European Archives of Oto-Rhino-Laryngology (2023) 280:1055–1062 https://doi.org/10.1007/s00405-022-07556-8 OTOLOGY Pre‑ and intra‑operative prognostic factors of facial nerve function in cerebellopontine angle surgery Francesco Gazia1,2 · Àngela Callejo1 · Marta Pérez‑Grau1 · Susana Lareo1 · José Prades1 · Francesc Roca‑Ribas1 · Emilio Amilibia1 Received: 24 April 2022 / Accepted: 12 July 2022 / Published online: 30 July 2022 © The Author(s) 2022 Abstract Purpose The study assesses whether pre- and intraoperative factors linked to electromyography and direct electrical stimu- lation (DES) of facial nerve can predict facial nerve function in the short- (12 days) and long-term (1 year) after cerebel- lopontine angle (CPA) tumor resection. Methods 157 patients who underwent surgical resection of CPA tumors with facial nerve monitoring. Pre-operative fac- tors (age, tumor size, pure tone average), surgical time and intra-operative parameters regarding facial function, minimum stimulation threshold (MST), compound muscle action potential (CMAP) and the difference between proximal and distal CMAP (DPDC) were evaluated. Results A correlation between tumor size, MST, CMAP and facial function in both short and long term was found. A higher grade of immediate facial paralysis corresponded to a higher risk of poor outcome after one year. A postoperative House– Brackmann (HB) score of V or VI was correlated with poor outcome in 88.8% and 93.8% of cases. A risk of HB 3 or more, in the long term, was correlated with a tumor size of 20.2 mm. Using an MST of 0.1 mA, for long-term predictions, sensitivity and specificity were 0.62 (95% CI 0.46–0.75) and 0.73 (95% CI 0.61–0.82), respectively. With a CMAP cut-off < 200 µV, for long-term prediction, sensitivity was 0.73 (95% CI 0.53–0.87) and specificity 0.73 (95% CI 0.55–0.85). Conclusion The assessment based on the cut-offs described increases the ability to predict facial function. Improving pre- dictive accuracy enables surgeons to address patients’ expectations and to establish an intervention timeline for planning facial reanimation. Keywords Electromyography · Stimulation threshold · Vestibular schwannoma · Muscle action potential · Prognosis · Nerve monitoring · Facial nerve Introduction have better nerve functional results, especially those with large tumors [1, 2]. Since the introduction of electromyo- Skull base surgery, and in particular the resection of cer- graphy (EMG) monitoring of the facial nerve by Delgado ebellopontine angle (CPA) tumors, entails a significant in 1979, the proportion of severe postoperative facial dys- risk of impaired facial nerve activity. Patients with surgi- function has dropped from 15 to 59% in the pre-monitoring cal removal of CPA tumors with facial nerve monitoring era to 10–33% [3]. Facial nerve monitoring is indispensable in skull base surgery today. According to recent literature reports, the anatomical conservation rate of the facial nerve * Emilio Amilibia in vestibular schwannoma (VS) surgery has improved by emilioamilibia@gmail.com up to 86–92%; even so, there are some patients who suffer 1 Department of Otorhinolaryngology, Hospital Universitari a facial dysfunction after surgery even though the nerve is Germans Trias i Pujol, Universitat Autònoma de Barcelona conserved [4–6]. (UAB), Carretera del Canyet, Badalona, 08916 Barcelona, One of the most important aims of skull base surgery Spain is the preservation of facial nerve function. Facial palsy 2 Unit of Otorhinolaryngology, Department of Adult is associated with increases in anxiety and depression, and Development Age Human Pathology “Gaetano Barresi”, and poorer quality of life [7, 8]. The capacity to predict University of Messina, Messina, Italy 13 Vol.:(0123456789) 1056 European Archives of Oto-Rhino-Laryngology (2023) 280:1055–1062 post-operative facial outcome is vital for the counselling those with alterations of the facial function before surgery between surgeon and patient regarding prognosis, expec- (HB > 1) were excluded. tations, and the prospects for rehabilitation. Several intra- The pre-operative factors likely to present significant operative monitoring techniques have been used with the associations with facial nerve outcome (age, tumor size and aim of saving and predicting facial nerve activity. However, pure tone average, PTA) were evaluated. Surgical time was the clinical application of EMG recordings has not been also evaluated. standardized [9]. The EMG criteria that allow the predic- As regards the facial nerve function, the following intra- tion of facial function are widely accepted, but the lack of operative parameters likely to present significant associa- standardization of the stimulation parameters has made it tions with facial nerve outcome were evaluated: MST, proxi- impossible to establish which method is the best [10]. The mal CMAP and the difference between proximal and distal two most frequently employed facial monitoring systems are CMAP (DPDC). direct electrical stimulation (DES) and free-running EMG; transcranial facial nerve motor-evoked potentials (FMEPs) Tumor size have been introduced more recently. DES implies the use of an impulse of constant voltage or constant current to the All cases included in this study were diagnosed by brain VII cranial nerve within the CPA through a mono- or bipo- MRI. Tumor size was recorded as the largest CPA diameter lar stimulator. The VII cranial nerve function obtained as measured in the axial, coronal, or sagittal view: the portion an effect of an electric impulse is perceived by electrodes of the tumor located inside the internal auditory canal (IAC)) which register muscle activity. It is amplified through a was not counted. Tumor size was evaluated using the Tos loudspeaker to show a compound muscle action potential and Thomson classification based on the largest extrameatal (CMAP) [11]. Intraoperative EMG monitorization is useful diameter only: intrameatal tumors, small tumors (1–10 mm not only in tumor resection and facial nerve preservation, but extrameatal size), medium tumors (11–20 mm), large tumors also for the prognosis of postoperative facial nerve outcome (21–40 mm), and giant tumors (41 mm or larger) [15]. [12, 13]. Several parameters can be used to predict the out- come of facial function. Measurement of the nerve minimum House–Brackmann grading system stimulation threshold (MST) near the brainstem is one of the most commonly applied. Postoperative facial nerve out- Facial nerve function was scored according to the comes are inversely proportional to the MST measured after House–Brackmann Grading System (Grades I–VI: Grade I, tumor removal, near the brainstem; a higher MST indicates normal function; Grade VI, total palsy). The target disorder, a worse outcome for facial function [14]. poor facial nerve outcome, was defined as HB III–VI, while The purpose of the present study is to retrospectively good facial nerve outcome was defined as HB I-II. Time- evaluate whether the intraoperative MST, CMAP and the points of study for evaluation of the facial activity were difference between proximal and distal CMAP from facial short-term (12 days post-surgery) and long-term (1 year monitoring can predict facial nerve function in the short post-surgery) [16]. (12 days) and the long term (1 year) after VS resection. We also analyse whether preoperative factors (age, size, tonal Audiological tests audiometry) or surgical time could predict facial function outcomes. Pure-tone audiometry was performed to assess the air conduction hearing threshold in a range between 125 and 8000 Hz and in a range between 250 and 4000 Hz for bone Materials and methods conduction. The PTA of hearing threshold levels at a set of specified frequencies (500, 1000, 2000, 4000 Hz) was A retrospective study was carried out at the otorhinolaryn- computed. gology department of a tertiary referral center, revising the local database and the medical charts of the patients with Facial nerve monitoring the approval of the Institutional Ethics Committee. The study included data on 157 patients who underwent surgi- Intraoperative 2-channel EMG monitoring was performed cal removal of a CPA tumor, with monitoring of facial nerve with the NIM response 2.0 system (Medtronic Xomed Inc., function by DSE and EMG. The facial nerve function was Jacksonville, FL, USA) in all patients. Two paired bipolar assessed immediately and at 12 days, 3 and 12 months post- subdermal electrodes were placed in the orbicularis oculi operatively. The surgery was always performed by the same and in the oris muscles. Differences in impedance between surgical team, between January 2013 and September 2020. electrodes remained less than 1 kΩ during the recordings Patients undergoing repeat surgery for the CPA tumor and on both channels. DES of the facial nerve was performed 13 European Archives of Oto-Rhino-Laryngology (2023) 280:1055–1062 1057 with a monopolar probe with a 0.5 mm tip. The stimulation A “p” level of ≤ 0.05 was considered significant. applied was square waves of 100 ms duration with 4 Hz fre- quency. The monitor reports when a response greater than 1 µV occurs with a signal filter of 3.10 ms. Results We consider the MST, the stimulus capable of producing 100 µV of CMAP response, recorded in at least one channel. The study population consisted of 157 patients (54.7% male, The stimulation protocol started with a 0.05 mA MST and 45.3% female), with a mean age of 51.9 years. Mean PTA was then decreased in intervals of 0.01 mA, up to a mini- before surgery was 61.7 dB and the mean surgical time was mum stimulation of 0.01 mA or increased up to a maximum 8.1 h. In almost all cases the translabyrinthine approach was stimulation of 5.0 mA. used (96.2%) with a final pathology result of VS in 93% and After tumor resection, the facial nerve proximal to the meningiomas in 7%. In some cases, a near total (35.7%) tumor, at the brain stem exit, was stimulated with the MST or a partial (3.8%) tumor resection was done to preserve to obtain the proximal CMAP. The facial nerve was then the facial nerve, since the tumor was closely attached to it stimulated with the same threshold at a point distal to the (Table 1). location of the tumor, in the IAC fundus, in order to obtain Most of the tumors were medium or large according to the distal CMAP. The peak-to-peak amplitude of each the the Tos and Thomsen Classification 124/157 (79%). Only proximal and distal CMAP was established and the differ- tumors which, after careful follow up, were seen to have ence was calculated (DPDC). increased in size or tumors which reached the brainstem When MST could not be detected we considered the were operated (Table 1 Supplemental material). CMAP response to be 0, and so did not calculate the DPDC. Anesthesia management Table 1  Study population Following the standard protocol for skull base surgery, Population M ± SD anesthesia was induced with intravenous application of the N (%) sedative drug propofol (2–3 mg/kg), the opioid analgesic remifentanil (0.5 mcg/kg/3 min), and the skeletal muscle Gender relaxant rocuronium (0.6 mg/kg). After intubation, rocuro- Male 86/157 (54.7%) nium was omitted because of its interference with electro- Female 71/157 (45.3%) physiological monitoring and mapping. Age (years) 51.9 ± 13.1 Side Statistical analysis Left 95/157 (60.5%) Right 62/157 (39.5%) Statistical analyses were performed using SPSS 25.0 (IBM PTA (dB) 61.7 ± 32.07 SPSS Statistics, New York, USA). Continuous variables Size (mm) were presented as means with standard deviations and Extracanalicular 20.34 ± 10.7 categorical variables were summarized with numbers and Intracanalicular 8.18 ± 3.7 proportions. Data normality was assessed using the Kol- Duration of surgery (h) 8.11 ± 2.4 mogorov–Smirnov test. The logistical regression was per- Approach formed to analyse the preoperative clinical predictors that Translabyrinthine 151/157 (96.2%) had impact on postoperative short- and long-term facial Transotic 3/157 (1.9%) nerve outcome (poor or good). Spearman 𝜌 Index was used Retrosigmoid 2/157(1.3%) to obtain the correlation between size, duration of surgery, Medial fossa 1/157 (0.6%) MST, CMAP, DPDC and HB. Tumor resection Two-by-two tables were used to determine the sensitiv- Total 95/157 (60.5%) ity and specificity of MST, CMAP and DPDC for predict- Near total 56/157 (35.7%) ing short and long-term facial nerve outcomes. Sensitivity Partial 6/157 (3.8%) reflects the probability that the parameters to be within the Anatomic pathology defined zone, given that the patient has a good outcome. Vestibular Schwannoma 146/157 (93%) Specificity is the probability that parameters will be outside Meningiomas 11/157 (7%) the defined zone, given that the patient has a poor outcome. Tumors associated with NF2 5/157 (3.1%) Odds ratio and their corresponding 95% confidence intervals N number, % percentage, M media, SD standard deviation, PTA pure were calculated. tone average, NF2 neurofibromatosis type 2 13 1058 European Archives of Oto-Rhino-Laryngology (2023) 280:1055–1062 Table 2  Logistical regression analysis of preoperative clinical predic- tors with regard to their impact on postoperative facial function long- 93.8 term outcomes (1 year) 88.8 Odds Ratio 95% CI p value Age 0.99 0.96–1.02 0.56 Size 1.06 1–1.07 0.04 Duration of surgery 1.16 0.99–1.37 0.06 Percentage % PTA 1.01 0.99–1.01 0.55 43.7 MST 114.12 4.02–3475.2 < 0.01 CMAP 0.99 0.99–0.99 < 0.01 29.4 Difference proximal 0.99 0.99–1 0.14 and distal CMAP 0 0 PTA pure tone average, MST minimum stimulation threshold, CMAP compound muscle action potential, CI confidence Interval 1 2 3 4 5 6 HB HB, House-Brackmann the long term was correlated with a mean size of 20.2 mm (Fig. 2). Fig. 1  Logistic regression model: Immediate post-operative HB As for surgical time, a positive correlation with short- and probability of long-term (1 year) negative outcome. HB House– term (r = 0.26, p < 0.01) HB grade was found (Table 3 Sup- Brackmann plemental material). The linear regression model showed that a risk of an HB 3 or more, in the short term, was cor- Short-term facial outcome was good in 44% of cases (HB related with an operation lasting more than 7.8 h (Fig. 4 I, 31%; HB II, 13%), poor outcome was obtained in 56% Supplemental material). (HB III, 12%; HB IV, 12%; HBV 23%; HB VI, 9%) (Fig. 1. Regarding the intra-operative DES during facial nerve Supplemental material). monitoring, a positive correlation between MST and short Long-term facial outcome was good in 61.1% of cases (r = 0.45, p < 0.01) and long-term (r = 0.40, p < 0.01) HB (HB I, 50.4%; HB II, 10.7%), and poor in 38.9% (HB III, grade was found. A negative correlation was calculated 18.5%; HB IV, 9.5%; HB V, 7%; HB VI, 3.9%) (Fig. 2. Sup- between proximal CMAP and short- (r = − 0.71, p < 0.01) plemental material). The worst long-term facial outcomes and long-term (r = − 0.62, p < 0.01) HB grade. A negative were obtained in giant and large tumors (Fig. 3. Supple- correlation between the DPDC and only the short-term mental material). (r = − 0.36; p < 0.01) HB grade (Table 3) was also found. Logistical regression was applied to analyse the preopera- The sensitivity and specificity of the parameters of the tive clinical predictors that had an impact on postoperative intra-operative DES were calculated for both short-term short- and long-term facial nerve outcome. The factors with (Table 4 Supplemental material) and long-term poor facial a statistically significant association were size, MST, CMAP outcome (Table 4). When an MST was not detected during for short- and long-term outcome (Table 2), and duration of the DES, and consequently not even one CMAP, the speci- surgery and DPDC only for short-term outcome (Table 2 ficity was 100% for both short- and long-term poor outcome. Supplemental material). No significant association with age and PTA (p > 0.05) was found. The logistical regression model was also used to associate Table 3  Correlation using Spearman 𝜌 Index between MST, proximal immediate post-operative HB and the probability of a long- CMAP, proximal and distal CMAP difference and HB term (1 year) negative outcome (Fig. 1). All patients with an MST Proximal CMAP Difference HB I-II on immediate post-surgery examination maintained r (p value) r (p value) proximal and a good facial outcome after 1 year of follow-up. A postop- distal CMAP r (p value) erative HB V or VI was correlated with a poor outcome in 88.8% and 93.8% of cases. Thus, the model demonstrated HB post-surgery 0.51 (< 0.01) – 0.71 (< 0.01) – 0.45 (< 0.01) that a higher grade of immediate facial paralysis corresponds HB after 12 days 0.45 (< 0.01) – 0.66 (< 0.01) – 0.36 (< 0.01) to a higher risk of poor outcome after one year. HB after 0.47 (< 0.01) – 0.63 (< 0.01) – 0.21 (0.13) Regarding tumor size, positive correlations with short- 3 months (r = 0.25, p = 0.01) and long-term (r = 0.18, p = 0.03) HB HB after 1 year 0.40 (< 0.01) – 0.62 (< 0.01) 0.20(0.16) grade were found (Table 3 Supplemental material). A linear MST minimum stimulation threshold, CMAP compound muscle regression model showed that a risk of a HB 3 or more in action potential, HB House–Brackmann, r Spearman 𝜌 Index 13 European Archives of Oto-Rhino-Laryngology (2023) 280:1055–1062 1059 45 The most significant results were obtained using a CMAP cut-off < 200 µV. For short-term function, a sensitivity of 40 0.62 (95% CI 0.46–0.76) and a specificity of 0.79 (95% CI 35 0.57–0.92) were calculated. For long-term prediction, sensi- tivity and specificity were 0.73 (95% CI 0.53–0.87) and 0.73 30 (95% CI 0.55–0.85), respectively. 25 Analysis of the logistical regression and the correlation Size (mm) data showed DPDC to be useful only for short-term out- 20 come. Setting a DPDC cut-off < 0 (distal stimulation higher 15 than the proximal one), a sensitivity of 0.89 (0.7–0.9) and a specificity of 0.58 (0.37–0.77) were obtained. 10 5 Discussion 0 0 1 2 3 4 5 6 7 HB Today, facial monitoring is an essential instrument in CPA surgery. It helps to prevent unintended injury to the facial Fig. 2  The size increase is correlated with postoperative House– nerve, as DES allows the recognition of the nerve and ena- Brackmann grade in the long term (𝜌 0.18, p = 0.03). The diagonal line represents a linear regression model to the data. The risk of an bles more careful tumor dissection. Even at experienced HB 3 or more begins with a size of 20.2 mm centers and with the use of facial nerve monitoring, the facial nerve is at risk in CPA tumor surgery. Therefore, a significant number of patients will have delayed or perma- Table 4  Long-term sensitivity and specificity of MST, proximal nent facial nerve dysfunction even though the nerve is pre- CMAP and proximal and distal CMAP difference in facial outcome served. The incidence of long-term facial dysfunction after Sensitivity (CI 95%) Specificity (CI 95%) VS treatment varies widely, from 4.8 to 41% in previous publications, with a mean of approximately 19% [17]. MTS cut-off (mA) In addition, the ability to accurately predict short- and Not detected 33% (0.2–0.49) 100% (0.93–1) long-term facial nerve outcomes remains unclear, especially 0.1 62% (0.46–0.75) 73% (0.61–0.82) in large tumors [1]. 0.05 91% (0.77–0.97) 21% (0.13–0.33) Several studies have shown that immediate post-operative CMAP (µV) preservation of facial nerve function (HB I–II) is an excel- < 500 97% (0.80–0.99) 30% (0.16–0.47) lent prognostic indicator of a good facial nerve outcome. < 200 73% (0.53–0.87) 73% (0.55–0.85) Patients with immediate facial dysfunction are at a greater Difference proximal and distal CMAP risk of poor recovery. In our study, 100% of patients who had (µV) intact eye closure on immediate post-surgery examination <0 79% (0.48–0.94) 38% (0.23–0.55) remained House–Brackmann grade I or II after one year of follow-up. A postoperative HB V or VI was correlated with MST minimum stimulation threshold, CMAP compound muscle poor outcome in 88.8% and 93.8% of cases, respectively, in action potential, CI confidence Interval agreement with other articles that showed the positive prog- nostic value of immediate postoperative HB to be between Using 0.10 mA MST as a cut-off point, a sensitivity of 94 and 98% [18–20]. In any case, patients with poor imme- 0.55 (95% CI 0.42–0.67) and a specificity of 0.75 (95% CI diate outcome (particularly HB III) may improve with time 0.61–0.85) for short-term facial nerve activity prediction was [21]. detected. For long-term prognosis, sensitivity and specificity With regard to tumor size, a positive correlation with were 0.62 (95% CI 0.46–0.75) and 0.73 (95% CI 0.61–0.82). short- and long-term facial outcome was found, with a risk Using an MST of 0.05 mA, for short-term function, a of HB III or more in tumors larger than 20.2 mm. Rinaldi sensitivity of 0.94 (95% CI 0.83–0.97) and a specificity of et al. reported an increased incidence of facial deficit in 0.28 were calculated. For long-term function, sensitivity was patients with stage IV tumors in the Zini-Magnan classi- 0.91 (95% CI 0.77–0.97) and specificity 0.21. fication (> 2 cm) [22] either in the immediate postopera- When used to predict short-term function, CMAP 500 µV tive period or 1 year after surgery [23]. Other authors have had a sensitivity of 0.95 (95% CI 0.82–0.99) and a specific- reported that the smaller the tumor, the better the subsequent ity of 0.37. To predict long-term function, sensitivity was facial function. Especially in the case of larger tumors, this 0.97 (95% CI 0.80–0.99) and specificity 0.3. association may be related to a pre-existing nerve weakness 13 1060 European Archives of Oto-Rhino-Laryngology (2023) 280:1055–1062 (due to the compression caused by the tumor) and to 200 µV, with which we obtained excellent predictive results, increased number of difficult surgical maneuvers during the more studies are needed for comparison. dissection of the tumor from the nerve [24, 25]. With respect to the evaluation of the difference between Regarding the duration of the intervention, a positive proximal and distal CMAP, the cut-off < 0 used in the pre- correlation was found with short-term facial outcome, with sent study obtained a sensitivity of 79% for short-term out- an increased risk of HB grade III or above with a duration comes. In the recent literature there are few studies on the longer than 7.8 h. Zaouche et al. stated that the case of hem- role of DPDC as a prognostic factor of postsurgical facial orrhagic tumors, adhesion between the tumor and the facial outcomes, but two articles valued a proximal-to-distal nerve, the cerebellum, or the brainstem lengthened the pro- CMAP amplitude ratio of 2:3, and another evaluated a ratio cedure and worsened the facial outcome [26]. of 1.0. A ratio of 2:3 was associated with a sensitivity for As regards MST, a positive correlation was found with short-term function of 0.67–0.93 and a specificity of 0.80 facial outcome in both the short and the long term. The pre- to 0.84. A ratio of 1.00 had sensitivities and specificities of sent study used 0.05 mA and 0.1 mA cut-offs, obtaining 0.2 and 0.89 for short-term function and 0.25 and 0.88 for a high sensitivity (91% short-term and 95% long-term) for long-term function, respectively [32–34]. 0.05 mA and a good specificity (75% short-term and 73% Electromyography monitoring of the facial nerve is based long term) and sensitivity (62% long-term) for 0.1 mA, in on the measurement of CMAP elicited by DES of the nerve agreement with other recently published studies. Huang proximally, close to the brain stem, after tumor resection. et al. found the sensitivity and specificity (12 months post- However, EMG could only be obtained from the point of operatively) of the “low current” of 0.05 mA for predicting nerve stimulation to the facial muscles, without tracking the facial function outcome were 87.2 and 48.0%, respectively. entire full-length pathway. Therefore, the recording of the In this study, for patients with HB IV in the first month post- CMAP may reflect a stimulation of the distal part even in surgery, the recovery rate in the group with MST 0.05 mA the case the facial is damaged proximally. Indeed, recent was significantly higher than that of the control group with studies suggest that the signals generated by the facial nerve a higher MST [27]. In the study by Schmitt et al., using an near the brainstem are the most significant and representa- MST of 0.1 mA, the sensitivity and specificity for long-term tive [35–37]. facial outcome were 94% and 12%, respectively [28]. The In some cases, a patient with a low MST and a high most recent meta-analysis, by Quimby et al., reports that an CMAP may develop a more severe facial dysfunction than MST cut-off of 0.10 mA had a sensitivity of 0.76 (95% CI expected, due to the event of delayed-onset facial nerve pare- 0.53–0.90) and a specificity of 0.68 (95% CI 0.42–0.87), and sis. Delayed-onset facial nerve dysfunction is a deteriora- an MST cut-off of 0.05 mA had a sensitivity of 0.73 (95% tion of at least one HB Grade between 3 and 30 days after CI 0.58–0.84) and a specificity of 0.74 (95% CI 0.59–0.85) surgery. The hypothesis proposed is that there is a latent for facial function in the long term [29]. reactivation of a dormant neurotrophic virus harbored in A negative correlation was found between proximal the facial nerve which can be seen in between 4.8 and 41% CMAP and facial outcome in both the short and the long of patients undergoing VS surgery. This process is not pre- term. CMAP cut-offs of 500 µV and 200 µV were used, dicted by intraoperative facial monitoring because it post- obtaining a high sensitivity (95% short-term and 97% long- dates the surgery, and it may be a factor leading to a poorer term) for 500 µV and a good sensitivity (62% short-term and than expected facial nerve outcome [38–40]. 73% long-term) and specificity (79% short-term and 73% The present study presents some limitations. First, it is a long-term) for 200 µV. Using a cut-off of 500 µV, Magliulo single-center retrospective study with a low number of cases. et al. reported the sensitivity and specificity for short-term Second, the grading of the facial function was evaluated by facial outcome to be 83% and 65%, respectively [30]. Neff the same medical team in the same department, and the et al. used logistic regression modelling to find a significant raters were not blinded to the DES parameters, which might relation between MST and CMAP, which was able to com- have increased the bias. Third, monopolar stimulation was pare between good and poor long-term results. The same used, although it’s been suggested that bipolar stimulation authors found that MST 0.05 mA and CMAP 240 mV had decreases the risk of false-negative responses and seems to 88% sensitivity and 85% specificity for good long-term be more accurate, to avoid this limitation monopolar stimu- facial prognosis [31]. In their meta-analysis, Quimby et al. lation was used with minimal threshold stimulation (MTS) reported a sensitivity of 0.87 (95% CI 0.78–0.93) and a spec- of 0.05–0.1 mA, when identifying facial nerve during tumor ificity 0.59 (95% CI 0.50– 0.68) [29] for CMAP 500 mV to removal. predict short-term function. The data regarding the sensitiv- To resolve some of these limitations, a larger prospective ity of CMAP 500 µV observed in the present study corrobo- blinded case-cohort study is needed to fully evaluate the rate the results of recent reports. As regards the cut-off of process of facial function recovery after VS removal. 13 European Archives of Oto-Rhino-Laryngology (2023) 280:1055–1062 1061 Conclusion 6. Silva J, Cerejo A, Duarte F, Silveira F, Vaz R (2012) Surgi- cal removal of giant acoustic neuromas. World Neurosurg. 77(5–6):731–735 Facial nerve paresis or paralysis after VS surgery continues 7. Arslan F, Gökgöz MC, Binar M, Aydemir E, Durmaz A (2018) to be a significant source of morbidity. Parameters obtained Anxiety, depression, and hopelessness in patients before and by DES of the facial nerve after tumor resection should be after treatment for peripheral facial paralysis. Ear Nose Throat J 97(4–5):E1–E4 an important advantage for prognosticating facial functional 8. Saadi R, Shokri T, Schaefer E, Hollenbeak C, Lighthall JG results in CPA surgery. Immediate HB, tumor size, MST, and (2019) Depression rates after facial paralysis. Ann Plast Surg proximal CMAP are correlated with long-term facial outcome. 83(2):190–194 Based on the different cut-offs analysed, it is possible to predict 9. Arnoldner C, Mick P, Pirouzmand F et al (2013) Facial nerve prognostication in vestibular schwannoma surgery: the con- the outcome of facial activity with considerable accuracy. cept of percent maximum and its predictability. Laryngoscope Refining predictive accuracy helps surgeons to meet 123(10):2533–2538 patients’ perspectives and to elaborate an intervention pro- 10. Schlake HP, Goldbrunner R, Milewski C et al (1999) Technical gram in order to plan facial rehabilitation or set up a surgical developments in intra-operative monitoring for the preservation of cranial motor nerves and hearing in skull base surgery. Neurol anastomosis. Res 21(1):11–24 11. Oh T, Nagasawa DT, Fong BM et al (2012) Intraoperative neu- Supplementary Information The online version contains supplemen- romonitoring techniques in the surgical management of acoustic tary material available at https://d​ oi.o​ rg/1​ 0.1​ 007/s​ 00405-0​ 22-0​ 7556-8. neuromas. Neurosurg Focus 33(3):E6 12. Nissen AJ, Sikand A, Curto FS, Welsh JE, Gardi J (1997) Value Funding Open Access Funding provided by Universitat Autonoma de of intraoperative threshold stimulus in predicting postoperative Barcelona. facial nerve function after acoustic tumor resection. Am J Otol 18(2):249–251 13. Duarte-Costa S, Vaz R, Pinto D, Silveira F, Cerejo A (2015) Declarations Predictive value of intraoperative neurophysiologic monitor- ing in assessing long-term facial function in grade IV vestibular Conflict of interest The authors declare no conflict of interest. schwannoma removal. Acta Neurochir (Wien) 157(11):1991–1997 (discussion 1998) Open Access This article is licensed under a Creative Commons Attri- 14. Mandpe AH, Mikulec A, Jackler RK, Pitts LH, Yingling CD bution 4.0 International License, which permits use, sharing, adapta- (1998) Comparison of response amplitude versus stimulation tion, distribution and reproduction in any medium or format, as long threshold in predicting early postoperative facial nerve function as you give appropriate credit to the original author(s) and the source, after acoustic neuroma resection. Am J Otol 19(1):112–117 provide a link to the Creative Commons licence, and indicate if changes 15. Tos M, Thomsen J (2003) Proposal for reporting size of vestibular were made. The images or other third party material in this article are schwannoma. In: Kanzaki J, Tos M, Sanna M, Moffat DA, Kuni- included in the article's Creative Commons licence, unless indicated hiro T, Inoue Y (eds) Acoustic Neuroma. Consensus on Systems otherwise in a credit line to the material. If material is not included in for Reporting Results Series. Keio University International Sym- the article's Creative Commons licence and your intended use is not posia for Life Sciencies and Medicine, 10. Springer, Tokyo, pp permitted by statutory regulation or exceeds the permitted use, you will 3–7 need to obtain permission directly from the copyright holder. To view a 16. House JW, Brackmann DE (1985) Facial nerve grading system. copy of this licence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/. Otolaryngol Head Neck Surg 93(2):146–147 17. Carlstrom LP, Copeland WR 3rd, Neff BA, Castner ML, Driscoll CL, Link MJ (2016) Incidence and risk factors of delayed facial palsy after vestibular schwannoma resection. Neurosurgery References 78(2):251–255 18. Arriaga M, Luxford W, Atkins J, Kwartler J (1993) Predicting 1. Acioly MA, Liebsch M, de Aguiar PH, Tatagiba M (2013) Facial long-term facial nerve outcome after acoustic neuroma surgery. nerve monitoring during cerebellopontine angle and skull base Otolaryngol Head Neck Surg 108:220–224 tumor surgery: a systematic review from description to current 19. Isaacson B, Kileny PR, El-Kashlan H (2003) Intraoperative moni- success on function prediction. World Neurosurg 80(6):e271-300 toring and facial nerve outcomes after vestibular schwannoma 2. Vivas EX, Carlson ML, Neff BA et al (2018) Congress of neuro- resection. Otol Neurotol 24:812–817 logical surgeons systematic review and evidence-based guidelines 20. Axon P, Ramsden R (1999) Intraoperative electromyography on intraoperative cranial nerve monitoring in vestibular schwan- for predicting facial function in vestibular schwannoma surgery. noma surgery. Neurosurgery 82(2):E44–E46 Laryngoscope 109:922–926 3. Delgado TE, Bucheit WA, Rosenholtz HR, Chrissian S (1979) 21. Lee S, Seol HJ, Park K et al (2016) Functional outcome of the Intraoperative monitoring of facial muscle evoked responses facial nerve after surgery for vestibular schwannoma: prediction obtained by intracranial stimulation of the facial nerve: a more of acceptable long-term facial nerve function based on immediate accurate technique for facial nerve dissection. Neurosurgery postoperative facial palsy. World Neurosurg 89:215–222 4(5):418–421 22. Zini C, Magnan J, Piazza F, Chays A, Passanisi E, Girard N (2020) 4. Sughrue ME, Yang I, Rutkowski MJ, Aranda D, Parsa AT (2010) New radiosurgical classification of acoustic neuromas. Roma: Preservation of facial nerve function after resection of vestibular Monduzzi Ed., pp. 117–120 schwannoma. Br J Neurosurg 24(6):666–671 23. Rinaldi V, Casale M, Bressi F, Potena M, Vesperini E, De Franco 5. Samii M, Gerganov VM, Samii A (2010) Functional outcome A et al (2012) Facial nerve outcome after vestibular schwannoma after complete surgical removal of giant vestibular schwannomas. surgery: our experience. J Neurol Surg B Skull Base 73(1):21–27 J Neurosurg 112(4):860–867 13 1062 European Archives of Oto-Rhino-Laryngology (2023) 280:1055–1062 24. Brackmann DE, Cullen RD, Fisher LM (2007) Facial nerve func- 33. Taha JM, Tew JM, Keith RW (1995) Proximal-to-distal facial tion after translabyrinthine vestibular schwannoma surgery. Oto- amplitude ratios as predictors of facial nerve function after acous- laryngol Head Neck Surg 136(5):773–777 tic neuroma excision. J Neurosurg 83:994–998 25. Darrouzet V, Martel J, Enée V, Bébéar JP, Guérin J (2004) Vestib- 34. Sobottka SB, Schackert G, May SA, Wiegleb M, Reiss G (1998) ular schwannoma surgery outcomes: our multidisciplinary experi- Intraoperative facial nerve monitoring (IFNM) predicts facial ence in 400 cases over 17 years. Laryngoscope 114(4):681–688 nerve outcome after resection of vestibular schwannoma. Acta 26. Zaouche S, Ionescu E, Dubreuil C, Ferber-Viart C (2005) Pre- Neurochir (Wien) 140(3):235–242 (discussion 242-3) and intraoperative predictive factors of facial palsy in vestibular 35. Nakatomi H, Miyazaki H, Tanaka M et al (2015) Improved pres- schwannoma surgery. Acta Otolaryngol 125(4):363–369 ervation of function during acoustic neuroma surgery. J Neurosurg 27. Huang X, Ren J, Xu J et al (2018) The utility of “low current” 122(1):24–33 stimulation threshold of intraoperative electromyography moni- 36. Hong RS, Kartush JM (2012) Acoustic neuroma neurophysiologic toring in predicting facial nerve function outcome after vestibu- correlates: facial and recurrent laryngeal nerves before, during, lar schwannoma surgery: a prospective cohort study of 103 large and after surgery. Otolaryngol Clin N Am 45(2):291–306 (vii-viii) tumors. J Neurooncol 138(2):383–390 37. Vega-Céliz J, Amilibia-Cabeza E, Prades-Martí J et al (2015) 28. Schmitt WR, Daube JR, Carlson ML et al (2013) Use of Nuestra experiencia con la monitorizacion del nervio facial en supramaximal stimulation to predict facial nerve outcomes follow- cirugia del schwannoma del vestibular bajo bloqueo neuromus- ing vestibular schwannoma microsurgery: results from a decade cular parcial. Acta Otorrinolaringol Esp 66(4):192–198 of experience. J Neurosurg 118(1):206–212 38. Gianoli GJ, Kartush JM (1996) Delayed facial palsy after acous- 29. Quimby AE, Lui J, Chen J (2021) Predictive ability of direct elec- tic neuroma resection: the role of viral reactiviation. Am J Otol trical stimulation on facial nerve function following vestibular 17:625–629 schwannoma surgery: a systematic review and meta-analysis. Otol 39. Megerian CA, McKenna MJ, Ojemann RG (1996) Delayed facial Neurotol 42(4):493–504 paralysis after acoustic neuroma surgery: factors influencing 30. Magliulo G, Zardo F (1998) Facial nerve function after cerebel- recovery. Am J Otol 17:630–633 lopontine angle surgery and prognostic value of intraoperative 40. Yawn RJ, Dedmon MM, Xie D et al (2019) Delayed facial nerve facial nerve monitoring: a critical evaluation. Am J Otolaryngol paralysis after vestibular schwannoma resection. J Neurol Surg B 19:102–106 Skull Base 80(3):283–286 31. Neff BA, Ting J, Dickinson SL, Welling DB (2005) Facial nerve monitoring parameters as a predictor of postoperative facial nerve Publisher's Note Springer Nature remains neutral with regard to outcomes after vestibular schwannoma resection. Otol Neurotol jurisdictional claims in published maps and institutional affiliations. 26(4):728–732 32. Berges C, Fraysse B, Yardeni E, Rugiu G (1993) Intraoperative facial nerve monitoring in posterior fossa surgery: prognostic value. Skull Base Surg 3(4):214–216 13