Facial Motor Evoked Potential Research Articles

Analysis of facial motor evoked potentials for assessing a central mechanism in hemifacial spasm.

OBJECTIVE Hemifacial spasm (HFS) is a cranial nerve hyperactivity disorder characterized by unique neurophysiological features, although the underlying pathophysiology remains disputed. In this study, the authors compared the effects of desflurane on facial motor evoked potentials (MEPs) from the spasm and nonspasm sides of patients who were undergoing microvascular decompression (MVD) surgery to test the hypothesis that HFS is associated with a central elevation of facial motor neuron excitability. METHODS Facial MEPs were elicited in 31 patients who were undergoing MVD for HFS and were administered total intravenous anesthesia (TIVA) with or without additional desflurane, an inhaled anesthetic known to centrally suppress MEPs. All measurements were completed before dural opening while a consistent mean arterial blood pressure was maintained and electroencephalography was performed. The activation threshold voltage and mean amplitudes of the MEPs from both sides of the face were compared. RESULTS There was a significantly lower mean activation threshold of facial MEPs on the spasm side than on the nonspasm side (mean ± SD 162.9 ± 10.1 vs 198.3 ± 10.1 V, respectively; p = 0.01). In addition, MEPs were also elicited more readily when single-pulse transcranial electrical stimulation was used on the spasm side (74% vs 31%, respectively; p = 0.03). Although desflurane (1 minimum alveolar concentration) suppressed facial MEPs on both sides, the suppressive effects of desflurane were less on the spasm side than on the nonspasm side (59% vs 79%, respectively; p = 0.03), and M waves recorded from the mentalis muscle remained unchanged, which indicates that desflurane did not affect the peripheral facial nerve or neuromuscular junction. CONCLUSIONS Centrally acting inhaled anesthetic agents can suppress facial MEPs and therefore might interfere with intraoperative monitoring. The elevated motor neuron excitability and differential effects of desflurane between the spasm and nonspasm sides support a mechanism of central pathophysiology in HFS. Clinical trial registration no.: B2012:099 ( clinicaltrials.gov ).

Microvascular decompression for glossopharyngeal neuralgia using intraoperative neurophysiological monitoring: Technical case report.

Background:Glossopharyngeal neuralgia (GN) is a rare functional disorder representing around 1% of cases of trigeminal neuralgia. Lancinating throat and ear pain while swallowing are the typical manifestations, and are initially treated using anticonvulsants such as carbamazepine. Medically refractory GN is treated surgically. Microvascular decompression (MVD) is reportedly effective against GN, superseding rhizotomy and tractotomy.Methods:We encountered three patients with medically refractory GN who underwent MVD using intraoperative neurophysiological monitoring (IONM). The offending vessels were the posterior inferior cerebellar arteries, which were confirmed intraoperatively via a transcondylar fossa approach to be affecting the root exit zones of the glossopharyngeal and vagus nerves. As IONM, facial motor-evoked potentials (MEPs) and brainstem auditory-evoked potentials were monitored during microsurgery in all three patients. Pharyngeal and vagal MEPs were added for two patients to avoid postoperative dysphagia.Results:GN disappeared immediately after surgery with complete preservation of hearing acuity and facial nerve function. Transient mild swallowing disturbance was observed in 1 patient without pharyngeal or vagal MEPs, whereas the remaining two patients with pharyngeal and vagal MEPs demonstrated no postoperative dysphagia.Conclusion:Although control of severe pain is expected in surgical intervention for GN, lower cranial nerves are easily damaged because of their fragility, even in MVD. IONM including pharyngeal and vagal MEPs appears very useful for avoiding postoperative sequelae during MVD for GN.

Intraoperative Monitoring of Facial Nerve Motor-Evoked Potentials in Children

To determine whether transcranial motor-evoked potential monitoring of the facial nerve (FNMEP) during eloquent tumor resection is feasible in children and can predict both immediate and postoperative facial nerve (FN) function. We included 24 consecutive procedures involving 21 patients (median age 5.5 years, range 5 months to 15 years, 8 female) who were operated on with FNMEP monitoring by the first author in 2013 and 2014. During surgery, we maintained a constant response amplitude by increasing the stimulation intensity and aimed to establish a warning criterion based on the "threshold-level" method. A threshold increase of greater than 20 mA for eliciting the FNMEP in the most reliable facial nerve target muscle was considered to be a prediction of reduced postoperative facial nerve function and consequently, a warning was given to the surgeon. The preoperative and early postoperative function was documented with the House-Brackmann grading system. Monitoring of the FNMEP was feasible in all the surgeries in at least one facial nerve target muscle. The orbicularis oris muscle yielded the best result (95% of the trials), followed by the mentalis (87%) and orbicularis oculi muscles (86%). The median stimulation threshold was initially 65 mA (range 40-110 mA) for the FNMEP and 60 mA (15-220 mA) for the motor-evoked potential of the thenar muscles. The FNMEP deterioration showed a sensitivity of 100% for House-Brackmann deterioration and specificity of 74%. Intraoperative FNMEP monitoring is feasible and safe in infants and children. We found no evidence that the procedures and thresholds should differ from FNMEP monitoring in adults. FNMEP monitoring provides valid evidence for FN function in pediatric eloquent area surgery; its use is complementary to direct electrical FN stimulation and continuous EMG monitoring of FN target muscles.

2-P-F-16. Multimodality monitoring during microvascular decompression for hemifacial spasm

Objective The present study investigated whether free-running electromyography (frEMG) monitoring correlated with abnormal muscle responses (AMRs), or facial motor evoked potentials (FMEP) in 26 hemifacial spasm patients who underwent microvascular decompression (MVD). Methods frEMGs were recorded after injection of 20 ml of saline onto the root exit zone of the facial nerve. Using a software tool, data were rectified and measured for frEMG activity from the orbicularis oculi or mentalis (ME) muscle as an integral value 10 s after injection. frEMG activity was measured before and after MVD. With regard to AMRs, patients were divided into two groups based on disappearance or persistence of AMRs after MVD. Ratios of final to baseline FMEP amplitudes were used for analysis. Results Thirteen patients with decreased frEMG activity after surgery had significantly smaller FMEP amplitude ratios in the ME muscles compared with 13 patients without decreased activity (65% vs. 94%, p Conclusions/significance Decreases in frEMG activity elicited by a saline injection after MVD night reflect normalization of facial nerve excitability.

Intraoperative neuromonitoring for removal of large vestibular schwannoma: Facial nerve outcome and predictive factors

BackgroundPreservation of facial nerve (FN) function is one of the major goals for resection of large vestibular schwannoma (VS) (≥30mm). Little is known about the FN outcome and its predictive factors due to limited data. ObjectiveTo explore the predictive factors affecting FN outcome following resection of large VS. Methods106 Large VS patients underwent surgical resection from 2010 to 2012 via intraoperative neuromonitoring for FN preservation approach. Postoperative FN function evaluation was conducted at the time points of 3–7th day, 3rd month and at the end of the 2nd year. Correlation between tumor size, intraoperative parameters and FN function were examined. ResultsThe ratios of total and subtotal resection were 82.1% and 14.2%, respectively. Acceptable FN function was achieved in 78% patients. Patients with good FN function showed much smaller (P<0.01) VS size than those of poor-FN function patients at 3–7th day, 3rd month and 2nd year. There was a significant correlation between facial motor evoked potential (FMEP) ratios and postoperative FN function at 3–7th day (r=−0.709, P<0.001) 3rd month (r=−0.709, P<0.001) and 2nd year (r=−0.750, P<0.001). Maximal response amplitude (MRA) ratio was a supplementary indicator for train time in predicting both immediate and long-term FN function in patients with large VS. ConclusionIndicative factors of both immediate and long-term postoperative FN function in large VSs include tumor size, intraoperative train time, start to final FMEP ratios and proximal to distal MRA ratios.

Intraoperative continuous monitoring of facial motor evoked potentials in acoustic neuroma surgery.

The preservation of facial nerve function is one of the primary objectives in acoustic neuroma surgery. We detail our method of continuous intraoperative facial motor evoked potential (MEP) monitoring and present criteria for the preservation of facial nerve function to avoid postoperative facial nerve palsy. Our study population was comprised of 15 patients who did not (group 1), and 20 who did (group 2) undergo facial MEP monitoring during surgery to remove acoustic neuromas. In group 2, we continuously stimulated the facial motor cortex at 5- or 10-s intervals throughout surgery. Electromyograms (EMGs) were recorded from the contralateral orbicularis oculi- and orbicularis oris muscles. Optimal anode and cathode placement was at the facial motor cortex and the vertex, respectively. Postoperative facial palsy occurred in 8 of the 15 group 1 patients; in 2 it improved to grade II at 6 months after the operation. Of the 20 group 2 patients, 7 suffered postoperative facial palsy. At 6 months after the operation, their facial nerve function was normal. At the end of the operation, the ratio of the amplitude of the supramaximal EMG to the amplitude at the dural opening was 39.6 % in patients with- and 94.3 % in patients without transient postoperative facial palsy. Continuous facial MEP monitoring not only alerts to surgical invasion of the facial nerves but also helps to predict postoperative facial nerve function. To preserve a minimum amplitude ratio of 50 %, even transient postoperative facial palsy must be avoided. MEP monitoring is an additional useful modality for facial nerve monitoring during acoustic neuroma surgery.

49. Transcranial facial motor evoked potentials: a continuous monitoring of the facial motor function during vestibular schwannoma surgery

Monitoring of facial nerve during vestibular schwannoma surgery is usually performed by using free-running EMG and by direct stimulation of the facial nerve. The main limit of free-running EMG is that a quantitative analysis is possible only after the surgery. On the other hand, the direct electrical stimulation of facial nerve can only be used intermittently and after the identification of the nerve. Facial motor evoked potentials of the orbicularis oris muscle turned out to be a very useful tool allowing continuous information about the actual functional state of facial nerve during the resection of the tumor. Forty-eight (n = 48) consecutive neurosurgical procedures for vestibular schwannoma were performed between July 2009 and June 2013 at the Department of Neurosurgery, Na Homolce Hospital, Prague. Free-running EMG and direct electrical stimulation were used to monitor function of the facial nerve in all the cases. Facial motor evoked potentials (FMEP) monitoring was performed in 42 cases. Stable FMEPs were reproducible in 39 of 42 patients (93%). All cases were divided into 3 groups according to the decrease of amplitude FMEPs at the end of the tumor resection. The first group (A) – 70–100% of the initial FMEP amplitude, the second group (B) – 30–70% of the initial FMEP amplitude and the third group (C) 0–30% of the initial FMEP amplitude. Functional result at the end of follow up was consider to be favorable when there was no or mild paresis of facial nerve (grade 1–3 according to House- Brackmann scale). The unfavorable results meant severe paresis (grade 4–6 according to House-Brackmann scale). A mean follow-up was 15, 4 months. Group A consisted of 18 cases with 17 (94%) favorable and 1 (6%) unfavorable results. Group B consisted of 10 cases with 9 (90%) favorable and 1 (10%) unfavorable results. Group C consisted of 10 cases with 2 (20%) favorable and 8 (80%) unfavorable results. Our study showed that the decrease of FMEP amplitude to 30% and lower at the end of tumor resection meant unfavorable prognosis for function of facial nerve.

Facial Motor Neuron Excitability in Hemifacial Spasm: A Facial MEP Study

Hemifacial spasm (HFS) may be due to peripheral axon ephapsis or central motor neuron hyperexcitability. Low facial motor evoked potential (MEP) thresholds or MEP responses to single pulse stimulation (normally multipulse stimulation is needed) may support the central hypothesis. We retrospectively compared response thresholds for facial MEPs in 65 patients undergoing surgical microvascular decompression (MVD) for HFS and 29 patients undergoing surgery for skull base tumors. Single pulse stimulation elicited facial Mep in up to 87% of HFS patients whereas only 10% of tumor patients responded to single pulse stimulation. When comparing facial MEP thresholds using multi-pulse stimulus trains the voltage required in the HFS group were significantly lower then in skull base tumor patients (p < 0.001). the MEP latencies and amplitudes at threshold stimulation were similar between the two groups. these results suggest the facial corticobulbar pathway demonstrates enhanced excitability in HFS.

89. Facial nerve monitoring during cerebellopontine angle surgery: Our experience

Intraoperative facial nerve neuromonitoring (FN-IOM) has been established as one of the methods by which neurosurgery can improve surgical results while reducing morbidity. Despite routine use of FN-IOM, FN injury is still a complication of major concern due to severe impact on patient’s quality of life. Currently, standard monitoring techniques comprise direct electrical stimulation (DES), free-running electromyography (EMG) and facial motor evoked potential (FMEP). Recently, intraoperative recording of Blink Reflex (BR) can also be used. We report our experience of FN-IOM in 56 patients who underwent surgery for cerebellopontine angle tumors, trigeminal neuralgias or hemifacial spasms. FN was monitored with DES, EMG and, in 19 patients, also with BR. Facial nerve outcomes were evaluated one week and six months after surgery using the House–Brackmann scale (HB). Nine patients (16,1%) had HB grade III–VI function at 6 months after surgery. Individually, DES and EMG can predict FN function at 6 months, but used together, in a multimodal monitoring approach, the sensibility is higher. BR can predict FN function only one week after surgery. With our experience we want to emphasize the role of multimodal FN monitoring and its impact on facial nerve function preservation.

Effectiveness of C5 or C6-Cz assembly in predicting immediate post operative facial nerve deficit

Intraoperative neurophysiology monitoring (IOM) is a valuable tool in cerebellopontine angle (CPA) surgeries posing risk to the cranial nerves. Transcranial electrical stimulation (TES) for cranial nerves has been performed in the last 7 years, for obtaining the facial nerve motor evoked potential (MEP), using either C3/C4-Cz or C3-C4 (or inverse) stimulating points, which have been correlated with facial nerve functional outcome. Intraoperative surgical and electrophysiological findings were documented prospectively. Patient files were reviewed for clinical data. We studied 23 patients undergoing CPA tumor resection using C5 or C6-Cz montage for TES, and were able to determine the correlation between facial nerve functional outcome and the amplitude drop of facial MEP above 50 %. Patients were evaluated for immediate facial nerve outcome and 6 months after the surgery. Follow-up was performed by structured telephone interviews with local physicians. The sensibility of the studied parameters was 92.8 % for amplitude drop of facial nerve MEP, with positive predictive value of 81.2 %. The absence of changes during IOM has shown a negative predictive value of 100 %. In this series, the used montage was effective in predicting new facial deficit.

S2-4. Facial motor evoked potentials elicited by transcranial electrical stimulation for intraoperative monitoring

We have employed facial motor evoked potentials (FMEPs) elicited by transcranial electrical stimulation to monitor facial nerve function during skull base surgery. Postoperative facial nerve function correlated significantly with the final/baseline FMEP ratios in the orbicularis oculi ( n = 92, r = −0.68, p n = 102, r = −0.74, p n = 29) who underwent surgery using both compound muscle action potential (CMAP) and FMEP monitorings had better facial nerve outcomes than those ( n = 117) using only CMAP monitoring 1 year after surgery ( p n = 15, r = 0.69, p n = 15, r = 0.83, p

O2-E-61. Correlation between postoperative facial nerve function and neurophysiological examinations in the perioperative period for vestibular schwannoma

The aim of this study was to examine the correlation between a change of blink reflex (BR) and facial motor evoked potential (FMEP) before and after the vestibular schwannoma (VS) operation and the postoperative facial nerve function (FNF). The study involved 53 patients who were operated for VS. Postoperative FNF was assessed at 1 and 12months after operation according to House and Blackmann grade. BR was recorded before and after operation within two weeks. Ipsilateral and contralateral differences in latency were defined as ΔR1. ΔR1 is classified by the following range (ΔR1≤q2ms, 2 p p p

Facial nerve motor evoked potentials during skull base surgery to monitor facial nerve function using the threshold-level method

During surgeries that put the facial nerve at risk for injury, its function can be continuously monitored by transcranial facial nerve motor evoked potentials (FNMEPs) in facial nerve target muscles. Despite their advantages, FNMEPs are not yet widely used. While most authors use a 50% reduction in FNMEP response amplitudes as a warning criterion, in this paper the authors' approach was to keep the response amplitude constant by increasing the stimulation intensity and to establish a warning criterion based on the "threshold-level" method. The authors included 34 consecutive procedures involving 33 adult patients (median age 47 years) in whom FNMEPs were monitored. A threshold increase greater than 20 mA for eliciting FNMEPs in the most reliable facial nerve target muscle was considered a prediction of reduced postoperative facial nerve function, and subsequently a warning was issued to the surgeon. Preoperative and early postoperative function was documented using the House-Brackmann grading system. Monitoring of FNMEPs was feasible in all 34 surgeries in at least one facial nerve target muscle. The mentalis muscle yielded the best results. The House-Brackmann grade deteriorated in 17 (50%) of 34 cases. The warning criterion was reached in 18 (53%) of 34 cases, which predicted an 83% risk of House-Brackmann grade deterioration. Sensitivity amounted to 88% (CI 64%-99%) and specificity to 82% (CI 57%-96%). Deterioration of FNMEPs and a worse House-Brackmann grade showed a high degree of association (p < 0.001). The impact of FNMEP monitoring on surgical strategy is exemplified in an illustrative case. In surgeries that put the facial nerve at risk, the intraoperative increase in FNMEP stimulation threshold was closely correlated to postoperative facial nerve dysfunction. Monitoring of FNMEPs is a valid indicator of facial nerve function in skull base surgery. It should be used as an adjunct to direct electrical facial nerve stimulation and continuous electromyographic monitoring of facial nerve target muscles.

Facial corticobulbar motor-evoked potential monitoring during the clipping of large and giant aneurysms of the anterior circulation

Surgical outcomes for large and giant intracranial aneurysms are suboptimal. Two important reasons for higher complication rates are either occlusion of perforators or parent arteries during aneurysm clipping, or prolonged temporary occlusion of the main arteries. Somatosensory-evoked potential (SSEP) monitoring and transcranial motor-evoked potential (TcMEP) monitoring are standard techniques for monitoring ischemia either during temporary arterial occlusion or after permanent clipping. In our study, facial corticobulbar motor-evoked potential (FCoMEP) monitoring was included to determine whether this modality improved intraoperative monitoring. FCoMEP were recorded intraoperatively in 21 patients undergoing surgical clipping of large and giant aneurysms of the anterior circulation. Valid TcMEP parameters were obtained for all patients. A correlation tending to significance between a prolonged temporary clipping time and TcMEP decrement was observed. In addition to this, the inclusion of FCoMEP improved the sensitivity of extremity muscle motor-evoked potential (ExMEP, which included TcMEP) monitoring (from 80% to 100%). In the long-term assessment, a favorable outcome was achieved in 16 of the 21 patients (76%). In conclusion, FCoMEP provides complementary corticobulbar tract information for detecting perforating vessel compromise that may lead to motor impairment and that is not identified by ExMEP.

Monitoring of abnormal muscle response and facial motor evoked potential during microvascular decompression for hemifacial spasm.

Background:To determine whether the monitoring of abnormal muscle response (AMR) and facial motor evoked potential (FMEP) during microvascular decompression (MVD) for hemifacial spasm (HFS) might be useful for predicting the postoperative clinical course and final outcomes.Methods:We analyzed 45 HFS patients who underwent both AMR and FMEP monitoring during MVD. Patients were divided into two groups on the basis of post-MVD disappearance (group AMR-A) or persistence (group AMR-B) of AMR. With regard to FMEPs, patients were classified into one of the two groups according to the ratio of the final to baseline FMEP amplitudes recorded for the orbicularis oculi muscle: one group with a ratio of <50% (group FMEP-A), and the other with a ratio of ≥50% (group FMEP-B).Results:Twenty-one of the 26 (81%) patients in group AMR-A were assigned to group FMEP-A, whereas 9 of the 17 (53%) patients in group AMR-B were assigned to FMEP-B (P < 0.05). In 38 of the 40 (95%) patients in whom the AMRs disappeared or persisted at amplitudes <50% that at the baseline, HFS had subsided at the final follow-up. Forty of the 42 (95%) patients whose FMEP amplitude ratios indicated reduction in the amplitudes from the baseline, had complete relief of the symptoms. Nineteen of the 20 (95%) patients whose AMRs disappeared after MVD experienced immediate relief of their symptoms after the operation. With regard to 14 of the 20 (70%) patients whose AMRs persisted at the final recordings, the symptoms of HFS improved over time and eventually subsided (P < 0.001).Conclusions:Intraoperative monitoring of both AMR and FMEP during MVD may be useful in predicting the postoperative outcomes in HFS patients. The AMR-related findings may help to predict whether HFS disappears immediately after surgery or some time later.

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

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

Facial motor evoked potentials in cerebellopontine angle surgery: Technique, pitfalls and predictive value

Objective To obtain information on functional integrity of the facial nerve by transcranial electrical motor evoked potentials independent of nerve visualization and to improve prediction of postoperative function. Patients and methods In a prospective clinical study, 68 patients with cerebello-pontine angle tumors and 5 patients with trigeminal neuralgia were investigated by facial motor evoked potentials (FMEP) elicited by multi-pulse transcranial electrical motor cortex stimulation. For recording the same electrode set-up was used as for continuous EMG monitoring of the orbicularis oculi and oris muscles. Pre-surgical FMEP amplitudes and latencies were correlated with tumor extensions. End to start amplitude ratios were compared to early and long-term facial nerve function by House–Brackmann-Grading (HB) documented by pre- and post-operative photo and video documentation. Results Reliable FMEP were obtained in 57 patients. FMEP responses at the start of surgery correlated with the degree of tumor extension. Largest FMEP amplitudes and shortest latencies were found in patients with trigeminal neuralgia. FMEP quality was reduced with increasing tumor extension ( P < 0.05). The ratio of end-operative to start-operative FMEP-amplitude showed a positive correlation with early and late facial nerve function. Correlation was especially close with early function: an amplitude preservation rate of 86% led to HB°1 or HB°2, of 67% to HB°3, at 33% to HB°4 and at 15% or lower to HB°5 or HB°6. Discussion Initial FMEP amplitudes correlate with the presumed pre-operative nerve affection by space occupying tumors, a phenomenon reported here for the first time. Intact FMEP are highly reliable for preserved nerve continuity and hereby are of special help to the neurosurgeon for those surgical phases where the facial nerve is not visible and still covered by tumor and where conventional EMG monitoring is of very limited use. The end-to-start amplitude ratio of the FMEP is closely related to early and late clinical function. Amplitude reduction by 30% or more should result in a change of microsurgical action to enable fast recovery. Conclusion As an adjunct to intraoperative EMG, FMEP are superior in two respects, first in identifying pre-surgical latent nerve lesions and second in monitoring nerve integrity without direct nerve visualization. FMEP are highly reliable in predicting early and late postoperative function.

Intraoperative facial nerve motor evoked potential monitoring during skull base surgery predicts long-term facial nerve function outcomes

Objectives: This study was designed to clarify whether facial nerve motor evoked potentials (FNMEPs) elicited by transcranial electrical stimulation during skull base surgery are useful for predicting long-term facial nerve function.Methods: We analyzed FNMEP findings in 35 patients with skull base tumors. Mean follow-up was 24·4 months. Corkscrew electrodes positioned at C3 or C4 and Cz were used to deliver supramaximal stimuli. FNMEPs were recorded from the orbicularis oculi and oris muscles.Results: The correlation between the final-to-baseline FNMEP ratio and initial or long-term facial nerve function was examined. Initial post-operative facial nerve function correlated significantly with the FNMEP ratios in the orbicularis oculi (r=−0·53, P<0·005) and orbicularis oris (r=−0·80, P<0·001) muscles. The correlations between FNMEP ratios and facial nerve function remained significant during long-term follow-up (orbicularis oculi muscle: r=−0·43, P<0·05; orbicularis oris muscle: r=−0·71, P<0·001). All patients in whom the FNMEP ratio in the orbicularis oculi muscles remained above 50% were assigned to the satisfactory facial nerve function (House-Brackmann Grades i and ii) group at the final examination.Discussion: FNMEP monitoring can predict facial nerve function not only immediately after surgery but also long-term.

Quantitative parameters of facial motor evoked potential during vestibular schwannoma surgery predict postoperative facial nerve function

Facial motor evoked potential (FMEP) amplitude ratio reduction at the end of the surgery has been identified as a good predictor for postoperative facial nerve outcome. We sought to investigate variations in FMEP amplitude and waveform morphology during vestibular schwannoma (VS) resection and to correlate these measures with postoperative facial function immediately after surgery and at the last follow-up. Intraoperative orbicularis oculi and oris muscles FMEP data from 35 patients undergoing surgery for VS resection were collected, then analysed by surgical stage: initial, dural opening, tumour dissection (TuDis), tumour resection (TuRes) and final. Immediately after surgery, postoperative facial function correlated significantly with the FMEP amplitude ratio during TuDis, TuRes and final stages in both the orbicularis oculi (p = 0.003, 0.055 and 0.028, respectively) and oris muscles (p = 0.002, 0.104 and 0.014, respectively). At the last follow-up, however, facial function correlated significantly with the FMEP amplitude ratio only during the TuDis (p = 0.005) and final (p = 0.102) stages for the orbicularis oris muscle. At both time points, postoperative facial paresis correlated significantly with FMEP waveform deterioration in orbicularis oculi during the final stage (immediate, p = 0.023; follow-up, p = 0.116) and in orbicularis oris during the TuDis, TuRes and final stages (immediate, p = 0.071, 0.000 and 0.001, respectively; follow-up, p = 0.015, 0.001 and 0.01, respectively). FMEP amplitude ratio and waveform morphology during VS resection seem to represent independent quantitative parameters that can be used to predict postoperative facial function. Event-to-baseline FMEP monitoring is quite useful to dictate when intraoperative changes in surgical strategy are warranted to reduce the chances of facial nerve injury.

Clinical study on intraoperative facial nerve motor evoked potentials monitoring under partial neuromuscular blockade

Objective To find optimal partial neuromuscular blockade used for intraoperative facial nerve motor evoked potential (FNMEP) monitoring and its feasibility for application in patients undergoing acoustic neuroma resection.Methods Two parties were included: the amplitude and latency of FNMEP and electrical stimulation-induced body movement scores were recorded under various degree of neuromuscular blockade in thirty neurosurgical patients.After the optimal partial neuromuscular blockade was obtained,intraoperative FNMEP monitoring was performed in fifteen patients undergoing acoustic neuroma resection .The sensitivity and specificity of FNMEP were analyzed according to postoperative/preoperative amplitude of FNMEP and postoperative facial nerve function.Results The FNMEP waveforms were discernible and less electrical stimulation induced body movement was observed when T1 =30%～40% was used.Fifteen patients were monitored successfully with FNMEP.And the sensitivity and specificity of postoperative/preoperative FNMEP amplitude ratio to predict postoperative facial nerve function were 90% and 91% respectively.Conclusions T1=30%～40% is optimal partial neuromuscular blockade used for intraoperative FNMEP monitoring and FNMEP monitoring under such partial neuromuscular block is feasible and reliable. Key words: Facial nerve; Motor evoked potentials; Intraoperative monitoring; Craniotomy; Partial neuromuscular block