Acoustic neuroma surgery: use of cochlear nerve action potential monitoring for hearing preservation

Monitoring Cochlear Nerve Action Potential for Hearing Preservation in Medium/Large Vestibular Schwannoma Surgery: Tips and Pitfalls

The diagnosis of large vestibular schwannomas (VS) with retained useful hearing has become increasingly common. Preservation of facial nerve (FN) function has improved using intraoperative EMG monitoring, hearing preservation remains challenging, with the recent use of cochlear nerve action potential (CNAP) monitoring. This prospective longitudinal series of VS with useful hearing operated on using a retrosigmoid approach included 37 patients with a mean largest extrameatal VS. diameter of 25 ± 8.7 mm (81% of Koos stage 4). CNAP was detected in 51% of patients, while auditory brainstem responses (ABR) were present in 22%. Patients were divided into two groups based on the initial intraoperative CNAP status, whether it was present or absent. FN function was preserved (grade I-II) in 95% of cases at 6 months. Serviceable hearing (class A + B) was preserved in 16% of the cases, while 27% retained hearing with intelligibility (class A-C). Hearing with intelligibility (class A-C) was preserved in 42% of cases when CNAP could be monitored in the early stages of VS resection versus 11% when it was initially absent. Changes in both the approach to the cochlear nerve and VS resection are mandatory in preserving CNAP and improve the rate of hearing preservation.

Auditory Brain Stem Response Predictors of Hearing Outcomes after Middle Fossa Resection of Vestibular Schwannomas

Objectives To analyze the relationship between preoperative and intraoperative auditory brain stem response (ABR) characteristics and hearing outcomes in patients with vestibular schwannomas (VS) undergoing hearing preservation (HP) surgery via a middle cranial fossa (MCF) approach.

Design Prospective study.

Setting Academic tertiary skull base referral center.

Methods Pre- and postoperative pure-tone average (PTA) and word recognition score (WRS) were examined. Intraoperative ABR wave III latency, wave V latency, and amplitude were recorded. HP was defined as postoperative WRS ≥50%.

Participants Adult patients with VS and WRS ≥50% who underwent MCF tumor resection between November 2017 and September 2019.

Main Outcome Measures Postoperative hearing outcomes.

Results Sixty patients were included. Mean tumor size was 9.2 mm (range, 3–17). HP rates were 56.7% for the cohort and 69.7% for tumors <10 mm. A complete loss of wave V was associated with an 82.9% increase in postoperative PTA (p < 0.001) and 97.2% decrease in WRS (p < 0.001), whereas a diminished wave V was correlated with 62.7% increase in PTA (p < 0.001) and 55.7% decrease in WRS (p = 0.006). A diminished or absent wave V, but not increased wave III/V latency or decreased wave V amplitude, was correlated with a decline in postoperative hearing class (r = 0.735, p < 0.001). Receiver-operating characteristic analysis demonstrated that a stable wave V has the highest accuracy in predicting HP (sensitivity of 82.6%, specificity of 84.8%).

Conclusion Of the examined preoperative and intraoperative ABR characteristics, a stable wave V intraoperatively was the strongest predictor of HP after MCF resection of VS.

Level of Evidence Level III.

Continuous intraoperative neuromonitoring (cIONM) in head and neck surgery-a review

Although the history of intraoperative neuromonitoring (IONM) dates back to the 19th century, the method did not evolve further than the mere differentiation of nerves until recently. Only the development of continuous IONM (cIONM) has allowed for non-stop analysis of excitation amplitude and latency during surgical procedures, which is nowadays integrated into the software of almost all commercially available neuromonitoring devices. The objective of cIONM is real-time monitoring of nerve status in order to recognize and prevent impending nerve injury and predict postoperative nerve function. Despite some drawbacks such as false-positive/negative alarms, technical artefacts, and rare adverse effects, cIONM remains a good instrument which is still under development. Active (acIONM) and passive (pcIONM) methods of cIONM are described in literature. The main fields of cIONM implementation are currently thyroid surgery (in which the vagal nerve is continuously stimulated) and surgery to the cerebellopontine angle (in which the facial nerve is either continuously stimulated or the discharge signal of the nerve is analyzed via pcIONM). In the latter surgery, continuous monitoring of the cochlear nerve is also established.

[Continuous intraoperative neuromonitoring (cIONM) in head and neck surgery-a review. German version]

Obwohl die Geschichte des intraoperativen Neuromonitorings (IONM) bereits in das 19. Jahrhundert zurückdatiert werden kann, hat sich diese Methode bis vor Kurzem nicht von der reinen Differenzierung des Nervs weiterentwickelt. Erst das kontinuierliche IONM (cIONM) ermöglichte die durchgehende Analyse der Reizamplituden und -latenzen, welche mittlerweile ebenfalls in die Software gängiger Monitoringsysteme integriert wurde. Zielsetzung des cIONM ist ein Real-Time-Monitoring des Nervenstatus während des Eingriffs, um so drohende Nervenverletzung erkennen und verhindern zu können und die postoperative Funktion des Nervs vorhersehbar zu erhalten. Trotz einiger Nachteile wie falsch-positiver oder -negativer Alarme, technischer Artefakte und seltener Nebenwirkungen bleibt das cIONM ein gutes Hilfsmittel, das noch weiterentwickelt wird. In der Literatur sind sowohl aktive (acIONM) als auch passive (pcIONM) Reiz- und Ableitmethoden des cIONM beschrieben. Derzeit gängige Anwendungsgebiete des cIONM umfassen die Schilddrüsenchirurgie mit der kontinuierlichen Stimulation des N. vagus sowie die Chirurgie des Kleinhirnbrückenwinkels (KHBW) mit dem Monitoring des N. facialis; hierbei werden neben kontinuierlicher Stimulation auch die Entladungsmuster des Nervs analysiert. Des Weiteren ist in die Chirurgie des KHBW das kontinuierliche Monitoring des Hörnervs etabliert.

Hearing Preservation in Vestibular Schwannoma Surgery

Modern imaging techniques allow early detection of small vestibular schwannomas (VSs) with minimal or no hearing impairment. While controversy surrounds the management of these tumors, given their benign nature and unpredictable natural history, microsurgical excision is the only modality that offers the opportunity to cure the tumor and preserve hearing. Hearing preservation in VS surgery may be accomplished via the middle fossa or retrosigmoid approaches. Appropriate patient selection and surgical approach is critical in achieving the best hearing outcomes. This article highlights the preoperative assessment, patient selection and prognostic factors, intraoperative monitoring of hearing, and surgical approaches to optimize hearing preservation during VS removal.

Cochlear Nerve Action Potential Monitoring for Preserving Function of an Unseen Cochlear Nerve in Vestibular Schwannoma Surgery

BACKGROUND

Intraoperative monitoring of cochlear nerve action potential (CNAP) has been used in patients with small vestibular schwannoma (<15 mm) to preserve cochlear nerve function. We performed surgery for a larger vestibular schwannoma under CNAP monitoring with the aim of preserving cochlear nerve function, and compared the data with findings from 10 patients with hemifacial spasm who underwent microvascular decompression surgery.

CASE DESCRIPTION

We report the case of a patient with a 26-mm vestibular schwannoma and normal hearing function who underwent neurosurgery under electrophysiological monitoring of the facial and cochlear nerves. Amplitudes of evoked facial muscle responses were maintained at approximately 70% during the operation. The latency of wave V on brainstem auditory evoked potential (BAEP) increased by 0.5 ms, and amplitude was maintained at approximately 70% of the value at the beginning of the operation. Latencies of P1, N1, and P2 on CNAP did not change intraoperatively. These latencies were comparable to those of 10 normal patients with hemifacial spasm. CNAP monitoring proved very useful in confirming the location of the cochlear nerve in the operative field and preserving cochlear nerve function. Both facial nerve function and hearing acuity were completely preserved after tumor removal, and wave V latency on BAEP returned to normal and was maintained in the normal range for at least 2 years.

CONCLUSIONS

CNAP monitoring is extremely useful for preserving the function of the unseen cochlear nerve during vestibular schwannoma surgery.

Preoperative characteristics of auditory brainstem response in acoustic neuroma with useful hearing: importance as a preliminary investigation for intraoperative monitoring

We classified the results of preoperative auditory brainstem response (ABR) in 121 patients with useful hearing and considered the utility of preoperative ABR as a preliminary assessment for intraoperative monitoring. Wave V was confirmed in 113 patients and was not confirmed in 8 patients. Intraoperative ABR could not detect wave V in these 8 patients. The 8 patients without wave V were classified into two groups (flat and wave I only), and the reason why wave V could not be detected may have differed between the groups. Because high-frequency hearing was impaired in flat patients, an alternative to click stimulation may be more effective. Monitoring cochlear nerve action potential (CNAP) may be useful because CNAP could be detected in 4 of 5 wave I only patients. Useful hearing was preserved after surgery in 1 patient in the flat group and 2 patients in wave I only group. Among patients with wave V, the mean interaural latency difference of wave V was 0.88 ms in Class A (n = 57) and 1.26 ms in Class B (n = 56). Because the latency of wave V is already prolonged before surgery, to estimate delay in wave V latency during surgery probably underestimates cochlear nerve damage. Recording intraoperative ABR is indispensable to avoid cochlear nerve damage and to provide information for surgical decisions. Confirming the condition of ABR before surgery helps to solve certain problems, such as choosing to monitor the interaural latency difference of wave V, CNAP, or alternative sound-evoked ABR.

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.

Preoperative, intraoperative, and postoperative auditory evaluation of patients with acoustic neuroma

This article is a concise clinical review of preoperative, intraoperative, and postoperative auditory evaluation of patients with acoustic neuroma. The author describes behavioral audiometry, auditory brainstem response, and otoacoustic emissions for preoperative evaluation; auditory brainstem and direct eighth-nerve intraoperative monitoring for intraoperative evaluation; and touches on postoperative auditory assessment.

Cochlear nerve action potential monitoring with the microdissector in vestibular schwannoma surgery

We developed a cochlear nerve action potential (CNAP) monitoring technique using a microdissector and compared the results of CNAP and auditory brainstem response (ABR) monitoring. Thirty-six patients underwent vestibular schwannoma resection via the retrosigmoid approach to preserve hearing. Both CNAP with the microdissector and surface ABR were recorded during the operation. We used the microdissector as an intracranial electrode for CNAP monitoring. The CNAP waveform was classified into four types: triphasic, biphasic, positive, and flat. At the completion of the tumor resection, the triphasic waveform was observed in 11 patients and the biphasic waveform was observed in 11 patients. Hearing function was preserved in all of them, although it was preserved in only two patients with other CNAP waveform types. The prognostic value of CNAP is significantly higher than that of ABR. We found that although CNAP with a microdissector does not provide real-time monitoring, with the classification of waveforms it can be used as predictable tool for postoperative hearing more accurately than ABR.

Acoustic neuroma surgery for preservation of hearing: technique and experience in the Chinese PLA General Hospital

CONCLUSIONS

Good hearing preservation can be achieved during acoustic neuroma surgery assisted by real-time dynamic auditory monitoring and ear endoscopic techniques. Preservation of the arachnoid and its blood supply are important for hearing preservation, and injury to the internal auditory artery is the most important cause of hearing loss.

OBJECTIVE

To explore techniques to preserve hearing during acoustic neuroma resection.

METHODS

This was a retrospective case review in a hospital setting. From July 2003 to July 2007, intraoperative auditory monitoring using auditory brainstem response (ABR) and electrocochleography (EcochG) was conducted in 18 of 138 patients undergoing surgery for acoustic neuroma who had preoperative hearing. The retrosigmoid approach was used for 16 complete resections and 2 patients underwent partial resections. Assisted endoscopic surgery was conducted for 10 ears. The main outcome measure was preservation of hearing, assessed using the classification method of the American Institute of Otolaryngology-Head and Neck Surgery.

RESULTS

Hearing was preserved in 11 of the 18 patients (61.1%): 2 of 5 patients whose tumors were larger than 20 mm (40%), and 9 of 13 patients with smaller tumors (69.2%). Among the 10 cases of ear endoscope-assisted surgery, hearing was preserved in 8 (80%). Intraoperative monitoring revealed that the waveform was influenced when the posterior labium of the internal acoustic meatus was ground and drilled, or when traction or electrocoagulation was performed near the opening of the internal acoustic meatus, especially when the internal auditory artery was clamped, the tumor in the internal acoustic meatus was treated, and the arachnoid vessels in the inner-most layer of the tumor surface were clamped or electrocoagulated.

Mapping clinical outcomes expectations to treatment decisions: an application to vestibular schwannoma management

OBJECTIVE

Complex medical decision making obligates tradeoff assessments among treatment outcomes expectations, but an accessible tool to perform the necessary analysis is conspicuously absent. We aimed to demonstrate methodology and feasibility of adapting conjoint analysis for mapping clinical outcomes expectations to treatment decisions in vestibular schwannoma (VS) management.

STUDY DESIGN

Prospective.

SETTINGS

Tertiary medical center and US-based otologists/neurotologists.

SUBJECTS

Treatment preference profiles among VS stakeholders-61 younger and 74 older prospective patients, 61 observation patients, and 60 surgeons-were assessed for the synthetic VS case scenario of a 10-mm tumor in association with useful hearing and normal facial function.

MAIN OUTCOME MEASURE

Treatment attribute utility.

METHODS

Conjoint analysis attribute levels were set in accordance to the results of a meta-analysis. Forty-five case series were disaggregated to formulate microsurgery facial nerve and hearing preservation outcomes expectations models. Attribute utilities were computed and mapped to the realistic treatment choices of translabyrinthine craniotomy, middle fossa craniotomy, and gamma knife radiosurgery.

RESULTS

Among the treatment attributes of likelihoods of causing deafness, temporary facial weakness for 2 months, and incurable cancer within 20 years, and recovery time, permanent deafness was less important to tumor surgeons, and temporary facial weakness was more important to tumor surgeons and observation patients (Wilcoxon rank-sum, p < 0.001). Inverse mapping of preference profiles to realistic treatment choices showed all study cohorts were inclined to choose gamma knife radiosurgery.

CONCLUSION

Mapping clinical outcomes expectations to treatment decisions for a synthetic clinical scenario revealed inhomogeneous drivers of choice selection among study cohorts. Medical decision engines that analyze personal preferences of outcomes expectations for VS and many other diseases may be developed to promote shared decision making among health care stakeholders and transparency in the informed consent process.

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.

Vestibular schwannoma: surgery or gamma knife radiosurgery? A prospective, nonrandomized study

OBJECTIVE

To conduct a prospective, open, nonrandomized study of treatment-associated morbidity in patients undergoing microsurgery or gamma knife radiosurgery (GKRS) for vestibular schwannomas.

METHODS

Ninety-one patients with vestibular schwannomas with a maximum tumor diameter of 25 mm in the cerebellopontine angle were treated according to a prospective protocol either by GKRS (63 patients) or open microsurgery (28 patients) using the suboccipital approach. Primary end points included hearing function, according to the Gardner-Robertson scale, and facial nerve function, according to the House-Brackmann scale at 2 years. Clinical data included a balance platform test, score for tinnitus and vertigo using a visual analog scale, and working ability. Patients responded to the quality-of-life questionnaires Short-Form 36 and Glasgow Benefit Inventory.

RESULTS

Three elderly GKRS patients withdrew; all remaining patients were followed for 2 years. Both primary end points were highly significant in favor of GKRS (P < 0.001). Evidence of reduced facial nerve function (House-Brackmann grade 2 or poorer) at 2 years was found in 13 of 28 open microsurgery patients and 1 of 60 GKRS patients. Thirteen of 28 patients who underwent surgery had serviceable hearing (Gardner-Robertson grade A or B) preoperatively, but none had serviceable hearing postoperatively. Twenty-five of 60 GKRS patients had serviceable hearing before treatment, and 17 (68%) of them had serviceable hearing 2 years after treatment. The tinnitus and vertigo visual analog scale score, as well as balance platform tests, did not change significantly after treatment, and working status did not differ between the groups at 2 years. Quality of life was significantly better in the GKRS group at 2 years, based on the Glasgow Benefit Inventory questionnaire. One GKRS patient required operative treatment within the 2-year study period.

CONCLUSION

This is the second prospective study to demonstrate better facial nerve and hearing outcomes from GKRS than from open surgery for small- and medium-sized vestibular schwannomas.

Waterjet dissection of the vestibulocochlear nerve: an experimental study

Object

Waterjet dissection has been shown to protect intracerebral vessels, but no experience exists in applying this modality to the cranial nerves. To evaluate its potential, the authors examined waterjet dissection of the vestibulocochlear nerve in rats.

Methods

Lateral suboccipital craniectomy and microsurgical preparation of the vestibulocochlear nerve were performed in 42 rats. Water pressures of 2–10 bar were applied, and the effect was microscopically evaluated. Auditory brainstem responses (ABRs) were used to define nerve function compared with preoperative values and the healthy contralateral side. The final anatomical preparation documented the morphological and histological effects of waterjet pressure on the nerve.

Results

In using up to 6 bar, the cochlear nerve was preserved in all cases. Eight bar moderately damaged the nerve surface. A 10-bar jet markedly damaged or even completely dissected the nerve. Time course analysis of the ABR demonstrated complete functional nerve preservation up to 6 bar after 6 weeks in all rats. Waterjet dissection with 8 bar was associated with a 60% recovery of ABR. In the 10-bar group, no recovery was seen.

Conclusions

Microsurgical dissection of cranial nerves is possible using waterjet dissection while preserving both morphology and function. The aforementioned jet pressures are known to be effective in neurosurgical treatment of tumors. Thus, waterjet dissection may be useful in skull base surgery including dissection of cranial nerves from tumors. Further studies on this subject are encouraged.

Cranial base approaches to inaccessible intracranial tumors

PURPOSE OF REVIEW

Craniotomy created through the base of the skull has improved exposure of many types of extraaxial tumors and thus enhanced both tumor control and preservation of neural function. The purpose of this article is to review recent advances in this emerging field.

RECENT FINDINGS

Use of microscopes and endoscopes has allowed these procedures to become progressively less invasive. Electrophysiological monitoring has enhanced neural identification and preservation. The increasingly documented efficacy of stereotactic radiation for certain tumor types (e.g. meningioma, schwannoma) has permitted nonoperative therapy for some individuals. In large tumors, selective use of less-than-complete microsurgical resection is establishing an increasing role, at times combined with focused radiotherapy of the surgical remnant. The role for transbasal craniotomy is well established in both benign tumors and vascular lesions, but has only limited applicability for high-grade malignant lesions. Today, the vast majority of procedures can be conducted in a single stage by a multidisciplinary team.

SUMMARY

Operative trajectories created through the cranial base, although technically demanding, have led to substantially improved outcomes for a wide variety of inaccessible intracranial lesions.

Intraoperative Cochlear Nerve Monitoring in Vestibular Schwannoma Surgery – Does It Really Affect Hearing Outcome?

OBJECTIVE

The objective of this study was to examine the usefulness of intraoperative cochlear nerve monitoring (ICNM) in the preservation of normal and social hearing in vestibular schwannoma (VS) removal.

METHODS

A retrospective chart review was conducted. Of 1315 patients operated for VS between June 1988 and December 2005, 150 patients were subjected to hearing preservation surgery. Among these, 99 patients with preoperative normal and social hearing (class A and B in the modified Sanna classification) and with a small tumor <1.5 cm in size were included in the analysis. The difference in hearing preservation rates between patients operated with and without ICNM was statistically examined using Fisher's exact test. An initial analysis was conducted for the total group. Patients were then divided into two subgroups according to the surgical approach (middle cranial fossa and retrosigmoid-retrolabyrinthine). The effectiveness of ICNM in each subgroup was analyzed.

RESULTS

The hearing preservation rate was 26.7% in cases operated with ICNM and 20.8% in cases without ICNM. The difference did not reach statistical significance (p = 0.79). In subgroup analyses, the ICNM did not prove to contribute to the significantly higher hearing preservation rate.

CONCLUSIONS

ICNM did not increase the ratio of patients with postoperative normal and social hearing in VS surgery.

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.

A Comparison of Direct Eighth Nerve Monitoring and Auditory Brainstem Response in Hearing Preservation Surgery for Vestibular Schwannoma

OBJECTIVE

The objective of this study was to compare the effectiveness of direct eighth nerve monitoring (DENM) and auditory brainstem response (ABR) in facilitating hearing preservation during vestibular schwannoma resection.

STUDY DESIGN

This was a retrospective study.

SETTING

: Tertiary referral center.

METHODS

We conducted a retrospective clinical study of the use of ABR and DENM during vestibular schwannoma removal. Tumors were removed through a retrosigmoid craniotomy. The rate of hearing preservation between the two monitoring modalities was compared. The additional outcome measures of facial nerve function and cerebral spinal fluid leak rate were also evaluated.

RESULTS

Hearing preservation was attempted in 77 patients with vestibular schwannomas. Tumor sizes ranged from 0.5 cm to 2.5 cm. Hearing was preserved in 71% of patients with tumors 1 cm or less and in 32% of patients with tumors between 1 and 2.5 cm when direct eighth nerve monitoring was used. Hearing preservation rates with ABR for tumors 1 cm or less were 41% and 10% in patients with tumors between 1 and 2.5 cm (p=0.03) Facial nerve preservations rates were 94% (House-Brackmann 1-2) for tumors less than 2 cm.

CONCLUSIONS

DENM provides significantly higher rates of hearing preservation during vestibular schwannoma resection when compared with ABR.

Monitoring auditif et chirurgie du neurinome de l’acoustique

Intraoperative auditory monitoring in acoustic neuroma surgery is used to improve residual hearing. Three techniques are available: monitoring of brainstem auditory evoked potentials, of electrocochleography or of direct eighth nerve compound action potential. The three techniques with their advantages and disadvantages are discussed. The current trend is to monitor brainstem auditory evoked potentials using digital filtering or to monitor the eighth nerve compound action potential.

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.

Hearing improvement after middle fossa resection of vestibular schwannoma

OBJECTIVE

The preoperative, intraoperative, and postoperative variables of patients experiencing hearing improvement after middle fossa resection of vestibular schwannomas were evaluated as potential prognostic indicators.

STUDY DESIGN

Retrospective case review with new objective postoperative data collected on patients with documented hearing improvement.

SETTING

California Ear Institute at Stanford and Stanford University Hospital, a tertiary referral center.

PATIENTS

The patient group consisted of 30 consecutive patients undergoing middle fossa approach to vestibular schwannoma between October 24, 1994, and November 11, 1998.

INTERVENTION

Hearing preservation surgery via the middle cranial fossa approach was performed on all patients.

MAIN OUTCOME MEASURES

Pure-tone averages (PTAs) and speech discrimination scores (SDS) were used to document hearing preoperatively and postoperatively. Preoperative electrophysiologic studies of auditory brainstem response, electronystagmography, electrical neuronography, transient evoked otoacoustic emissions, and distortion product otoacoustic emissions were evaluated. Intraoperative continuous nerve action potential and auditory brainstem response tracings were reviewed. Postoperative auditory brainstem response and transient evoked otoacoustic emissions were obtained when possible on patients whose hearing improved. Statistical analysis was completed using Student's t test and chi-square test.

RESULTS

Seventeen (57%) of 30 patients with tumors ranging from 2 to 31 mm maintained hearing postoperatively. Among patients with hearing preservation, 7 (41% or 23% of the overall group) exhibited hearing improvement (PTA 2 improved by > or =5 dB and/or SDS improved by > or =12%). Three of these 7 patients moved from nonfunctional (AAOHNS class C/D) to functional (AAOHNS class A/B) categories. All patients who enjoyed postoperative hearing improvement had preoperative absence or abnormality of ABRs. No patient with normal preoperative ABR experienced hearing improvement. Hearing improvement patients also had lower preoperative caloric function on electronystagmography compared with the entire group (p < 0.02) and were more likely to have superior vestibular nerve tumors. No differences were noted for electrical neurography and otoacoustic emissions.

CONCLUSIONS

Middle fossa resection of vestibular schwannoma offers patients the possibility of hearing improvement after treatment. The chance of hearing improvement is significantly higher than with other forms of treatment such as radiation therapy or translabyrinthine surgery. Although preoperative ABR abnormality may be an indicator of poor prognosis for hearing preservation, those patients who enjoy hearing improvement come from the group of patients with abnormal preoperative ABRs. Other factors identified as associated with hearing improvement include poor SDS with more normal PTA 2, and significantly decreased electronystagmographic caloric function (as an indicator of superior vestibular nerve tumors). Hearing improvement to the functional range after surgical resection is possible in some patients previously thought to be poor candidates for hearing preservation attempts. Hearing improvement may continue for many months after surgery.