB levels while a linear negative correlation was present between EMG amplitudes and NMB levels.

Conclusions The facial nerve monitoring via facial electromyographic responses can be obtained when an intraoperative partial neuromuscular blockade is induced to provide an adequate immobilization of the patient. The 50% NMB should be considered as the choice of anesthetic management for facial nerve monitoring in otologic microsurgery based on the relationship of correlation.

Edited by

Department of Anesthesiology (Cai YR, Xu J and Chen LH), Department of Otolaryngology (Chi FL), Eye, Ear, Nose and Throat Hospital, Fudan University, Shanghai 200031, China

Correspondence to: Prof. CHEN Lian-hua, Department of Anesthesiology, Eye, Ear, Nose and Throat Hospital, Fudan University, Shanghai 200031, China (Tel: 86–13816880840. Email:

CAI Yi-rong and XU Jing contributed equally to this work.

This study was supported by a grant from the National Natural Science Foundation of China (No. 30872428).

(Received September 20, 2008)

Postoperative facial nerve paralysis is a devastating complication after facial or ear surgeries. Iatrogenic facial nerve injury occurs in 0.6%-3.6% of patients who had primary otologic surgeries, and in 4%-10% of patients who had repeated otologic surgeries.1 Intraoperative facial electromyographic (EMG) monitoring is a helpful tool for surgeon to identify facial nerve and map its course during the surgery, and this technique has been shown to reduce the rate of iatrogenic facial nerve injury significantly.2–7 The use of such a monitoring unit may place an additional restraint on the anesthetic management with great worries of administration of muscle relaxant. The facial EMG monitoring needs a functional and intact facial neuromuscular junction to enable facial muscles responsive to nerve stimulation. Neuromuscular blocking agents, as an important ingredient for maintaining general anesthesia, act directly on the nerve muscle junction and block the signal transmission. It leads to patient's paralysis and immobilization as required in general anesthesia, but at the same time, it may interfere the facial nerve monitoring during the surgery. So theoretically these agents should be avoided to exclude any compromise of facial EMG monitoring capability. However, delicate microsurgical interventions in otologic surgery demand absolute immobility of the patient. Usually, this goal is achieved using large doses of narcotics and volatile anesthetic agents without giving muscle relaxant. This anesthetic technique is poorly tolerated by some patients with severe hemodynamic instability, necessitating the use of vasopressors to support circulatory system.8 So it is essential to keep balance between hemodynamic stability and absolute immobility, while preserving an optimal condition for facial EMG monitoring.

With the introduction of the peripheral nerve stimulator using train-of-four (TOF) stimulation, the anesthesiologist can measure and titrate the level of neuromuscular blockade (NMB). Several reports have suggested that partial neuromuscular blockade (PNMB) can be used to prevent patient movement while still retaining the ability to elicit EMG responses with facial nerve stimulation, most of them were investigated during resection of acoustic neuromas.9–11 There are several differences of the operative management and the condition of the facial nerve between resection of acoustic neuromas and otologic surgery, which may affect the role of PNMB on facial nerve monitoring. Moreover, the relationship between facial EMG responses and peripheral NMB levels has not been well established.

Therefore, the current study was designed to examine the feasibility of controlled levels of neuromuscular blockade on preserving adequate ability for facial nerve EMG monitoring; to determine the appropriate level of neuromuscular blockade for facial nerve monitoring in middle ear surgery; to evaluate the relationship between facial EMG responses and peripheral NMB levels.

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General data

Forty patients of ASA I—II (American Society of Anesthesiologists, grade I—II, 19 men, 21 women, 20–59 years old) undergoing open cavity tympanoplasty were included in the study. Preoperatively, all patients demonstrated clinically normal facial nerve function confirmed by House-Brackmann grading (Class I). Markedly obese patients and patients with neuromuscular disease were excluded. Approval of this study was obtained from the Hospital Human Research Committee and written informed consent was obtained from all patients.

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Facial nerve and ulnar neuromuscular blockade monitoring

Intraoperative facial nerve evoked EMG and ulnar NMB monitoring were performed simultaneously. The facial nerve evoked EMG was record by NIM-ResponseTM system (Medtronic XOMED, USA). After the patient was anesthetized, the subcutaneous needle electrodes were inserted into the upper and lower orbicularis oris and orbicularis oculi muscles on the operative side. The facial nerve was directly stimulated with a Prass monopolar stimulator (square wave constant current pulses, 0.1–3.0 mA, 4 Hz, 100 μs) and the response was measured and recorded in millivolts. The maximal stimulation current was limited within 1 mA to avoid the iatrogenic facial nerve injury. All surgical manipulations and facial nerve stimulations were executed by the same surgeon.

The level of peripheral neuromuscular blockade was determined using the TOF-Watch®SX instrument (Organon, Holland). Stimulation on the ulnar nerve was performed using surface electrodes placed over the ulnar nerve 2 and 9 cm proximal to the distal end of the ulna. The stimulation was composed of a train of four stimulations (60 mA, 4 Hz, constant current of four twitches at 2 Hz every 0.5 second over 2 seconds). The adduction of the thumb was assessed by an accelerometer. The percentage of the first twitch response compared to baseline (T1%) was used to determine the level of peripheral neuromuscular blockade. i.e. NMB=1-T1%.

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Anesthetic management

Anesthesia was induced intravenously with midazolam 2–3 mg, fentanyl 2 μg/kg, and propofol 2 mg/kg. After the patient was anesthetized, the baseline response of the thumb in the non-muscle relaxant condition was calibrated to 100%. The tracheal intubation was facilitated with rocuronium 0.6 mg/kg. Anesthesia was maintained with a continuous intravenous infusion of remifentanil 0.2 μg·kg-1 ·min-1 and inhalation of isoflurane (0.6 MAC). Electrocardiography, sphygmomanometry, pulse oximetry and temperature were monitored in patients. Ventilation was mechanically controlled and PETCO2 was kept within 35 and 40 mmHg throughout the surgical procedure by adjusting the end tidal volume. Controlled hypotension (MAP 55–65 mmHg) to reduce surgical bleeding was applied to achieve a clear operation field by changing the depth of anesthesia. Electric blanket was used to keep patients’ temperature within 36°C-37°C.

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Patients were divided into two groups according to the surgical manipulation, facial nerve exposure group (group A) and non-exposure group (group B). In group A the tympanic segment of facial nerve was exposed by dehiscence of fallopian's channel and the stimulations were applied through the crevice, while in group B the fallopian's channel was kept intact and the stimulations were performed on the surface of the channel. After complete recovery of peripheral neuromuscular functions from the muscle relaxant used during the induction, the baseline of facial nerve response evoked by electrical stimulation was recorded. Then, the intravenous infusion rate of Rocuronium was gradually increased to reach the targeted NMB levels at 0, 25%, 50%, 75%, 90% and 100%. The stimulation thresholds and amplitudes of EMG responses were recorded at above-mentioned NMB levels respectively. Stimulation thresholds were determined by increasing stimulating intensity in increments of 0.05 mA initiated from 0.05 mA until recordable EMG responses were obtained. According to the preliminary experiments, stimulating with 0.3–0.5 mA and 0.8–1.0 mA could evoke stable and repeatable EMG responses without postoperative facial nerve injury in both exposure and non-exposure groups. The response amplitudes were determined by stimulating facial nerve with 0.5 mA in group A, while 1.0 mA in group B.

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Statistical analysis

Analyses were performed by using SPSS 11.5 software. Average values and standard deviations were calculated, and statistical analyses were performed by analysis of variance (ANOVA) with Student-Newman-Keuls Test for multiple comparison. Spearman's Rank Correlation analysis was employed to assess the correlation of facial EMG monitoring results and different NMB levels. A P value <0.05 was considered statistically significant.

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Of all cases, 31 patients suffered from chronic otitis media and 9 patients suffered from cholesteatoma. Among them, 36 cases were primary surgeries and the other 4 cases were revisions. In the operative field, the facial nerve was found to be exposed in 16 of the cases, then Group A contained 16 cases and Group B contained 24 cases. The characteristics and pathologies of the patients are presented in Table.



All of the patients had recordable EMG responses when the levels of peripheral NMB were ≤50%. However, when the levels of peripheral NMB were ≥75%, no response to facial nerve stimulation was detected in 4 patients, and all of whom were in group B.

The recordings of facial nerve stimulation thresholds and EMG amplitudes for different NMB levels are shown in Figures 1 and 2. The stimulation threshold in Group A was significantly lower than that in Group B at each NMB level, P <0.01. Significant differences were found between each NMB levels for stimulation thresholds and EMG amplitudes in two groups, P <0.05. An increase in the stimulation thresholds and a decrease in the facial EMG amplitudes generally occurred as NMB levels increased in both groups. There was a linear positive correlation between stimulation thresholds and NMB levels (rA=0.38, rB=0.26, P <0.01), while there was a linear negative correlation between EMG amplitudes and NMB levels (rA=-0.66, rB=-0.55, P <0.01).

Figure 1.

Figure 1.

Figure 2.

Figure 2.

The stable hemodynamics was maintained and the immobility was achieved in all patients during the operation. Postoperatively, all 40 patients demonstrated clinically normal facial nerve function.

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This study showed that all of the patients had recordable EMG responses when the levels of peripheral NMB were lower than 50%. Clinically, the purpose to use intraoperative facial EMG monitoring is to identify and map the facial nerve, so we consider the results of patients who had non-dehiscent Fallopian's channel to be more important. In group B, 4 out of 24 patients’ EMG response was unable to be detected when the levels of peripheral NMB were greater than 75%. The stimulation threshold of the facial nerve for a recordable response at 75% NMB was significantly higher than that at 50% NMB, while the EMG amplitude was significantly smaller at 75% NMB compared with that at 50%. Based on these findings, we conclude that the NMB levels greater than 75% are not suitable for facial EMG monitoring during tympanoplasty. Theoretically, the least NMB would provide an optimal condition for facial EMG monitoring, but the risk for a potential body movement may be dramatically increased if the patient was not adequately relaxed with NMB lower than 25%. For the consideration that adequate surgical relaxation is generally obtained at 55%-60% twitch height reduction,12 we feel that the 50% NMB is the most appropriate level for facial nerve monitoring in middle ear microsurgery. Our conclusion concurs with previous reports which had demonstrated that a 50% level of peripheral NMB with atracurium provided sufficient muscle relaxation and the reliable facial nerve monitoring in otologic surgery9 and acoustic neuroma surgery.11 Our results will provide a better explanation of the relationship between the NMB and facial nerve monitoring because we observed both stimulation threshold and EMG amplitude simultaneously while EMG amplitude was only measured in most of previous studies.

Although studies had demonstrated the effect of PNMB on facial nerve monitoring, the contradictory conclusions still exist among the reports about their relationship. Blair EA et al8 found a strong linear correlation between both T1 and T4/T1 (T4: the fourth twitch response of TOF) to the percent of baseline amplitude of facial EMG in patients undergoing cerebellopontine angle tumor resection, and but, the correlation between the degree of peripheral NMB and the magnitude of the facial action potential could not be found in another study conducted in patients undergoing acoustic neuroma resection.13 The author attributed the disagreement to some methodological problems. There have been no reports regarding the relationship between facial EMG responses and peripheral NMB levels in middle ear surgery. In the results from the current study, a significant linear positive correlation was proved between stimulation thresholds and NMB levels, while a linear negative correlation between EMG amplitudes and NMB levels. Despite there are differences between the otologic and neurologic procedures, our results presented significance in this area.

Studies had showed that there was a distinct difference in sensitivity to non-depolarizing muscle relaxants between the facial nerve and ulnar nerve. There were evidence to suggest that the facial musculature seems to be less sensitive to neuromuscular blocking agents14–16 compared with hypothenar muscle, and that is, at a given level of blockade, the response of facial nerve is always stronger than the one of ulnar nerve. This provides a theoretical support for the use of PNMB on intraoperative monitoring of the facial nerve. The explanation for this relative insensitivity remains unclear. It is said that the facial muscles have a greater number of neuromuscular junctions than other muscles in the body.17 Whether a different affinity of the facial acetylcholine-receptor for acetylcholine or non-depolarizing muscle relaxants plays an additional role has not been established yet. Further studies will be designed to investigate these questions.

In conclusion, this study suggests that partial neuromuscular blockade can provide reliable conditions for intraoperative facial EMG monitoring as well as adequate immobilization. It meets the clinical desires of limiting the facial nerve damage by identifying it clearly during the surgery, and of keeping patient motionless by adequate muscle relaxation. The 50% NMB should be considered as the choice of anesthetic management for facial nerve monitoring in otologic microsurgery.

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neuromuscular blockade; facial nerve; evoked electromyography

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