Quantitative parameters of intraoperative electromyography predict facial nerve outcomes for vestibular schwannoma surgery

Predictive value of Blink reflex and facial corticobulbar motor evoked potential in cerebellopontine angle tumor surgery

OBJECTIVE

The current study examined the efficacy of the facial corticobulbar motor evoked potentials (FCoMEPs) and blink reflex (BR) on predicting postoperative facial nerve function during cerebellopontine angle (CPA) tumor surgery.

METHODS

Data from 110 patients who underwent CPA tumor resection with intraoperative FCoMEPs and BR monitoring were retrospectively reviewed. The association between the amplitude reduction ratios of FCoMEPs and BR at the end of surgery and postoperative facial nerve function was determined. Subsequently, the optimal threshold of FCoMEPs and BR for predicting postoperative facial nerve dysfunction were determined by receiver operating characteristic curve analysis.

RESULTS

Valid BR was record in 103 of 110 patients, whereas only 43 patients successfully recorded FCoMEP in orbicularis oculi muscle. A reduction over 50.3% in FCoMEP (O. oris) amplitude was identified as a predictor of postoperative facial nerve dysfunction (sensitivity, 77.1%; specificity, 83.6%). BR was another independent predictor of postoperative facial nerve deficit with excellent predictive performance, especially eyelid closure function. Its optimal cut-off value for predicting long-term postoperative eyelid closure dysfunction was was 51.0% (sensitivity, 94.4%; specificity, 94.4%).

CONCLUSIONS

BR can compensate for the deficiencies of the FCoMEPs. The combination of BR and FCoMEPs can be used in CPA tumor surgery.

SIGNIFICANCE

The study first proposed an optimal cut-off value of BR amplitude deterioration (50.0%) for predicting postoperative eyelid closure deficits in patients undergoing CPA tumor surgery.

[Utility of the intraoperative neurophysiological monitoring as a prognostic value of postoperative facial paresis in vestibular schwannomas]

BACKGROUND AND OBJECTIVE

Intraoperative neurophysiological monitoring allows us to predict the functional status of the facial nerve after vestibular schwannoma surgery. Due to the great variability of the neurophysiological protocols used for it, the goal of this study is to determine the prognostic ability of our neurophysiological protocol.

MATERIAL AND METHODS

We have performed a statistical analysis of the neurophysiological monitoring data collected from patients operated between March 2009 and July 2021 at the Neurosurgery Service of Salamanca according to their functional status, both in the immediate post-surgical period and one year after surgery.

RESULTS

A number of 51 patients between 46 and 63 years old (median: 54) were analyzed. We have found significant differences studying the threshold value of the stimulation intensity of the facial nerve and the variation of the Cortico-bulbar Evoked Motor Potentials (P=0.043 and P=0.011, respectively) between the patients with good and bad clinical situation after surgery. The most discriminating intensity threshold value was 0.35mA (Sensitivity: 85%; Specificity: 48%). No statistical relationship was found in the study group one year after surgery.

CONCLUSIONS

Our intraoperative monitoring protocol allows us to predict the clinical situation of patients in the immediate postoperative period and improve information for the patient and her relatives after surgery. We cannot, however, use these parameters to predict the functional situation one year after surgery and make clinical decisions in this regard.

Facial nerve outcome score: a new score to predict long-term facial nerve function after vestibular schwannoma surgery

Introduction

Patients' quality of life (QoL), facial nerve (FN), and cochlear nerve (CN) (if conserved) functions should be pursued as final outcomes of vestibular schwannoma (VS) surgery. In regard to FN function, different morphologic and neurophysiological factors have been related to postoperative outcomes. The aim of the current retrospective study was to investigate the impact of these factors on the short- and long-term FN function after VS resection. The combination of preoperative and intraoperative factors resulted in designing and validating a multiparametric score to predict short- and long-term FN function.

Methods

A single-center retrospective analysis was performed for patients harboring non-syndromic VS who underwent surgical resection in the period 2015-2020. A minimum follow-up period of 12 months was considered among the inclusion criteria. Morphological tumor characteristics, intraoperative neurophysiological parameters, and postoperative clinical factors, namely, House-Brackmann (HB) scale, were retrieved in the study. A statistical analysis was conducted to investigate any relationships with FN outcome and to assess the reliability of the score.

Results

Seventy-two patients with solitary primary VS were treated in the period of the study. A total of 59.8% of patients showed an HB value < 3 in the immediate postoperative period (T1), reaching to 76.4% at the last follow-up evaluation. A multiparametric score, Facial Nerve Outcome Score (FNOS), was built. The totality of patients with FNOS grade A showed an HB value < 3 at 12 months, decreasing to 70% for those with FNOS grade B, whereas 100% of patients with FNOS grade C showed an HB value ≥ 3. The ordinal logistic regression showed three times increasing probability to see an HB value ≥ 3 at 3-month follow-up for each worsening point in FNOS score [Exp(B), 2,999; p < 0.001] that was even more probable [Exp(B), 5.486; p < 0.001] at 12 months.

Conclusion

The FNOS score resulted to be a reliable score, showing high associations with FN function both at short- and long-term follow-up. Although multicenter studies would be able to increase its reproducibility, it could be used to predict the FN damage after surgery and the potential of restoring its function on the long-term period.

Tailored Approach and Multimodal Intraoperative Neuromonitoring in Cerebellopontine Angle Surgery

The cerebellopontine angle (CPA) is a highly complex anatomical compartment consisting of numerous nervous and vascular structures that present mutual and intricate spatial relationships. CPA surgery represents, therefore, a constant challenge for neurosurgeons. Over the years, neurosurgeons have developed and refined several solutions with the aim of maximizing the surgical treatment effects while minimizing the invasiveness and risks for the patient. In this paper, we present our integrated approach to CPA surgery, describing its advantages in treating pathologies in this anatomical district. Our approach incorporates the use of technology, such as neuronavigation, along with advanced and multimodal intraoperative neuromonitoring (IONM) techniques, with the final goal of making this surgery safe and effective.

Bilateral and Optimistic Warning Paradigms Improve the Predictive Power of Intraoperative Facial Motor Evoked Potentials during Vestibular Schwannoma Surgery

Simple Summary

During surgery for vestibular schwannomas, the facial nerve is monitored via motor evoked potentials (facial nerve MEP). The established warning criteria for facial nerve MEP signal changes mostly refer to the ipsilateral side and disregard the contralateral side. Furthermore, the surgeon is warned as soon as the signal of a single facial muscle deteriorates. We examined how the predictive power of the facial nerve MEP would change if we used the percent change in ipsilateral versus contralateral MEP stimulation intensity over time as warning criterion; additionally, if we warned in a novel optimistic manner, a manner in which the surgeon would be warned only if all derived facial muscles deteriorate significantly, as opposed to the traditional method, in which the surgeon is warned as soon as a single muscle deteriorates. We retrospectively compared this approach to actual intraoperative warnings (based on unilateral threshold change, A-trains, and MEP loss) and show that with our method, the facial nerve MEP was significantly more specific and triggered fewer unnecessary warnings.

Abstract

Facial muscle corticobulbar motor evoked potentials (FMcoMEPs) are used to monitor facial nerve integrity during vestibular schwannoma resections to increase maximal safe tumor resection. Established warning criteria, based on ipsilateral amplitude reduction, have the limitation that the rate of false positive alarms is high, in part because FMcoMEP changes occur on both sides, e.g., due to brain shift or pneumocephalus. We retrospectively compared the predictive value of ipsilateral-only warning criteria and actual intraoperative warnings with a novel candidate warning criterion, based on “ipsilateral versus contralateral difference in relative stimulation threshold increase, from baseline to end of resection” (BilatMT ≥ 20%), combined with an optimistic approach in which a warning would be triggered only if all facial muscles on the affected side deteriorated. We included 60 patients who underwent resection of vestibular schwannoma. The outcome variable was postoperative facial muscle function. Retrospectively applying BilatMT, with the optimistic approach, was found to have a significantly better false positive rate, which was much lower (9% at day 90) than the traditionally used ipsilateral warning criteria (>20%) and was also lower than actual intraoperative warnings. This is the first report combining the threshold method with an optimistic approach in a bilateral multi-facial muscle setup. This method could substantially reduce the rate of false positive alarms in FMcoMEP monitoring.

Improving facial nerve outcome and hearing preservation by different degrees of vestibular schwannoma resection guided by intraoperative facial nerve electromyography

BACKGROUND

Facial nerve (FN) outcome has been improved by deciding upon the degree of vestibular schwannoma (VS) resection guided by intraoperative facial nerve monitoring (IOFM). This study aimed to evaluate hearing and FN outcomes following IOFM-guided VS removal through a retrosigmoid approach (RS).

METHODS

Between January 2015 and August 2017, 77 patients undergoing VS resection through RS, regardless of their preoperative hearing or tumor size, were included in this monocentric retrospective study. Total or near-/subtotal resection was decided intraoperatively according to IOFM. Partial resection was planned preoperatively. Pre- and postoperative FN function (House-Brackmann) and hearing level (AAO-HNS and Gardner-Robertson (GR) scales) were reported. Predictive factors for hearing preservation were analyzed according to a score developed to take into account pre-/postoperative hearing evolution.

RESULTS

The mean extrameatal diameter was 17 ± 8.5 mm. VS resection was total, near-/subtotal, and partial in 71%, 21%, and 8% of cases, respectively. Postoperative grade I and II FN function was achieved in 91% of patients. Serviceable (A+B or GR I+II) hearing and hearing with intelligibility (A+B+C or GR I-III) were preserved in 30% and 43% of cases, respectively. In multivariate analysis, preoperative synchronized auditory brainstem responses (ABR) were the only predictive factor (p = 0.006) imparting a five times greater chance of hearing preservation.

CONCLUSION

IOFM-guided VS excision through RS, whatever the VS size and hearing level, achieved a high rate of good postoperative FN function together with significant hearing preservation, and it should be used if hearing with some intelligibility is still present preoperatively, even in large VS.

Estimation of Intraoperative Stimulation Threshold of the Facial Nerve in Patients Undergoing Microvascular Decompression

Introduction  Facial weakness can result from surgical manipulation of the facial nerve. Intraoperative neuromonitoring reduces functional impairment but no clear guidelines exist regarding interpretation of intraoperative electrophysiological results. Most studies describe subjects with facial nerves encumbered by tumors or those with various grades of facial nerve weakness. We sought to obtain the neurophysiological parameters and stimulation threshold following intraoperative facial nerve triggered electromyography (t-EMG) stimulation during microvascular decompression for trigeminal neuralgia to characterize the response of normal facial nerves via t-EMG. Methods  Facial nerve t-EMG stimulation was performed in seven patients undergoing microvascular decompression for trigeminal neuralgia. Using constant current stimulation, single stimulation pulses of 0.025 to 0.2 mA intensity were applied to the proximal facial nerve. Compound muscle action potentials, duration to onset, and termination of t-EMG responses were recorded for the orbicularis oculi and mentalis muscles. Patients were evaluated for facial weakness following the surgical procedure. Results  Quantifiable t-EMG responses were generated in response to all tested stimulation currents of 0.025, 0.05, 0.1, and 0.2 mA in both muscles, indicating effective nerve conduction. No patients developed facial weakness postoperatively. Conclusions  The presence of t-EMG amplitudes in response to 0.025 mA suggests that facial nerve conduction can take place at lower stimulation intensities than previously reported in patients with tumor burden. Proximal facial nerve stimulation that yields responses with thresholds less than 0.05 mA may be a preferred reference baseline for surgical procedures within the cerebellopontine angle to prevent iatrogenic injury.

Natural History of Facial Weakness Following Surgery of the Cerebellopontine Angle: A Tertiary Care Cohort

OBJECTIVE

Facial function is a key outcome in surgery of the cerebellopontine angle (CPA). This study describes the natural history of facial weakness after CPA surgery.

STUDY DESIGN AND SETTING

Retrospective study of two tertiary centers.

PATIENTS AND INTERVENTION

Patients undergoing surgery for CPA tumors from 2003 to 2017 with preoperatively normal facial function and without subsequent surgical or adjuvant therapy.

MAIN OUTCOME MEASURE

Serial facial nerve examinations using the House-Brackmann (HB) scale.

RESULTS

Of 301 patients examined, 149 (49.5%) had facial weakness postoperatively and 74.5% of these occurred within 24 hours. Of patients with HB-1 function within 24 hours after surgery, 95.3% had HB-1 function and 100% had HB-3 or better function at last follow-up. Of patients with HB-4 or worse function within 24 hours after surgery, 22.6% ultimately achieved HB-1 function and 83.9% ultimately achieved HB-3 or better function. By 180 days postoperatively, 83.7% of patients' facial function had stabilized. At last follow-up, 3.0% of patients had facial function poorer than HB-3, which was associated with subtotal resections (12.9% vs. 4.4% for near total and 0.6% for gross total resections, p = 0.002), aspirin use (10.6% vs. 1.6%, p = 0.001), larger tumors (p < 0.0005), longer surgery (p < 0.002), and immediate versus delayed postoperative facial weakness (p = 0.002).

CONCLUSIONS

Facial function over time after CPA surgery is presented. While facial function immediately after surgery correlates with future function, delayed weakness or improvement is common. These data inform patient counseling both preceding and following surgery.

Electromyographic Response of Facial Nerve Stimulation Under Partial Neuromuscular Blockade During Resection of Vestibular Schwannoma

OBJECTIVE

To evaluate whether use of partial nondepolarizing neuromuscular blocking agents, at a train-of-four level 1, compromise facial nerve monitoring during vestibular schwannoma (VS) resection.

METHODS

Sixty consecutive patients undergoing VS resection were enrolled into a partial peripheral neuromuscular blockade group or free of neuromuscular blockade group. Stimulation threshold to elicit an electromyographic response amplitude of at least 100 μV was recorded at the proximal and distal facial nerve after VS removal. The proximal-to-distal ratio of amplitude of the orbicularis oculi and oris muscles was calculated.

RESULTS

All patients successfully passed the electromyography monitoring test. Mean electrical stimulation thresholds were higher in the peripheral neuromuscular blockade group than in the free of neuromuscular blockade group (0.12 mA vs. 0.06 mA at proximal site, P = 0.001; 0.08 mA vs. 0.03 mA at distal site, P = 0.0002). The differences in median proximal-to-distal amplitude ratios were not statistically significant in both groups. There was a trend toward more patients needing phenylephrine. Recovery profiles were comparable in the 2 groups.

CONCLUSIONS

Although mean stimulation threshold to elicit a response amplitude was higher in the peripheral neuromuscular blockade group than in the free of neuromuscular blockade group at the proximal site, the stimulation thresholds in both groups were sufficient for facial nerve monitoring in VS surgery, indicating no clinical difference in both groups.

How Deleterious Is Facial Nerve Dissection for the Facial Nerve in Parotid Surgery: An Electrophysiological Evaluation

OBJECTIVE

This study was conducted to investigate subclinical electrophysiological deleterious effect due to microtrauma to the nerve in response to the dissection of a tumor and parotid tissue from the facial nerve and its branches and surgical traction experienced during the operation.

METHODS

The study included 34 adult patients who underwent parotidectomy operations under intraoperative facial nerve monitoring. Three measurements were taken from each patient to evaluate facial nerve functions, with 3 stimuli of different intensities applied with different timing. An initial stimulus of 1 mA was applied to confirm the identification of the main trunk of the facial nerve (Group 1: Initial-Normal). Then, a threshold value was found by stimulating the main trunk until muscle fasciculations were observed on facial muscles (Group 2: Basal-Minimal). The same procedure was repeated after the tumor was resected (Group 3: Final-Minimal).

RESULTS

There was no significant difference between the stimulus thresholds of Group 2 (0.31 mA) and Group 3 (0.30 mA). The highest amplitude in all 3 groups was observed at the mental branch, and the lowest at the frontal. The highest latency value was measured at the frontal branch and the lowest at the mental branch. Five (14.7%) of the patients developed postoperative pareses that was completely resolved by the seventh postoperative day visit in these patients.

CONCLUSION

This study demonstrated that the surgical trauma of a meticulously conducted dissection and surgical traction did not cause any deleterious electrophysiological alteration on the facial nerve.

Facial functional outcome in monitored versus not-monitored patients in vestibular schwannomas surgery

Objective:

Even though advances in surgical techniques have improved facial nerve outcomes, functional preservation is still an issue because injury to the facial nerve has significant physical and psychological consequences for the patient. We retrospectively review our data in VS surgery to compare the facial outcome in intraoperative facial monitored versus not-monitored patients.

Materials and Methods:

51 consecutive patients with unilateral vestibular schwannoma in the period from 2005 to 2010 were treated in our Institution. In according to the type of neurophysiological tool used during surgical procedures, two patients groups were identified: Group 1 (facial stimulator only) and Group 2 (stimulator and facial monitoring). Statistical comparison of the two groups was made with the t- test, and facial function results were evaluated with the Fisher's exact test.

Results:

In the Group 1, of the 22 patients with anatomically preserved facial nerves, 3 (13.6%) showed excellent facial nerve function, 14 (63.6%) showed intermediate function, and 5 (22.7%) showed poor function. In the Group 2, all the 27 patients got anatomically preserved facial nerves, and 18 (66.7%) showed excellent facial nerve function, 9 (33.3%) showed intermediate function, and no one showed poor function.

Conclusions:

We found that retrosigmoid approach associated with continuous EMG facial monitoring combined with the use of bipolar stimulation is a safe and effective treatment for vestibular schwannomas.

Intraoperative monitoring: normative range associated with normal postoperative glottic function

OBJECTIVES/HYPOTHESIS

Despite increasing use of intraoperative nerve monitoring (IONM), there is limited information on normative electrophysiologic electromyographic (EMG) parameters. The objective of this study was to define normative parameters of recurrent laryngeal nerve (RLN) intraoperative neuromonitoring during thyroid surgery associated with normal postoperative vocal cord function.

STUDY DESIGN

Prospective data collection in a tertiary care center.

METHODS

Quantitative analysis of evoked waveform amplitude and threshold was performed on 125 patients with 167 nerves at risk. Values were displayed as a mean with 5th percentile and 95th percentiles (5th-95th). Postoperative vocal cord function in all patients was documented.

RESULTS

All patients had normal postoperative laryngeal function (group I-normal) except for two patients who had postoperative transient vocal cord paralysis (group II-abnormal/outlier). The final amplitude between 247 and 3607 μV at the end of dissection/end of surgery was associated in all group I patients with a normal postoperative neural function. Final intraoperative amplitude measures for group II averaged just 97.5 μV, significantly different than our normative ranges obtained for group I, and fell outside of the group I 5% to 95% percentile range (P = .016). Final amplitude adequately predicted postoperative RLN impaired function immediately after surgery.

CONCLUSIONS

We propose IONM EMG data criteria that predict normal postoperative vocal cord function monitoring and provide information about nerve functioning at the end of the operation, thereby allowing adaptation of the surgical strategy when a bilateral procedure is indicated to avoid bilateral nerve paralysis.

Radiological and Clinical Factors Predicting the Facial Nerve Outcome following Retrosigmoid Approach for Large Vestibular Schwannomas (VSs)

Objective The aim of our study was to identify the radiological and clinical factors that predict postoperative facial nerve outcome following retrosigmoid approach for large vestibular schwannomas (VSs). Methods A total of 72 patients with large (≥ 3 cm) vestibular schwannomas was included in this retrospective study. Various parameters evaluated were age, gender, clinical presentation, tumor diameter in three planes, intrameatal extension, and pattern of growth. Results Age of the patient and presenting symptoms such as headache, ataxia, or preoperative facial nerve dysfunction correlated with poorer facial nerve outcome (p < 0.05). Patients with larger tumor volumes and extrameatal growth experienced a worse outcome (p < 0.05). Anterior and caudal extension (p = 0.001) correlated with poorer outcome, as well. Intrameatal extension and bony changes of the internal acoustic meatus did not correlate with the outcome (p > 0.05). Of the various examined factors, preoperative facial nerve function independently predicted postoperative facial nerve outcome. Conclusion Our study suggests that young patients with small tumor volume and normal facial nerve function at presentation are more likely to experience a good postoperative facial nerve outcome. These clinical and radiological parameters can be used to predict facial nerve outcome prior to surgery.

Evoked electromyography to rocuronium in orbicularis oris and gastrocnemius in facial nerve injury in rabbits

BACKGROUND

Muscles innervated by the facial nerve show different sensitivities to muscle relaxants than muscles innervated by somatic nerves, especially in the presence of facial nerve injury. We compared the evoked electromyography (EEMG) response of orbicularis oris and gastrocnemius in with and without a non-depolarizing muscle relaxant in a rabbit model of graded facial nerve injury.

METHODS

Differences in EEMG response and inhibition by rocuronium were measured in the orbicularis oris and gastrocnemius muscles 7 to 42 d after different levels of facial nerve crush injuries in adult rabbits.

RESULTS

Baseline EEMG of orbicularis oris was significantly smaller than those of the gastrocnemius. Gastrocnemius was more sensitive to rocuronium than the facial muscles (P < 0.05). Baseline EEMG and EEMG amplitude of orbicularis oris in the presence of rocuronium was negatively correlated with the magnitude of facial nerve injury but the sensitivity to rocuronium was not. No significant difference was found in the onset time and the recovery time of rocuronium among gastrocnemius and normal or damaged facial muscles.

CONCLUSIONS

Muscles innervated by somatic nerves are more sensitive to rocuronium than those innervated by the facial nerve, but while facial nerve injury reduced EEMG responses, the sensitivity to rocuronium is not altered. Partial neuromuscular blockade may be a suitable technique for conducting anesthesia and surgery safely when EEMG monitoring is needed to preserve and protect the facial nerve. Additional caution should be used if there is a risk of preexisting facial nerve injury.

Electromyographic response of facial nerve stimulation under different levels of neuromuscular blockade during middle-ear surgery

OBJECTIVE

To investigate facial nerve monitoring in patients receiving the partial nondepolarizing neuromuscular blocking agents (NMBAs), remifentanil and propofol.

METHODS

Patients with normal facial function and advanced middle-ear disease were enrolled. For total intravenous anaesthesia (TIVA), propofol and remifentanil were infused as induction/maintenance anaesthesia. Stimulation thresholds and amplitudes were recorded at each train-of-four (TOF) nerve stimulation level. Time differences between start of TOF and electromyographic (EMG) amplitude decreases (Ti), and between complete recovery of TOF and EMG amplitudes (Tr), were calculated.

RESULTS

Fifteen patients were enrolled. Mean ± SD Ti was 3.4 ± 1.28 min; Tr was 18.7 ± 4.41 min. Amplitude of stimulation was apparent mostly at TOF level 1. In most cases, no or a weak response (<100 µV) was observed at TOF 0. Mean ± SD threshold of electrical stimulation was 0.31 ± 0.10 mA at TOF 1. At TOF > 2, all cases showed EMG response on electrical stimulation.

CONCLUSIONS

Induction of TIVA using propofol and remifentanil provided reliable conditions for delicate microsurgery. Minimal NMBA use, considered as producing TOF levels >1, was sufficient for facial nerve monitoring in neuro-otological surgery.

Intraoperative neuromonitoring techniques in the surgical management of acoustic neuromas

Unfavorable outcomes such as facial paralysis and deafness were once unfortunate probable complications following resection of acoustic neuromas. However, the implementation of intraoperative neuromonitoring during acoustic neuroma surgery has demonstrated placing more emphasis on quality of life and preserving neurological function. A modern review demonstrates a great degree of recent success in this regard. In facial nerve monitoring, the use of modern electromyography along with improvements in microneurosurgery has significantly improved preservation. Recent studies have evaluated the use of video monitoring as an adjunctive tool to further improve outcomes for patients undergoing surgery. Vestibulocochlear nerve monitoring has also been extensively studied, with the most popular techniques including brainstem auditory evoked potential monitoring, electrocochleography, and direct compound nerve action potential monitoring. Among them, direct recording remains the most promising and preferred monitoring method for functional acoustic preservation. However, when compared with postoperative facial nerve function, the hearing preservation is only maintained at a lower rate. Here, the authors analyze the major intraoperative neuromonitoring techniques available for acoustic neuroma resection.

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.

Use of supramaximal stimulation to predict facial nerve outcomes following vestibular schwannoma microsurgery: results from a decade of experience

Object

The goal of vestibular schwannoma surgery is tumor removal and preservation of neural function. Intraoperative facial nerve (FN) monitoring has emerged as the standard of care, but its role in predicting long-term facial function remains a matter of debate. The present report seeks to describe and critically assess the value of applying current at supramaximal levels in an effort to identify patients destined for permanent facial paralysis.

Methods

Over more than a decade, the protocol for stimulating and assessing the FN during vestibular schwannoma surgery at the authors' institution has consisted of applying pulsed constant-current stimulation at supramaximal levels proximally and distally following tumor resection to generate an amplitude ratio, which subtracted from 100% yields the degree to which the functional integrity of the FN “dropped off” intraoperatively. These data were prospectively collected and additional variables that might impact postoperative FN function were retrospectively reviewed from the medical record. Only patients with anatomically intact FNs and > 12 months of follow-up data were analyzed.

Results

There were 267 patients available for review. The average posterior fossa tumor diameter was 24 mm and the rate of long-term good (House-Brackmann Grade I–II) FN function was 84%. Univariate logistic regression analysis revealed that prior treatment, neurofibromatosis Type 2 status, tumor size, cerebellopontine angle extension, subjectively thinned FN at the time of operation, minimal stimulation threshold, percent dropoff by supramaximal stimulation (SMS), and postoperative FN function all correlated statistically (p < 0.05) with long-term FN function. When evaluating patients with significant FN weakness at the time of hospital discharge, only the percent dropoff by SMS remained a significant predictor of long-term FN function. However, the positive predictive value of SMS for long-term weakness is low, at 46%.

Conclusions

In a large cohort of patients, the authors found that interrogating intraoperative FN function with SMS is safe and technically simple. It is useful for predicting which patients will ultimately have good facial function, but is very limited in identifying patients destined for long-term facial weakness. This test may prove helpful in the future in tailoring less than gross-total tumor removal to limit postoperative facial weakness.

Neurophysiologic monitoring in neurosurgery

This article focuses on the application of neurophysiologic monitoring in uniquely neurosurgical procedures. Neurophysiologic monitoring provides functional testing and mapping to identify neural structures. Once identified, the functionality of the central and peripheral nervous system areas at risk for neurosurgical injury can be monitored. It discusses the use of motor-evoked potentials, sensory evoked potentials, electromyography and electroencephalography to assess neurologic change.

Neurophysiologic monitoring of the spinal accessory nerve, hypoglossal nerve, and the spinomedullary region

This review of hypoglossal nerve, spinal accessory nerve, and spinomedullary region intraoperative monitoring details pertinent central and extramedullary anatomy, an updated understanding of proper free-run EMG recording methods and recent developments in stimulation technique and instrumentation. Mapping and monitoring the floor of the fourth ventricle, especially the vagal/hypoglossal trigone region, are emphasized. Although cranial nerve transcranial electrical motor evoked potential recordings can afford appreciation of corticobulbar/corticospinal tract function and secure a more dependable measure of proximate extramedullary somatoefferents, the sometimes difficult implementation and the, as yet, unresolved alert criteria of these recordings demand critical appraisal. Nearby and intimately associated cardiochronotropic and barocontrol neural networks are described; their better understanding is recommended as an important adjunct to "routine" neural monitoring. Finally, an Illustrative case is presented to highlight the many strengths and weaknesses of "state of the art" lower cranial nerve/spinomedullary region monitoring.

Differences in sensitivity to rocuronium among orbicularis oris muscles innervated by normal or damaged facial nerves and gastrocnemius muscle innervated by somatic nerve in rats: combined morphological and functional analyses

OBJECTIVES/HYPOTHESIS

To evaluate mechanisms of discrepant responses to the nondepolarizing muscle relaxant rocuronium among normal and injured facial nerve-innervated orbicularis oris and tibial nerve-innervated gastrocnemius, and to provide information for the proper use of muscle relaxants to balance evoked electromyography (EEMG) monitoring and immobility in general anesthesia.

STUDY DESIGN

Randomized controlled study.

METHODS

Right-sided facial nerve injury was induced by crush axotomy in 18 Sprague-Dawley rats. At different rocuronium concentrations, muscular tension amplitude (MTA) was determined in vitro for normal and injured facial nerve-innervated orbicularis oris and gastrocnemius; the number of unsaturated acetylcholine receptors (AChRs) at end plates was determined by (125) I-α-bungarotoxin staining followed with gamma spectroscopy. The morphological composition of muscle fibers was determined by histological examination.

RESULTS

Following rocuronium incubation, the percentage of MTA inhibition (MTAI%) of gastrocnemius was significantly higher than the corresponding values of orbicularis oris (P < .05), and the degree of saturation of AChR in gastrocnemius was significantly greater than that in orbicularis oris (P < .05). The baseline MTA and AChR density of injured-side orbicularis oris was significantly smaller than those of the normal side, whereas no significant difference was found regarding MTAI% and the degree of AChR saturation between the normal and injured side.

CONCLUSIONS

The affinity of AChR at end plates and different number of AChR per unit fiber cross-sectional area may be the mechanisms for differential sensitivities to neuromuscular blockers between facial nerve-innervated muscles and somatic nerve-innervated muscles. The lower EEMG responses in the impaired facial nerve-innervated muscles may result from the lower AChR density at end plates compared with the normal facial nerve-innervated muscles.

Pharyngeal motor evoked potentials elicited by transcranial electrical stimulation for intraoperative monitoring during skull base surgery

OBJECT

The purpose of this study was to determine whether monitoring of pharyngeal motor evoked potentials (PhMEPs) elicited by transcranial electrical stimulation during skull base tumor surgery might be useful for predicting postoperative swallowing deterioration.

METHODS

The authors analyzed PhMEPs in 21 patients during 22 surgical procedures for the treatment of skull base tumors. Corkscrew electrodes positioned at C3 or C4 and Cz were used to deliver supramaximal stimuli (220-550 V). Pharyngeal MEPs were recorded from the posterior wall of the pharynx through a modified endotracheal tube. The correlation between the final/baseline PhMEP ratio and postoperative swallowing function was examined.

RESULTS

Postoperative swallowing function was significantly (p < 0.05), although not strongly (r = -0.47), correlated with the final/baseline PhMEP ratio. A PhMEP ratio < 50% was recorded during 4 of 22 procedures; in all 4 of these cases, the patients experienced postoperative deterioration of swallowing function. After 18 procedures, the PhMEP ratios remained > 50%; nevertheless, after 4 (22.2%) of these 18 procedures, patients showed deterioration of swallowing function.

CONCLUSIONS

Intraoperative PhMEP monitoring can be useful for predicting swallowing deterioration following skull base surgery, especially in patients with swallowing disturbances that are mainly due to reduction in the motor functions of the pharyngeal muscles.

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

BACKGROUND

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.

METHODS

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.

FINDINGS

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).

CONCLUSIONS

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.

Intraoperative neurophysiological monitoring in vestibular schwannoma surgery: advances and clinical implications

OBJECT

Intraoperative neurophysiological monitoring has become an integral part of vestibular schwannoma surgery. The aim of this article was to review the different techniques of intraoperative neurophysiological monitoring in vestibular schwannoma surgery, identify the clinical impact of certain pathognomonic patterns on postoperative outcomes of facial nerve function and hearing preservation, and highlight the role of postoperative medications in improving delayed cranial nerve dysfunction in the different reported series.

METHODS

The authors performed a review of the literature regarding intraoperative monitoring in acoustic/vestibular schwannoma surgery. The different clinical series representing different monitoring techniques were reviewed. All the data from clinical series were analyzed in a comprehensive and comparative model.

RESULTS

Intraoperative brainstem auditory evoked potential monitoring, direct cochlear nerve action potential monitoring, and facial nerve electromyography are the main tools used to assess the functional integrity of an anatomically intact cranial nerve. The identification of pathognomonic brainstem auditory evoked potential and electromyography patterns has been correlated with postoperative functional outcome. Recently, perioperative administration of intravenous hydroxyethyl starch and nimodipine as vasoactive and neuroprotective agents was shown to improve vestibular schwannoma functional outcome in few reported studies.

CONCLUSIONS

Recent advances in electrophysiological technology have considerably contributed to improvement in functional outcome of vestibular neuroma surgery in terms of hearing preservation and facial nerve paresis. Perioperative intravenous nimodipine and hydroxyethyl starch may be valuable additions to surgery.

Prognostic clinical and radiological parameters for immediate facial nerve function following vestibular schwannoma surgery

OBJECTIVE

The paper aims to define the parameters available before surgery which could predict immediate facial nerve function after excision of a vestibular schwannoma (VS).

METHODS

Ninety-nine patients with VS operated consecutively by a single surgeon using an identical surgical technique have been evaluated retrospectively. Data were collected regarding patients' sex, age at onset of symptoms and at surgery, initial symptoms, neurological status at presentation, early post-operative neurological status and complications. The main radiological parameters included in the study were tumour extension pattern, diameters, shape, and volume, as well as extent of bony changes of the internal auditory canal.

RESULTS

As the tumour stage and volume increase, facial nerve function is worse after surgery (p < 0.001 and p < 0.05, respectively). Concomitantly, larger extra-meatal tumour diameters in three dimensions (sagittal, coronal and axial) led to worse function (p < 0.01). Anterior and/or caudal tumour extension (p = 0.001 and p = 0.004, respectively) had more significant correlation than posterior and/or cranial extension (p = 0.022 and p = 0.353, respectively). Polycyclic VS had the worst prognosis, followed by the tumours with oval shape. The extent of intra-meatal tumour growth does not correlate with immediate facial nerve outcome. The different angles, lengths and diameters of the internal auditory channel showed no significant correlation with facial nerve outcome. Patients with headache as an initial symptom and those with gait instability and/or pre-operative poor facial nerve function had significantly worse immediate facial nerve outcome.

CONCLUSION

Our data suggests that the analysis of the radiological and neurological patient data prior to surgery could give reliable clues regarding the immediate post-operative facial nerve function.

A Novel Method in Predicting Immediate Postoperative Facial Nerve Function Post Acoustic Neuroma Excision

To determine whether the percentage calculated by dividing the amplitude of postexcision direct facial nerve stimulus responses (at pontomedullary junction) by the amplitude of distal ipsilateral transcutaneous (stylomastoid region) maximal stimulus responses and response amplitude progression by increasing stimulus intensities have predictive value for determining normal or near-normal (House-Brackmann Grade 1 or 2) immediate postoperative facial nerve function.

STUDY DESIGN

Intraoperative recordings of three muscle groups: 1) frontalis, 2) orbicularis oculi, and 3) orbicularis oris. Postexcision direct facial nerve stimulation at the pontomedullary junction and transcutaneous maximal facial nerve stimulation at the ipsilateral stylomastoid region and their associated response amplitudes were recorded.

SETTING

Tertiary referral center.

PATIENTS AND METHODS

Patients who underwent acoustic neuroma surgery from January 2004 to March 2006 with intraoperative facial nerve monitoring and an intact facial nerve after tumor excision were included. Recordings were available for 38 patients.

RESULTS

With a stimulus intensity of 0.3 mA at the root exit zone, there was an 81% positive predictive value in patients that exhibited a compound action potential of greater than 20% of maximum (sensitivity, 81%). This increased to 93% when the compound action potential was greater than 50% of maximum. When the amplitude increase was greater than 5 microV, there was a 77% positive predictive value (sensitivity, 87%).

CONCLUSION

The percentage of the response amplitude of direct facial nerve stimulation at the pontomedullary junction when compared with the maximum response amplitude of ipsilateral transcutaneous stimulation at the stylomastoid foramen is a good predictor of normal to near-normal immediate postoperative facial nerve function. Progression of amplitude response also seems to be a good predictor of normal to near-normal immediate postoperative facial nerve function.

Continuous EMG recordings and intraoperative electrical stimulation for identification and protection of cervical nerve roots during foraminal tumor surgery

OBJECTIVE

Spinal cord function is now routinely monitored with somatosensory evoked potentials (SEPs) and motor evoked potentials (MEPs) during surgery for intraspinal cervical dumbbell and foraminal tumors. However, upper extremity nerve roots are also at risk during these procedures. Anatomic relations are frequently difficult to interpret because the nerve roots may be displaced by the tumor. We used electrical stimulation with compound muscle action potential (CMAP) recordings at multiple sites to identify the location and course of the involved nerve root and to provide real-time information regarding the functional status of the roots to predict postoperative outcome.

METHODS

Ten patients were monitored during surgery for cervical dumbbell or foraminal tumors. SEPs and MEPs were monitored as a routine procedure. CMAPs were recorded from needle electrodes placed in the deltoid, biceps, triceps, and flexor carpi ulnaris muscles. Spontaneous electromyography (EMG) muscle activity was also continuously monitored. A handheld monopolar stimulation electrode was used to elicit evoked EMG responses to identify and trace the course of nerves in relation to the tumor. In four patients, the stimulation threshold was tested before and after tumor resection to predict postoperative nerve root function.

RESULTS

Electrical stimulation with CMAP recording was successful in localizing nerve roots during tumor resection in all 10 patients. Monitoring predicted postoperative nerve root preservation after tumor removal in each case. It was possible to identify either by using low-level stimulation (<2.0 V) or by observing changes in spontaneous EMG amplitude if activation was present during surgical dissection. The monitoring of spontaneous muscle activity in response to direct or indirect surgical manipulation during tumor resection also provided continuous assessment of nerve root function and identified any physiologic disturbance induced by surgical manipulation.

CONCLUSIONS

Electrical stimulation in the operating field and recording of CMAPs facilitated nerve root identification and predicted postoperative function during dissection and separation from ligamentous or neoplastic tissue in 10 patients. Electrical stimulation might also be useful to predict postoperative preservation of function when nerve root sacrifice is necessary and no motor response is detected intraoperatively.

Comparison between intraoperative observations and electromyographic monitoring data for facial nerve outcome after vestibular schwannoma surgery

A four-channel device may enhance the sensitivity of electromyography (EMG). Determination of stimulation thresholds (STs) below 0.05 mA improves facial prognostic information after vestibular schwannoma (VS) surgery. To compare intraoperative observations with electromyographic data for predicting the immediate facial function outcome after VS surgery.A total of 111 VS patients operated on and intraoperatively monitored using four-channel facial EMG between October 2002 and September 2003 were included in this multicentre, prospective study. Mascular activity detection was performed in the frontal, orbicularis oculi, orbicularis oris and platysma muscles. Intraoperative observations were made concerning adhesion and nerve stretch, and facial function was graded (House-Brackmann classification) at postoperative Days 1 and 8.Facial function at postoperative Days 1 and 8 was related to the intraoperative nerve STs at the brainstem and in the adhesion zone (range 0.01-3 mA for a response > 100 microV). At postoperative Day 8, good facial function (Grades 1 or 2) was observed in 93% of cases for STs in the adhesion zone of 0.01-0.04 mA, 85% for STs of 0.05-0.3 mA and 79% for STs > 0.3 mA. These STs were related to the degree of tumor adhesion and not to the nerve stretch. The maximal EMG response was detected in the frontal or platysma muscles in 26% of cases, and in the orbicularis oris and orbicularis oculi muscles in 74%.

Intraoperative Neurophysiologic Monitoring: Indications and Techniques for Common Procedures in Otolaryngology–Head and Neck Surgery

Intraoperative cranial nerve monitoring can be an effective adjunct in otolaryngology-head and neck surgery. Monitoring is not considered standard of care, despite indications of cost effectiveness and improved functional outcomes. Lessons learned performing facial nerve monitoring are applicable to upper and lower cranial motor nerves. Auditory nerve monitoring can be modified accord-ing to need for selected otologic and neurotologic surgery. Process standardization and effective communication can lead to improved patient outcomes.

Facial Nerve Monitoring Parameters As a Predictor of Postoperative Facial Nerve Outcomes after Vestibular Schwannoma Resection

OBJECTIVE

To evaluate whether the intraoperative stimulus threshold and response amplitude measurements from facial electromyography can predict facial nerve function at 1 year after vestibular schwannoma resection.

STUDY DESIGN

Prospective study.

SETTING

Tertiary academic center.

PATIENTS

Seventy-four consecutive vestibular schwannoma patients.

INTERVENTION

The minimal stimulus intensity (in milliamperes) and electromyographic response amplitude (in microvolts) were recorded during stimulation applied to the proximal facial nerve after vestibular schwannoma removal.

MAIN OUTCOME MEASURE

Facial nerve outcomes at 1 year were evaluated using the House-Brackmann scale. Analysis was then performed to evaluate whether these electrophysiologic recordings and tumor size could predict facial nerve functional outcomes.

RESULTS

Of the 74 patients, 66 of 74 (89%) had House-Brackmann Grade I or II facial nerve function and 8 of 74 (11%) had House-Brackmann Grade III-VI function at 1 year after surgery. If standards were set for intraoperative minimal stimulus intensity of 0.05 mA or less and response amplitude of 240 microV or greater, the authors were able to predict a House-Brackmann Grade I or II outcome in 56 of 66 (85%) patients at 1 year after surgery. With these same electrophysiologic parameters, only 1 of 8 (12%) House-Brackmann Grade III-VI patients also met this standard and thus gave a false-positive result. Logistic regression analysis of the data showed that both a stimulus threshold of 0.05 mA or less and a response amplitude of 240 microV or greater predicted a House-Brackmann Grade I or II outcome with a 98% probability. However, stimulus threshold or response amplitude alone had a much lower probability of the same result. In addition, although tumor size was found to independently predict facial nerve outcomes at 1 year, it did not improve the ability to predict facial nerve function over a model using stimulus intensity and amplitude alone.

CONCLUSION

Individually, minimal stimulus intensity or response amplitude was less successful in predicting long-term postoperative facial nerve function. However, if both parameters are considered together, the study demonstrates that they are good prognostic indicators for facial nerve function at 1 year after surgery.

Localized Transcranial Electrical Motor Evoked Potentials for Monitoring Cranial Nerves in Cranial Base Surgery

OBJECTIVE:

To describe a novel monitoring technique that allows “functional” assessment of cranial nerve continuity during cranial base surgery.

METHODS:

Facial motor evoked potentials (MEP) in 71 consecutive patients were obtained by localized transcranial electrical stimulation in all patients requiring facial nerve monitoring during the period from November 2002 to August 2004. With transcranial electrical stimulation localized to the contralateral cortex, facial nerve MEPs are obtained through stimulation of more proximal intracranial structures.

RESULTS:

Logistic regression revealed that the final-to-baseline facial MEP ratio predicted satisfactory (House-Brackmann Grade 1 and 2 function) immediate postoperative facial function (0.005 &gt; P &gt; 0.0005). Contingency table analysis showed high correlation (χ2, P ≤ 2 × 108) and acceptable test characteristics using a 50% final-to-baseline MEP ratio.

CONCLUSION:

Facial nerve MEPs recorded intraoperatively during cranial base surgery using the proposed technique predicts immediate postoperative facial nerve outcome. This technique can also be used to monitor other motor cranial nerves in cranial base surgery.

Intraoperative facial motor evoked potential monitoring with transcranial electrical stimulation during skull base surgery

OBJECTIVE

To address the limitations of standard electromyography (EMG) facial nerve monitoring techniques by exploring the novel application of multi-pulse transcranial electrical stimulation (mpTES) to myogenic facial motor evoked potential (MEP) monitoring.

METHODS

In 76 patients undergoing skull base surgery, mpTES was delivered through electrodes 1cm anterior to C1 and C2 (M1-M2), C3 and C4 (M3-M4) or C3 or C4 and Cz (M3/M4-Mz), with the anode contralateral to the operative side. Facial MEPs were monitored from the orbicularis oris muscle on the operative side. Distal facial nerve excitation was excluded by the absence of single pulse responses and by onset latency consistent with a central origin.

RESULTS

M3/M4-Mz mpTES (n=50) reliably produced facial MEPs while M1-M2 (n=18) or M3-M4 (n=8) stimulation produced 6 technical failures. Facial MEPs could be successfully monitored in 21 of 22 patients whose proximal facial nerves were inaccessible to direct stimulation. Using 50, 35 and 0% of baseline amplitude criteria, significant facial deficits were predicted with a sensitivity/specificity of 1.00/0.88, 0.91/0.97 and 0.64/1.00, respectively.

CONCLUSIONS

Facial MEPs can provide an ongoing surgeon-independent assessment of facial nerve function and predict facial nerve outcome with sufficiently useful accuracy.

SIGNIFICANCE

This method substantially improves facial nerve monitoring during skull base surgery.

Prediction of Long-Term Facial Nerve Outcomes with Intraoperative Nerve Monitoring

OBJECTIVE

Evaluate the utility of a statistical equation using two independent intraoperative monitoring parameters in predicting long-term facial nerve function.

STUDY DESIGN

Retrospective case review.

SETTING

Tertiary care hospital.

PATIENTS

Sixty patients undergoing resection of vestibular schwannomas with intraoperative facial nerve monitoring at a single institution.

INTERVENTION

All patients underwent microsurgical resection of vestibular schwannomas with the use of intraoperative cranial nerve monitoring.

MAIN OUTCOME MEASURE

Final facial nerve outcome measured using the House-Brackmann scale at least 6 months after microsurgical resection.

RESULTS

Five out of 60 (8.3%) patients demonstrated significant long-term weakness (i.e., House-Brackmann grade III or worse). Intraoperative monitoring parameters (proximal stimulation threshold, proximal-to-distal response amplitude ratio) were accurate in predicting increased risk of long-term facial nerve dysfunction when used in a logistic regression model. A Student's t test confirmed the equation result was statistically significant in differentiating long-term facial nerve outcomes.

CONCLUSION

Patients with immediate weakness are at higher risk of having long-term poor facial nerve function. The use of intraoperative monitoring parameters was reliable in predicting facial nerve outcomes. Patients with permanent facial nerve dysfunction often require rehabilitative procedures. The ability to predict facial nerve outcomes with intraoperative monitoring may allow early rehabilitative procedures to improve quality of life and prevent ocular complications.

Four-Channel Electromyography of the Facial Nerve in Vestibular Schwannoma Surgery: Sensitivity and Prognostic Value for Short-Term Facial Function Outcome

OBJECTIVE

To evaluate the short-term facial prognostic value of a four-channel facial electromyographic device in vestibular schwannoma surgery.

STUDY DESIGN

Eighty-nine vestibular schwannomas operated on and intraoperatively monitored by a four-channel facial electromyographic device between October 2002 and September 2003 were included in this prospective study. Detection was performed in frontal, orbicularis oculi, orbicularis oris, and platysma muscles.

MAIN OUTCOME MEASURE

Facial function grading at postoperative Days 1, 8, 30, and 180 (House-Brackmann classification).

SETTING

Tertiary referral center.

RESULTS

Postoperative facial function at Day 180 was assessed as Grade 1 or 2 in 80%, as Grade 3 or 4 in 16%, and as Grade 5 or 6 in 4% (n=80). The postoperative facial function was related to the intraoperative nerve stimuli thresholds (range, 0.01-3 mA for a response >100 microV) near the brainstem and the proximal-to-distal ratio of the stimulation threshold. A proximal threshold between 0.01 and 0.04 mA had a positive predictive value of 94% for good facial function (Grade 1 or 2). The proximal threshold was lower in patients with improving or stable facial function in comparison with those with a delayed deterioration between Days 8 and 30. The stimulation threshold at the adhesion zone was related to the immediate facial function outcome. The maximal electromyographic response was detected in the frontal muscle or the platysma in 27% of cases and in orbicularis oris and oculi in 73% of cases.

CONCLUSION

A four-channel device may enhance electromyographic sensitivity. Determination of stimulation threshold below 0.05 mA yields facial prognostic information.

Intraoperative cranial nerve monitoring

The purpose of intraoperative monitoring is to preserve function and prevent injury to the nervous system at a time when clinical examination is not possible. Cranial nerves are delicate structures and are susceptible to damage by mechanical trauma or ischemia during intracranial and extracranial surgery. A number of reliable electrodiagnostic techniques, including nerve conduction studies, electromyography, and the recording of evoked potentials have been adapted to the study of cranial nerve function during surgery. A growing body of evidence supports the utility of intraoperative monitoring of cranial nerve nerves during selected surgical procedures.