Intraoperative cranial nerve monitoring during posterior skull base surgery

Impact of Latency Variations on the Predictive Value of Facial Nerve Proximal-to-Distal Amplitude Ratio during Vestibular Schwannoma Surgery

Introduction  This study highlights the relation between compound muscle action potential (CMAP) latency variations and the predictive value of facial nerve (FN) proximal-to-distal (P/D) amplitude ratio measured at the end of vestibular schwannoma resection. Methods  Forty-eight patients underwent FN stimulation at the brainstem (proximal) and internal acoustic meatus (distal) using a current intensity of 2 mA. The proximal latency and the P/D amplitude ratio were assessed. House-Brackmann grades I & II indicated good FN function, and grades III to VI were considered fair/poor function. A P/D amplitude ratio > 0.6 was used as a cutoff to indicate a good FN function, while a ratio of ≤ 0.6 indicated a fair/poor FN function. Results  The P/D amplitude ratio was measured for all patients, and the calculated sensitivity (SE), specificity (SP), positive predictive value (PPV), and negative predictive value (NPV) were 85.2, 85.7, 88.5, and 81.8%, respectively. The CMAPs from the mentalis muscle were then classified based on their proximal latency into group I (< 6 ms), group II (6-8 ms), and group III (> 8 ms). The SE, SP, PPV, and NPV became 90.5, 90.9, 95, and 83.3%, respectively, in group II. In group I, SE and NPV increased, whereas SP and PPV decreased. While in group III, SP and PPV increased, whereas SE and NPV decreased. Conclusion  At a latency between 6 and 8 ms, the P/D amplitude ratio was predictive of outcomes with high SE and SP. When latency was < 6 ms or > 8 ms, the same predictive ability was not observed. Knowing the strengths and limitations is important for understanding the predictive value of the P/D amplitude ratio.

Toward Intraoperative Visual Intelligence: Real-Time Surgical Instrument Segmentation for Enhanced Surgical Monitoring

BACKGROUND

Open surgery relies heavily on the surgeon's visual acuity and spatial awareness to track instruments within a dynamic and often cluttered surgical field.

METHODS

This system utilizes a head-mounted depth camera to monitor surgical scenes, providing both image data and depth information. The video captured from this camera is scaled down, compressed using MPEG, and transmitted to a high-performance workstation via the RTSP (Real-Time Streaming Protocol), a reliable protocol designed for real-time media transmission. To segment surgical instruments, we utilize the enhanced U-Net with GridMask (EUGNet) for its proven effectiveness in surgical tool segmentation.

RESULTS

For rigorous validation, the system's performance reliability and accuracy are evaluated using prerecorded RGB-D surgical videos. This work demonstrates the potential of this system to improve situational awareness, surgical efficiency, and generate data-driven insights within the operating room. In a simulated surgical environment, the system achieves a high accuracy of 85.5% in identifying and segmenting surgical instruments. Furthermore, the wireless video transmission proves reliable with a latency of 200 ms, suitable for real-time processing.

CONCLUSIONS

These findings represent a promising step towards the development of assistive technologies with the potential to significantly enhance surgical practice.

Intraoperative Neurophysiologic Monitoring and Mapping in Children Undergoing Brainstem Surgery

SUMMARY

Intraoperative neurophysiologic monitoring during surgery for brainstem lesions is a challenge for intraoperative neurophysiologists and surgeons. The brainstem is a small structure packed with vital neuroanatomic networks of long and short pathways passing through the brainstem or originating from it. Many central pattern generators exist within the brainstem for breathing, swallowing, chewing, cardiovascular regulation, and eye movement. During surgery around the brainstem, these generators need to be preserved to maintain their function postoperatively. This short review presents neurophysiologic and neurosurgical experiences of brainstem surgery in children.

Frontiers of Cranial Base Surgery: Integrating Technique, Technology, and Teamwork for the Future of Neurosurgery

The landscape of cranial base surgery has undergone monumental transformations over the past several decades. This article serves as a comprehensive survey, detailing both the historical and current techniques and technologies that have propelled this field into an era of unprecedented capabilities and sophistication. In the prologue, we traverse the historical evolution from rudimentary interventions to the state-of-the-art neurosurgical methodologies that define today's practice. Subsequent sections delve into the anatomical complexities of the anterior, middle, and posterior cranial fossa, shedding light on the intricacies that dictate surgical approaches. In a section dedicated to advanced techniques and modalities, we explore cutting-edge evolutions in minimally invasive procedures, pituitary surgery, and cranial base reconstruction. Here, we highlight the seamless integration of endocrinology, biomaterial science, and engineering into neurosurgical craftsmanship. The article emphasizes the paradigm shift towards "Functionally" Guided Surgery facilitated by intraoperative neuromonitoring. We explore its historical origins, current technologies, and its invaluable role in tailoring surgical interventions across diverse pathologies. Additionally, the digital era's contributions to cranial base surgery are examined. This includes breakthroughs in endoscopic technology, robotics, augmented reality, and the potential of machine learning and AI-assisted diagnostic and surgical planning. The discussion extends to radiosurgery and radiotherapy, focusing on the harmonization of precision and efficacy through advanced modalities such as Gamma Knife and CyberKnife. The article also evaluates newer protocols that optimize tumor control while preserving neural structures. In acknowledging the holistic nature of cranial base surgery, we advocate for an interdisciplinary approach. The ecosystem of this surgical field is presented as an amalgamation of various medical disciplines, including neurology, radiology, oncology, and rehabilitation, and is further enriched by insights from patient narratives and quality-of-life metrics. The epilogue contemplates future challenges and opportunities, pinpointing potential breakthroughs in stem cell research, regenerative medicine, and genomic tailoring. Ultimately, the article reaffirms the ethos of continuous learning, global collaboration, and patient-first principles, projecting an optimistic trajectory for the field of cranial base surgery in the coming decade.

Long-Term Outcomes of the Electrically Unresponsive, Anatomically Intact Facial Nerve Following Vestibular Schwannoma Surgery

Objective The study aimed to determine long-term outcomes in patients with intraoperative electrical conduction block in an anatomically intact facial nerve (FN).

Methods Single center retrospective review of prospectively collected database of all vestibular schwannoma surgeries between January 1, 2008 and August 25, 2015. Operative notes were reviewed and patients with anatomically intact FNs, but complete conduction block at the end of surgery were included for analysis.

Results In total, 371 patients had vestibular schwannoma surgery of which 18 met inclusion criteria. Mean follow-up was 34.28 months and average tumor size was 28.00 mm. Seventeen patients had House-Brackmann Grade VI facial palsy immediately postoperatively and one patient was grade V. At 1 year, three patients remained grade VI (17%), two improved to grade V (11%), seven to grade IV (39%), six to grade III (33%), and one patient to grade II (6%). On extended follow-up, five patients (28%) had additional 1 to 2 score improvement in facial function. Subset analysis revealed no correlation of tumor size, vascularity, adherence to nerve, operative approach, extent of resection, splaying of FN, and recurrent tumor or sporadic tumors to the extent of FN recovery.

Conclusion Intraoperative conduction block does not condemn a patient to permanent FN palsy. There is potential for a degree of recovery comparable with those undergoing nerve grafting. Our data do not clearly support a policy of same-surgery or early-postoperative primary nerve grafting in the event of a complete conduction block, and instead we favor monitoring for recovery in an anatomically intact nerve.

Nerve Integrity Monitor Responses to Direct Facial Nerve Stimulation During Facial Nerve Decompression Surgery Can Predict Postoperative Outcomes

OBJECTIVE

To test whether the threshold of nerve integrity monitor (NIM) responses during facial nerve decompression surgery can predict the postoperative outcome.

STUDY DESIGN

Retrospective study.

SETTING

University hospital.

PATIENTS

Twenty peripheral facial palsy patients who underwent transmastoid decompression surgery.

INTERVENTION

During decompression surgery, thresholds of NIM responses were measured via direct facial nerve stimulation at three sites: the geniculate ganglion (GG), the second genu (2 G), and the stylomastoid foramen.

MAIN OUTCOME MEASURES

Facial nerve function was evaluated before and 6 months after surgery using the Yanagihara grading score (maximum score = 40 points). Complete recovery was defined as an improvement of the grading score to ≥ 36 points without synkinesis. Variables including age, sex, disease (Bell's palsy or Ramsay Hunt syndrome), time after onset, Yanagihara grading score, and electroneurography before surgery, and the thresholds of NIM responses during surgery were compared in the complete and incomplete recovery groups. NIM responders were defined as those exhibiting a NIM response of < 1.5 mA at any site. Postoperative Yanagihara grading scores in NIM responders and NIM nonresponders were compared.

RESULTS

No variables differed significantly in the complete and incomplete recovery groups before surgery. NIM response thresholds in the complete recovery group at the GG and the 2nd G were significantly lower than the corresponding thresholds in the incomplete recovery group. The postoperative Yanagihara grading scores of NIM responders were significantly better than those of NIM nonresponders.

CONCLUSION

NIM responses to intraoperative direct facial nerve stimulation were useful for predicting outcomes after decompression surgery.

Continuous dynamic mapping to avoid accidental injury of the facial nerve during surgery for large vestibular schwannomas

In vestibular schwannoma (VS) surgery postoperative facial nerve (CN VII) palsy is reducing quality of life. Recently, we have introduced a surgical suction device for continuous dynamic mapping to provide feedback during tumor resection without switching to a separate stimulation probe. The objective was to evaluate the reliability of this method to avoid CN VII injury. Continuous mapping for CN VII was performed in large VS (08/2014 to 11/2017) additionally to standard neurophysiological techniques. A surgical suction-and-mapping probe was used for surgical dissection and continuous monopolar stimulation. Stimulation was performed with 0.05-2 mA intensities (0.3 msec pulse duration, 2.0 Hz). Postoperative CNVII outcome was assessed by the House-Brackmann-Score (HBS) after 1 week and 3 months following surgery. Twenty patients with Koos III (n = 2; 10%) and Koos IV (n = 18; 90%) VS were included. Preoperative HBS was 1 in 19 patients and 2 in 1 patient. Dynamic mapping reliably indicated the facial nerve when resection was close to 5-10 mm. One week after surgery, 7 patients presented with worsening in HBS. At 3 months, 4 patients' facial weakness had resolved and 3 patients (15%) had an impairment of CN VII (HBS 3 and 4). Of the 3 patients, near-total removal was attempted in 2. The continuous dynamic mapping method using an electrified surgical suction device might be a valuable additional tool in surgery of large VS. It provides real-time feedback indicating the presence of the facial nerve within 5-10 mm depending on stimulation intensity and may help in avoiding accidental injury to the nerve.

Contemporary Opinions on Intraoperative Facial Nerve Monitoring

Objective

To examine the current trend in intraoperative facial nerve monitoring (IOFNM) training, performance, and reimbursement by subspecialists.

Study Design

Cross-sectional survey of the American Neurotology Society, American Otological Society, American Society of Pediatric Otolaryngology, and program directors of otolaryngology-head and neck surgery programs accredited by the Accreditation Council on Graduate Medical Education.

Setting

American Academy of Otolaryngology-Head and Neck Surgery Intraoperative Nerve Monitoring Task Force.

Subjects and Methods

The task force developed 2 surveys, which were implemented through Surveymonkey.com: (1) a 10-question survey sent to 1506 members of the societies listed to determine IOFNM practice and reimbursement patterns and (2) a 10-question survey sent to the 107 accredited US otolaryngology residency program directors to examine the state of resident training on facial nerve monitoring.

Results

Response rates were 18% for practicing physicians and 15% for residency program directors. The majority agreed that IOFNM was indicated for most otologic and neurotologic procedures. In addition to facial nerve monitoring, facial nerve stimulation was used in complex skull base and temporal bone procedures. When queried about reimbursement by Medicare, only 4.4% of surgeons responded that they received reimbursement. Program directors indicated universal exposure of residents to IOFNM, with 61% of programs giving residents formal training.

Conclusions

IOFNM is widely used among otologists and neurotologists in the United States. The majority of residents receive formal training, and all residents are exposed to the setup, use, monitoring, and troubleshooting of the device. Reimbursement for IOFNM is reported by a paucity of those surveyed.

Electrical stimulation-based nerve location prediction for cranial nerve VII localization in acoustic neuroma surgery

INTRODUCTION

Cranial nerve (CN) VII localization is a critical step during acoustic neuroma surgery because the nerve is generally hidden due to the tumor mass. The patient can suffer from Bell's palsy if the nerve is accidentally damaged during tumor removal. Surgeons localize CN VII by exploring the target area with a stimulus probe. Compound muscle action potentials (CMAPs) are elicited when the probe locates the nerve. However, false positives and false negatives are possible due to unpredictable tissue impedance in the operative area. Moreover, a single CMAP amplitude is not correlated with probe-to-nerve distance.

OBJECTIVES

This paper presents a new modality for nerve localization. The probe-to-nerve distance is predicted by the proposed nerve location prediction model.

METHODS

Input features are extracted from CMAP responses, tissue impedance, and stimulus current. The tissue impedance is calculated from the estimated resistance and capacitance of the tissue equivalent circuit. In this study, experiments were conducted in animals. A frog's sciatic nerve and gastrocnemius were used to represent CN VII and facial muscle in humans, respectively. Gelatin (2.8%) was used as a mock material to mimic an acoustic neuroma. The %NaCl applied to the mock material was used to emulate uncontrollable impedance of tissue in the operative area.

RESULTS

The 10-fold cross-validation results revealed an average prediction accuracy of 86.71% and an average predicted error of 0.76 mm compared with the measurement data.

CONCLUSION

The proposed nerve location prediction model could predict the probe-to-nerve distance across various impedances of the mock material.

Peripheral nerve function estimation by linear model of multi-CMAP responses for surgical intervention in acoustic neuroma surgery

Nerve function assessments are crucial for surgical intervention during acoustic neuroma surgery. Cranial nerves such as acoustic and facial nerves, can be possibly damaged during tumor dissection. Proper surgical intervention should prevent neurological deficit and achieve total tumor removal. Conventionally, nerve function is qualitatively evaluated by surgeon and neurologist. Facial nerves can be preserved by monitoring the compound muscle action potential (CMAP) response. The differences in the amplitude and latency of CMAP are used as indicators during surgical interventions. However, baseline CMAPs cannot be recorded in the presence of large acoustic tumors. This paper presents a new way of estimating nerve function. Instead of a single CMAP examination, multi‐CMAP responses are obtained from a train of varied stimulus intensities and these are applied a mathematical model. Shifts in the mathematical model parameters reflect changes in facial nerve function. In this study, experiments conducted in frog revealed that shifts in the linear model parameters were related to the level of induced nerve injury. Significant differences in the slope parameter of the linear model were found between each nerve condition. The identification of healthy and severed nerves via a support vector machine (SVM) corresponded to 94% accuracy. This classification criterion could be used with surgical intervention to prevent severed facial nerve palsy in acoustic neuroma surgery. The proposed method could be used to estimate nerve outcomes without prior information of a CMAP baseline.

Intraoperative neural monitoring in thyroid surgery: lessons learned from animal studies

Recurrent laryngeal nerve (RLN) injury remains a significant morbidity associated with thyroid and parathyroid surgery. In the past decade, surgeons have increasingly used intraoperative neural monitoring (IONM) as an adjunct technique for localizing and identifying the RLN, detecting RLN injury, and predicting the outcome of vocal cord function. In recent years, many animal studies have investigated common pitfalls and new applications of IONM. For example, the use of IONM technology in animal models has proven valuable in studies of the electrophysiology of RLN injury. The advent of animal studies has substantially improved understanding of IONM technology. Lessons learned from animal studies have immediate clinical applications in establishing reliable strategies for preventing intraoperative RLN injury. This article gives an overview of the research progress on IONM-relevant animal models.

Endotracheal Tube Electrodes to Assess Vocal Cord Motor Function During Surgery in the Cerebellopontine Angle

BACKGROUND

The 10th cranial nerve (CN X) is at risk during surgery in the lower cerebellopontine angle (CPA).

OBJECTIVE

To evaluate endotracheal surface electrodes for assessment of CN X motor function during CPA surgery.

METHODS

Twenty patients were enrolled. Electrophysiological recordings were analyzed and retrospectively correlated with clinical, imaging, and intraoperative data.

RESULTS

Recordings from endotracheal surface electrodes were reliable and eligible for analyses in 17 patients; in 3 patients, no surface electrode compound motor action potentials (CMAPs) could be obtained. Those patients with sufficient recordings underwent surgery in the CPA for tumors in 14 patients and for nontumor pathologies in 3 patients. In 12 patients, bipolar stimulation of motor rootlets in the CPA resulted in simultaneous CMAPs recorded from both surface electrodes and needle electrodes placed in the soft palate. Coactivation was particularly seen in patients with an intricate relationship between lower cranial nerves and tumor formations (n = 9/10). Amplitudes and latencies of vocal cord CMAPs showed high interindividual but low intraindividual variability. Parameters were not well correlated with the type of surgery (tumor vs nontumor surgery) and lower CN anatomy (displaced vs undisplaced). In 2 patients, vocal cord CMAPs were lost during tumor surgery, which was associated with postoperative dysphagia and hoarseness in 1 patient.

CONCLUSION

Endotracheal surface electrodes allow identification of vocal cord motor rootlets in the CPA. Worsening of CMAP parameters might indicate functional impairment. These aspects support the use of endotracheal surface electrodes in selected patients in whom the vagus nerve might be at risk during CPA surgery.

Intraoperative neurophysiological monitoring of microvascular decompression for glossopharyngeal neuralgia

PURPOSE

To evaluate if adding cranial nerves (CNs) V and VI to standard intraoperative neurophysiological monitoring (IONM) of microvascular decompressions for glossopharyngeal neuralgia improve its efficacy.

METHODS

We reviewed all patients who received a microvascular decompression for glossopharyngeal neuralgia at our institution between January 2008 and August 2012. All received upper extremity somatosensory evoked potentials, brainstem auditory evoked potentials, and free-running electromyography of muscles innervated by ipsilateral CNs VII, IX, and X. The sample was divided into 12 patients who received additional monitoring of CNs V and VI and 15 who did not.

RESULTS

No difference on neurotonic activity presence was found on CN V (standard IONM: 0% versus additional CNs IONM: 8.33%; p = 0.423), CN VI (never present on the additional CN patients), CN VII (standard IONM: 73.33% versus additional CNs IONM: 66.64%; p = 0.973), CN IX (standard IONM: 40.0% versus additional CNs IONM: 25.0%; p = 0.683), or CN X (standard IONM: 46.67% versus additional CNs IONM: 33.33%; p = 0.701) between groups. Additionally, no differences of brainstem auditory evoked potentials wave V's delay, and amplitude at the end of the decompression, or closing of the case were found between groups.

CONCLUSIONS

Monitoring free-running electromyography of additional CNs V and VI does not improve the efficacy of IONM of microvascular decompressions for glossopharyngeal neuralgia.

Surgery of the ear and the lateral skull base: pitfalls and complications

Surgery of the ear and the lateral skull base is a fascinating, yet challenging field in otorhinolaryngology. A thorough knowledge of the associated complications and pitfalls is indispensable for the surgeon, not only to provide the best possible care to his patients, but also to further improve his surgical skills. Following a summary about general aspects in pre-, intra-and postoperative care of patients with disorders of the ear/lateral skull base, this article covers the most common pitfalls and complications in stapes surgery, cochlear implantation and surgery of vestibular schwannomas and jugulotympanal paragangliomas. Based on these exemplary procedures, basic "dos and don'ts" of skull base surgery are explained, which the reader can easily transfer to other disorders. Special emphasis is laid on functional aspects, such as hearing, balance and facial nerve function. Furthermore, the topics of infection, bleeding, skull base defects, quality of life and indication for revision surgery are discussed. An open communication about complications and pitfalls in ear/lateral skull base surgery among surgeons is a prerequisite for the further advancement of this fascinating field in ENT surgery. This article is meant to be a contribution to this process.

Improved facial nerve outcomes using an evolving treatment method for large acoustic neuromas

OBJECTIVE

To describe a successful paradigm for the treatment of large acoustic neuromas (vestibular schwannomas).

STUDY DESIGN

Retrospective case review.

SETTING

Tertiary referral center.

PATIENTS

The charts of 2,875 acoustic neuroma patients at Michigan Ear Institute were reviewed to identify 153 patients who underwent surgical resection for large acoustic neuromas (>=3 cm) between 2000 and 2009.

INTERVENTION(S)

Staged surgical resection or single stage surgery with or without adjuvant stereotactic radiosurgery.

MAIN OUTCOME MEASURE(S)

Postoperative facial nerve outcomes are reported using the House-Brackmann (HB) facial nerve grading scale and compared with historical controls from a literature review. Rates of adverse outcomes are also reported.

RESULTS

Seventy-five patients underwent staged surgical resection of their tumors, whereas 78 patients underwent either single stage surgery or surgery with subsequent stereotactic radiosurgery. Eighty-one percent of patients in the staged surgical resection group had a postoperative HB Grade I or II facial nerve function compared with 75% in the single stage surgical group. Overall, 78% of patients in the current study had HB Grade I or II after treatment compared with a mean of 53% in the literature for similar sized tumors. Our methods including the decision to use staged surgery when necessary, dissection of tumor with stimulating dissector-directed intraoperative monitoring, and use of adjuvant stereotactic radiosurgery are described.

CONCLUSION

Using the described paradigm, large acoustic neuromas can be successfully treated with either staged or single-stage surgical resection with or without adjuvant radiosurgery to obtain more favorable facial nerve outcomes than historically reported controls while minimizing morbidity for the patient.

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.

Vagal Nerve Monitoring during Parapharyngeal Space Tumor Removal

The vagus nerve innervates the intrinsic and extrinsic laryngeal musculature as well as the complex pharyngeal plexus. Acute paralysis of this nerve results in dysfunctional speech, deglutition, and airway protection. These untoward effects, which lead to additional infectious and aerodigestive complications, may arise following manipulation of the vagus nerve during the surgical removal of a variety of neoplasms found in the parapharyngeal space.The vagal nerve has been intraoperatively monitored in an effort to maintain its anatomic and functional integrity. Bipolar hook-wire electrodes are introduced transcutaneously through the cricothyroid membrane and are guided into the vocalis muscle by an assistant performing direct laryngoscopy. Continuous, real-time monitoring of the vagal nerve is provided by audio and visual feedback to the operating surgeon. Potentially injurious stretching, heating, and compression of the nerve are easily detected, and monopolar stimulation of the nerve is used to map the nerve's course through the tumor bed.This presentation outlines our technique for vagal nerve monitoring in patients with tumors of the parapharyngeal space and intact preoperative vocal cord mobility. Selected cases are presented and illustrated through intraoperative and postoperative videotapes.

Anterior and Posterior Facial Nerve Rerouting: A Comparative Study

The infratemporal fossa type A (IFT-A) and the modified transcochlear approach are used to remove glomus jugular tumors type C and petroclival tumors extending to the prepontine cistern, respectively. Anterior rerouting is a key step in IFT-A, whereas the modified transcochlear approach involves posterior rerouting. The aim of this study was to analyze facial nerve function after application of the two procedures. One year after surgery, anterior rerouting was associated with excellent to good outcomes (grade I to III) in 94 % of the cases, while posterior rerouting was associated with good outcomes (grade III) in 70 % of the cases.

Facial nerve function after cerebellopontine angle surgery and prognostic value of intraoperative facial nerve monitoring: a critical evaluation

PURPOSE

Facial nerve monitoring is often used to predict postoperative facial function after acoustic neuroma tumor removal. In this study, three methods of predicting facial nerve function were compared. These methods used various parameters of the evoked electromyographic monitoring.

MATERIALS AND METHODS

Thirty-four patients who underwent surgery for acoustic neuroma were retrospectively reviewed. Amplitude of ongoing electromyographic activity, stimulation current thresholds, and the amplitude of evoked response were analyzed. The predictive value of the three methods was compared with actual postoperative facial nerve function.

RESULTS

One method predicted the final postoperative facial function in 90% of the patients, one method in 84%, and the final method failed to predict the final VIIth nerve function in patients with current stimulation thresholds greater than 0.05 mA.

CONCLUSION

Analysis of prognostic value showed that one of the three studied proved superior in predicting facial nerve function.

Intra-operative facial nerve monitoring. Its predictive value after skull base surgery

PURPOSE

Facial nerve monitoring can be used to predict post-operative facial function after skull base surgery. In this study three methods of prediction of facial function were compared. These methods utilize various parameters of the evoked electromyographic monitoring.

MATERIAL AND METHODS

Twenty-three patients who underwent surgery for skull base diseases were retrospectively reviewed. Amplitude of ongoing electromyographic activity, stimulation current thresholds and amplitude of evoked response were analysed. The predictive value of the three methods was correlated with post-operative facial nerve function.

RESULTS

The method that used only the stimulation thresholds predicted the final post-operative facial function in 86.9 per cent of the patients. The second employed a mathematical ratio which combined the amplitude of evoked response and the stimulation current thresholds and confirmed the prediction of the facial function in 91.3 per cent of the patients. The last method does not consider the stimulation thresholds greater than 0.05 mA and failed to predict the final VIIth nerve function in patients in whom the stimulation was greater than 0.05 mA.

CONCLUSION

Analysis of prognostic value demonstrates that the first two methods had the smaller degree of variation showing the better sensitivity.

Facial nerve injury in acoustic neuroma (vestibular schwannoma) surgery: etiology and prevention

Facial nerve injury associated with acoustic neuroma surgery has declined in incidence but remains a clinical concern. A retrospective analysis of 611 patients surgically treated for acoustic neuroma between 1973 and 1994 was undertaken to understand patterns of facial nerve injury more clearly and to identify factors that influence facial nerve outcome. Anatomical preservation of the facial nerve was achieved in 596 patients (97.5%). In the immediate postoperative period, 62.1% of patients displayed normal or near-normal facial nerve function (House-Brackmann Grade 1 or 2). This number rose to 85.3% of patients at 6 months after surgery and by 1 year, 89.7% of patients who had undergone acoustic neuroma surgery demonstrated normal or near-normal facial nerve function. The surgical approach appeared to have no effect on the incidence of facial nerve injury. Poor facial nerve outcome (House-Brackmann Grade 5 or 6) was seen in 1.58% of patients treated via the suboccipital approach and in 2.6% of patients treated via the translabyrinthine approach. When facial nerve outcome was examined with respect to tumor size, there clearly was an increased incidence of facial nerve palsy seen in the immediate postoperative period in cases of larger tumors: 60.8% of patients with tumors smaller than 2.5 cm had normal facial nerve function, whereas only 37.5% of patients with tumors larger than 4 cm had normal function. This difference was less pronounced, however, 6 months after surgery, when 92.1% of patients with tumors smaller than 2.5 cm had normal or near normal facial function, versus 75% of patients with tumors larger than 4 cm. The etiology of facial nerve injury is discussed with emphasis on the pathophysiology of facial nerve palsy. In addition, on the basis of the authors' experience with these complex tumors, techniques of preventing facial nerve injury are discussed.

Intraoperative Vagal Nerve Monitoring

A variety of benign and malignant neoplasms occur in the superior cervical neck, parapharyngeal space or the infratemporal fossa. The surgical resection of these lesions may result in postoperative iatrogenic injury to the vagus nerve with associated dysfunctional swallowing and airway protection. Anatomic and functional preservation of this critical cranial nerve will contribute to a favorable surgical outcome.

Fourteen patients with tumors of the cervical neck or adjacent skull base underwent intraoperative vagal nerve monitoring in an attempt to preserve neural integrity following tumor removal. Of the 11 patients with anatomically preserved vagal nerves in this group, seven patients had normal vocal cord mobility following surgery and all 11 patients demonstrated normal vocal cord movement by six months. In an earlier series of 23 patients with tumors in the same region who underwent tumor resection without vagal nerve monitoring, 18 patients had anatomically preserved vagal nerves. Within this group, five patients had normal vocal cord movement at one month and 13 patients demonstrated normal vocal cord movement at six months.

This paper will outline a technique for intraoperative vagal nerve monitoring utilizing transcricothyroid membrane placement of bipolar hook-wire electrodes in the vocalis muscle. Our results with the surgical treatment of cervical neck and lateral skull base tumors for patients with unmonitored and monitored vagal nerves will be outlined.

Facial nerve monitoring in skull base surgery

The purpose of this work was to compare pre- and post-operative facial nerve function between unmonitored and monitored cases of skull base lesions. The study involved 32 patients suffering from lateral skull base tumours (10 unmonitored and 22 monitored). Facial nerve function was monitored intraoperatively by an acoustic facial electromyographic system (NIM-2). Post-operative facial function was graded according to the House-Brackmann scale. In the group of monitored cases, facial nerve function was normal (Grade 1 or 2) in 79 per cent of the patients, while the unmonitored patients showed normal function in only 50 per cent of the cases. These results confirm previous observations that audible evoked electromyographic monitoring significantly reduces permanent facial nerve dysfunction.