Use of intraoperative auditory evoked potentials to preserve hearing in unilateral acoustic neuroma removal

Diagnostic accuracy of intraoperative brainstem auditory evoked potential for predicting hearing loss after vestibular schwannoma surgery

Objective

This meta-analysis evaluated the diagnostic value of intraoperative brainstem auditory evoked potential (BAEP) for predicting post-operative hearing loss.

Methods

Research articles in MEDLINE, Embase, and Cochrane Library databases were searched and selected up to 20 January 2022, and data were extracted following a standard procedure. A diagnostic accuracy test meta-analysis was performed using a mixed-effect binary regression model.

Results

A total of 693 patients from 15 studies were extracted. The change in intraoperative BAEP showed high sensitivity (0.95) but low specificity (0.37), with an area under the curve of 0.83. Diagnostic accuracy of the loss of potentials showed high sensitivity (0.82) and specificity (0.79). The area under the curve was 0.88. No factor was found to account for the heterogeneity of the results according to the meta-regression and subgroup analyses (all P-values > 0.05).

Conclusions

Our results showed that the loss of BAEP has meaningful value for predicting hearing loss after vestibular schwannoma surgery. The change in BAEP is also important for its high sensitivity during hearing preservation surgery.

Vagal schwannomas of the head and neck: A comprehensive review and a novel approach to preserving vocal cord innervation and function

BACKGROUND

Schwannomas, benign tumors arising from neurolemmocytes, are the most common type of peripheral nerve tumors. Extracranial schwannomas are most often found in the parapharyngeal space, commonly involving the vagus nerve to cervical sympathetic trunk. Vagal schwannomas present several unique clinical and therapeutic challenges.

METHODS

A comprehensive literature review was conducted on 197 articles reporting 235 cases of cervical vagal schwannomas. Presenting symptoms, treatment approach, and postoperative outcomes were recorded and analyzed.

RESULTS

Vagal schwannomas commonly present as asymptomatic neck masses. When they become symptomatic, surgical resection is the standard of care. Gross total resection is associated with higher postoperative morbidity compared to subtotal resection. Initial reports using intraoperative nerve monitoring have shown improved nerve preservation. Recurrence rates are low.

CONCLUSION

The combination of intermittent nerve mapping with novel continuous vagal nerve monitoring techniques may reduce postoperative morbidity and could represent the future standard of care for vagal schwannoma treatment.

Electrophysiology of Cranial Nerve Testing: Auditory Nerve

The electrocochleogram and brainstem auditory evoked potentials (BAEPs) are electrophysiologic signals used to assess the auditory nerve. The electrocohleogram includes the cochlear microphonic, the cochlear summating potential, and the eighth nerve compound action potential. It is used predominantly for hearing assessment and for diagnosis of Ménière disease and auditory neuropathy. Brainstem auditory evoked potentials are used for hearing assessment, diagnosis of dysfunction within the cochlea, the auditory nerve, and the brainstem auditory pathways up to the level of the mesencephalon, and intraoperative monitoring of these structures. The earliest BAEP component, wave I, and the eighth nerve compound action potential reflect the same process-the initial depolarization in the distal auditory nerve. Brainstem auditory evoked potential wave II receives contributions from the region of the cochlear nucleus and from the second depolarization in the distal auditory nerve. Wave III and later components are entirely generated rostral to the auditory nerve. Interpretation of BAEP studies is based on waves I, III, and V; auditory nerve dysfunction is manifested as prolongation of the I-III interpeak interval or absence of waves III and V. Eighth nerve tumors can cause a variety of BAEP abnormalities depending on which structures they affect. Adverse intraoperative BAEP changes can have many etiologies, including direct mechanical or thermal injury of tissue, ischemia (including cochlear ischemia or infarction due to compromise of the internal auditory artery), eighth nerve stretch, systemic or localized hypothermia, and artifactual BAEP changes due to technical factors.

Preliminary application of intra-operative hearing monitoring by tone pip ABR via loudspeakers

CONCLUSION

This method might be an effective intra-operative way to access hearing threshold under general anesthesia in the operating room.

OBJECTIVE

To develop an intra-operative hearing monitoring method for surgeons to access hearing threshold under general anesthesia in the operating room.

METHOD

System establishment and calibration was done firstly. Normal hearing (NH) and conductive hearing loss (CHL) participants were recruited. This study applied pure tone audiometry (PTA) to all subjects and tone pip ABR (tp-ABR) via loudspeakers in sound booth and operating room for NH and CHL subjects, respectively. For NH subjects, Bland Altman was conducted to compare 1k Hz PTA and tp-ABR threshold. For CHL participants, Paired t-test, Satterth waite t'-test, and linear correlation analysis were used to compare the two methods. p < .05 was considered statistically different.

RESULTS

(1) Bland Altman showed all plots were inside the 95% confidence interval, indicating that there was no difference between the two methods for NH subjects. (2) The two methods had a high correlation (Pearson's coefficient =0.872 > 0.6, p < .05) in assessing hearing threshold in the operating room. (3) Satterth waite t'-test showed the ossicular chain malfunction enlarged the deviation of threshold by the two methods (.01 < p < .025).

Real-time intraoperative monitoring of brainstem auditory evoked potentials during microvascular decompression for hemifacial spasm

OBJECTIVE The aim of this study was to define a new protocol for intraoperative monitoring (IOM) of brainstem auditory evoked potentials (BAEPs) during microvascular decompression (MVD) surgery to treat hemifacial spasm (HFS) and to evaluate the usefulness of this new protocol to prevent hearing impairment. METHODS To define the optimal stimulation rate, estimate the number of trials to be averaged, and identify useful warning criteria in IOM of BAEPs, the authors performed a preliminary study of 13 patients with HFS in 2010. They increased the stimulation rate from 10.1 Hz/sec to 100.1 Hz/sec by 10-Hz increments, and they elevated the average time from 100 times to 1000 times by 100-unit increments at a fixed stimulus rate of 43.9 Hz. After defining the optimal stimulation rate and the number of trials that needed to be averaged for IOM of BAEPs, they also identified the useful warning criteria for this protocol for MVD surgery. From January to December 2013, 254 patients with HFS underwent MVD surgery following the new IOM of BAEPs protocol. Pure-tone audiometry and speech discrimination scoring were performed before surgery and 1 week after surgery. To evaluate the usefulness of the new protocol, the authors compared the incidence of postoperative hearing impairment with the results from the group that underwent MVD surgery prior to the new protocol. RESULTS Through a preliminary study, the authors confirmed that it was possible to obtain a reliable wave when using a stimulation rate of 43.9 Hz/sec and averaging 400 trials. Only a Wave V amplitude loss > 50% was useful as a warning criterion when using the new protocol. A reliable BAEP could be obtained in approximately 9.1 seconds. When the new protocol was used, 2 patients (0.8%) showed no recovery of Wave V amplitude loss > 50%, and only 1 of those 2 patients (0.39%) ultimately had postoperative hearing impairment. When compared with the outcomes in the pre-protocol group, hearing impairment incidence decreased significantly among patients who underwent surgery with the new protocol (0.39% vs 4.02%, p = 0.002). There were no significant differences between the 2 surgery groups regarding other complications, including facial palsy, sixth cranial nerve palsy, and vocal cord palsy. CONCLUSIONS There was a significant decrease in postoperative hearing impairment after MVD for HFS when the new protocol for IOM of BAEPs was used. Real-time IOM of BAEPs, which can obtain a reliable BAEP in less than 10 seconds, is a successful new procedure for preventing hearing impairment during MVD surgery for HFS.

Electrophysiologic auditory tests

Auditory stimulation elicits a complex series of electric signals in the ear and nervous system which can be used for hearing assessment, audiologic and neurologic diagnosis, intraoperative monitoring, and neurophysiologic research. The earliest components comprise the electrocochleogram. The cochlear microphonic arises from receptor potentials from cochlear hair cells, and is used to assess hair cell function in patients with auditory neuropathy. The summating potential becomes larger in Ménière's disease. The eighth-nerve compound action potential is useful for objective audiometry. Brainstem auditory evoked potentials, a series of components generated in the eighth nerve and the brainstem auditory pathways, can be used for diagnostic assessment and intraoperative monitoring of the ears and of the auditory pathways up through the mesencephalon. They are relatively easy to record, highly consistent in normal subjects, and little unaffected by surgical anesthesia. Middle-latency and long-latency auditory evoked potentials are generated in multiple areas of cerebral cortex that are activated by auditory stimulation. Anesthetic effects limit their utility for intraoperative monitoring, and substantial intersubject variability limits their utility as a diagnostic test in individual patients, but they are important research tools for the study of memory processes and the way in which the brain analyzes auditory stimuli.

Superiority of tympanic ball electrodes over mastoid needle electrodes for intraoperative monitoring of hearing function

Object

Recording the auditory brainstem response (ABR) is a common method for monitoring the integrity of auditory pathways during surgery in the cerebellopontine angle. Electrocochleography (ECochG) is an alternative means of intraoperative neuromonitoring. In the present study the authors compared the practicability and prognostic significance of these two methods by performing simultaneous recordings in the operating room.

Methods

Between 2006 and 2011, 125 patients (mean age 55 years) underwent surgery in the cerebellopontine angle. Seventy-one percent of the patients presented with a hearing deficit, and 37% had useful hearing but with slight functional impairment. Auditory brainstem response was recorded with a subdermal needle electrode at the mastoid. For ECochG recording, a noninvasive ball electrode was attached to the tympanic membrane. Amplitudes obtained from both ECochG and ABR audiometry were compared and correlated to pre- and postoperative hearing deficits.

Results

Simultaneous intraoperative monitoring via ABR and ECochG was possible in 114 cases (91%). Postoperatively, 42% of patients showed some degree of new hearing deficit, whereas 4% had improvement. The mean amplitudes in ECochG monitoring were significantly higher (0.18 ± 0.04 μV) than the ABR potentials (0.08 ± 0.006 μV; p < 0.05). All waves recorded at the mastoid needle electrode could be recognized in the potentials of the tympanic ball electrode. Hearing outcome correlated more reliably with the relative amplitude changes in Waves III and V in ECochG (III: p = 0.0008, V: p = 0.0015) than in ABR monitoring (III: p = 0.2075, V: p = 0.0398).

Conclusions

Intraoperative monitoring of the auditory system by recording with noninvasive tympanic ball electrodes is more practicable than with subcutaneous needle electrodes at the tragus. Since there is also a reliable correlation between ECochG and clinical outcome, the method can replace common ABR recording during surgery in the cerebellopontine angle.

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.

Intraoperative cranial nerve monitoring during posterior skull base surgery

Intraoperative monitoring of neurophysiologic function is rapidly evolving as an important adjunct during skull base surgery to reduce the incidence of neurologic deficit. Facial nerve monitoring is an excellent model, since electrical and mechanical evoked potentials can be directly presented to the surgeon in real-time through an acoustic loudspeaker display. The lower cranial nerves may also be monitored using similar electromyographic techniques. Auditory system monitoring is more difficult due to the low amplitude response that requires averaging and filtering to extract the evoked potential. In conjunction with auditory monitoring, improved hearing preservation may be further enhanced by concomitant facial nerve monitoring, since the surgeon is alerted to traumatic manipulations that may affect both facial and cochlear nerves. Techniques and interpretative issues are presented to maximize the efficacy and safety of cranial nerve monitoring.

Changes in directly recorded cochlear nerve compound action potentials during acoustic tumor surgery

Damage to labyrinthine, neural, and vascular structures may possibly account for failure to achieve hearing preservation after removal of small acoustic neuromas. The pathogenesis of auditory impairment is, however, sometimes unclear. Intraoperative monitoring of auditory evoked potentials enables us to study possible correlations between surgical maneuvers and the function of auditory structures and pathways. To this end, simultaneous recording of brainstern auditory evoked potentials and direct cochlear nerve action potentials (CNAPs) was utilized in 14 patients with small (12 to 18 mm) unilateral acoustic neuromas. The latency of the first negative component (N(1)) of the CNAPs proved extremely sensitive in detecting intraoperative auditory damage and was a good predictive index of postoperative auditory outcome. Evaluation of temporal and morphologic CNAP patterns allowed identification of coagulation close to the cochlear nerve, drilling of the internal auditory canal, and removal of the intrameatal portion of the tumor, the most critical steps in hearing preservation. The mechanisms of damage are analyzed and discussed.

Hearing preservation after acoustic neuroma surgery

Acoustic neuromas (AN) are benign tumors that arise from the vestibular nerve within the internal auditory canal, where hearing loss is the most common symptom. This retrospective study was done to determine the results of hearing preservation in patients operated for AN at the University of Minnesota, as well as the factors affecting them. One hundred-eighty patients with AN were operated between 1988 and 1998, of whom 91 (50.5%) underwent hearing preservation surgery by either the middle fossa (MF) or the suboccipital (SO) approach. Preoperative and postoperative pure-tone averages (1, 2, and 4 K), speech discrimination scores (SDS), and acoustic reflex thresholds (ART) were noted and classified according to the Shelton's and the Gardner's classifications. The overall rate of hearing preservation was 23.1%. The outcome of hearing results was better with the MF approach compared with the SO approach. Small tumor size and better preoperative hearing levels favored a better postoperative hearing result. The rate of hearing improvement over time was better for the MF patients. Patients for whom intraoperative auditory monitoring was performed seemed to have better hearing outcomes.

Iatrogenic impairment of hearing during surgery for acoustic neuroma

Isolated or combined labyrinthine, neural, and vascular damage account for failure to preserve hearing during removal of acoustic neuromas. However, the specific mechanisms of auditory impairment remain unclear unless surgical maneuvers can be related to peri- and postoperative hearing on the basis of intraoperative monitoring of auditory function.Among the different auditory monitoring techniques, recording of cochlear nerve action potentials (CNAPs) from the intracranial portion of the nerve has proven particularly useful for identifying the mechanisms of iatrogenic auditory injury.The present investigation analyzes intra- and postoperative auditory impairment in relation to surgical steps in a group of 38 subjects with acoustic neuroma (size ranging from 5 to 24 mm) undergoing removal via a retrosigmoid approach.Coagulation close to the cochlear nerve, drilling of the internal auditory canal, and removal of the intrameatal portion of the acoustic neuroma have prove to be the most critical surgical steps in hearing preservation.Changes were correlated with intra- and extrameatal tumor size, the relationship between the internal auditory canal and vestibule, and internal auditory canal enlargement, anatomic involvement of the cochlear nerve, preoperative auditory level, and ABR and ENG test findings.Changes in CNAP morphology and latency are detailed, and mechanisms of injury are analyzed and discussed as a function of these variables.

Preservation of facial nerve function after resection of vestibular schwannoma

OBJECTIVE

Most data regarding facial nerve function in patients undergoing microsurgical resection of vestibular schwannomas predominantly include series performed at a single institution. In an effort to minimise individual surgeon or institutional bias, we performed an analysis of the published literature on facial nerve outcomes following microsurgical resection of vestibular schwannomas. The objective of this study was to provide a comprehensive assessment of reported outcomes for facial nerve preservation after VS surgery.

MATERIALS AND METHODS

We identified a total of 296 studies involving over 25,000 patients that included outcome data for facial nerve function of vestibular schwannoma patients treated surgically. Data regarding surgical approach, tumour size, patient age, and use of intra-operative monitoring were extracted and correlated with facial nerve function after surgery. Patients with preoperative facial nerve dysfunction (House-Brackmann score 3 or higher) were excluded and 'facial nerve preservation' was defined as grade I or II House-Brackmann function at last follow-up visit.

RESULTS

A total of 79 articles reporting on 11,873 patients met our inclusion criteria contributing to our analysis. Patients treated with the middle cranial fossa approach had a trend towards higher overall facial nerve preservation rate (85%), compared to the translabyrinthine approach (81%, p = 0.07) , and did statistically better than the retrosigmoid approach (78%, p < 0.0001). Patients with an average tumour size <20 mm had significantly improved facial nerve preservation rates, compared to larger tumours (90% vs. 67%, p < 0.0001). Patients under 65 years of age had a lower facial nerve preservation rate (71% vs. 84%, p < 0.001). Finally, the use of intra-operative monitoring improved the facial nerve preservation rate (76% vs. 71%, p < 0.001).

CONCLUSION

Factors that appear to be associated with facial nerve preservation after microsurgical resection of a vestibular schwannoma include tumour size <20 mm, use of the middle fossa approach and use of neuromonitoring during surgery. These data provide a summary assessment of the published literature regarding facial nerve preservation after microsurgical resection of vestibular schwannoma.

The patterns and risk factors of hearing loss following microvascular decompression for hemifacial spasm

OBJECTIVE

The aim of this study was to reveal the risk factors including intraoperative brain stem auditory evoked potential (BAEP) changes and to define parameter and warning values of BAEP beyond which the probability of hearing impairment rises significantly.

METHODS

From April 1997 to February 2009, 1156 patients underwent microvascular decompression (MVD) for hemifacial spasm (HFS) and their medical records and audiologic data. The intraoperative BAEP monitoring was performed in all operations during surgery from the time of administration of general anesthesia until the time of skin closure. Pure tone audiometry (PTA) and Speech Discrimination Score (SDS) were performed on all patients before and after surgery for categorizing the patterns of hearing loss. There were 825 females and 331 males with a mean age of 48.7 years (range 17-75 years). The mean symptom duration was 67.8 months (range 1-420 months).

RESULTS

At the 1-year follow-up examination, 1091 (94.4%) patients of the total 1156 patients exhibited a cured state, and 65 (5.6%) patients had residual spasms. Hearing loss occurred in 46 patients (3.9%). In 26 patients, PTA was decreased more than 15 dB with a proportional decrease of the SDS. In 10 patients, poor SDS without hearing loss occurred. Total deafness was developed in 10 patients. A higher incidence of BAEP change and a poor recovery especially amplitude in wave V during surgery was observed in patients with poor SDS (eight patients) and total deafness (seven patients) (p =  0.000). Reduction of amplitude more than 50% in wave V was a strong indicator for a worse outcome of the hearing capacity. The difference in other risk factors according to hearing loss pattern was not statistically significant (p  >  0.05). Only female was significant (p = 0.005).

CONCLUSIONS

The intraoperative BAEP change and a poorer recovery, especially reduction of amplitude more than 50% in wave V, was a strong indicator for a worse outcome of the hearing capacity. Vigilant intraoperative monitoring of the BAEP and adequate steps for recovery of the BAEP change could prevent hearing loss after MVD for HFS.

Diagnosis and therapy of vestibular schwannoma

MRI studies are of paramount importance for diagnosis and follow-up measurements during conservative and postinterventional management of vestibular schwannomas (VS). MRI findings that convey important information for hearing-preservation VS surgery are: length of tumor-cochlear nerve contact, involvement of the internal auditory canal, incomplete filling of internal auditory canal, tumor size less than 15 mm and the intralabyrinthine signal intensity on 3DFT-CISS gradient-echo images. Functional neuro-otologic studies of facial nerve function, hearing and vestibular/balance function provide a valuable means of assessment of the actual impairment of the functional status of the VS patient. Intraoperative monitoring of facial nerve function and hearing has been refined, resulting in improved final postoperative facial nerve and hearing outcomes in VS patients treated with microsurgery. Long-term results reported by teams practicing stereotactic radiosurgery or fractionated stereotactic radiotherapy have been very encouraging. On the other hand, conservative management appears to be a viable option for a select group of VS patients. The refinement of surgical technique has rendered surgery safer and less invasive, resulting in better functional outcomes. Steroid use is currently used postinterventionally to improve final hearing outcome, although with questionable effectiveness. Physical rehabilitation programs are applied to accelerate vestibular functional recovery postoperatively and there is weak evidence that early physical rehabilitation may improve the final facial nerve outcome. Quality-of-life measures have emerged as important determinants of final therapeutic decision-making. More studies with high levels of evidence are needed to support clinical decisions.

Hearing preservation and facial nerve function after microsurgery for intracanalicular vestibular schwannomas: comparison of middle fossa and retrosigmoid approaches

PURPOSE

Therapeutic options for vestibular schwannomas (VS) include microsurgery, stereotactic radiosurgery and conservative management. Early treatment of intracanalicular vestibular schwannomas (IVS) may be advisable because their spontaneous course will show hearing loss in most cases. Advanced microsurgical techniques and continuous intraoperative monitoring of cranial nerves may allow hearing preservation (HP) without facial nerve damage. However, there are still controversies about the definition of hearing preservation, and the best surgical approach that should be used.

METHODS

In this study, we reviewed the main data from the recent literature on IVS surgery and compared hearing, facial function and complication rates after the retrosigmoid (RS) and middle fossa (MF) approaches, respectively.

RESULTS

The results showed that the average HP rate after IVS surgery ranged from 58% (RS) to 62% (MF). HP varied widely depending on the audiometric criteria that were used for definition of serviceable hearing. There was a trend to show that the MF approach offered a better quality of postoperative hearing (not statistically significant), whereas the RS approach offered a better facial nerve preservation and fewer complications (not statistically significant).

CONCLUSIONS

We believe that the timing of treatment in the course of the disease and selection between radiosurgical versus microsurgical procedure are key issues in the management of IVS. Preservation of hearing and good facial nerve function in surgery for VS is a reasonable goal for many patients with intracanalicular tumors and serviceable hearing. Once open surgery has been decided, selection of the approach mainly depends on individual anatomical considerations and experience of the surgeon.

Management strategies for bilateral vestibular schwannomas

BACKGROUND

Bilateral vestibular schwannomas (VS) are rare. Most patients in India present late in the course of illness with large tumors and disabling deafness. Clinical presentation and management goals are different from that of unilateral VS.

AIMS

To highlight the differences in clinical presentations and surgical results of bilateral VS compared to unilateral VS; and, to propose a management strategy for these tumors with reference to tumor size, extent of growth and the presence or absence of hearing impairment.

METHOD

This is a retrospective study of 16 patients with bilateral VS treated over 10 years in a tertiary referral hospital. Assessment of VIIth and VIIIth cranial nerve function, tumor size, volume and extent of growth was performed in all patients. The management strategy was based on Samii's classification of tumor extent. All patients were operated using a retromastoid suboccipital approach. Postoperative results were analyzed and compared with those of unilateral VS.

RESULTS

The mean age of presentation was 25.7 years. Hearing impairment was the commonest symptom. Headache with features of raised intracranial pressure were present in 10 (62.5%) patients. Giant tumors were present in seven (43.7%) patients; large tumors in eight (50%) and a medium-sized tumor in one (6.3%). Total tumor resection was achieved in 13 patients and subtotal resection in two. One patient was managed conservatively and followed up with serial CT scans. On the contralateral side, one large tumor required total excision. One medium sized tumor underwent sub-capsular excision in an attempt to preserve hearing. The facial nerve was anatomically preserved in seven (46.7%) patients and in one, the cochlear nerve was anatomically preserved. There was no peri-operative mortality.

CONCLUSIONS

Patients with bilateral schwannomas are younger, have larger lesions, poorer preoperative hearing and are more likely to lose either auditory and/or facial nerve function during attempted total resection of the tumor. Classifying the tumors into two groups by extent, that is, tumors extending to the cerebellopontine angle cistern (T1-T3a) and, tumors extending to or compressing the brainstem (T3b to T4b), allows the surgical strategy to be defined.

Brainstem Auditory Evoked Potential Monitoring during Microvascular Decompression for Hemifacial Spasm: Intraoperative Brainstem Auditory Evoked Potential Changes and Warning Values to Prevent Hearing Loss—Prospective Study in a Consecutive Series of 84 Patients

OBJECTIVE

The nerve function of Cranial Nerve VIII is at risk during microvascular decompression for hemifacial spasm. Intraoperative monitoring of brainstem auditory evoked potentials (BAEPs) can be a useful tool to decrease the danger of hearing loss. The aim of this study was 1) to assess the side effects of surgery on hearing and describe the main intraoperative BAEP changes observed in the authors' series, and 2) to define warning values beyond which the probability of hearing impairment rises significantly. These values were calculated by correlating the (possible) postoperative hearing disturbances evaluated in terms of pure tone average with intraoperative BAEP changes (especially delay in Wave V latency).

METHODS

This series included 84 consecutive patients affected with hemifacial spasm who underwent microvascular decompression during which BAEPs were monitored. During surgery, Wave I, I to V interpeak interval, latency, and amplitude of Wave V were recorded and measured. Auditory function was studied before and after surgery and expressed as a pure tone average in all patients. Then, correlations were made between hearing impairment after surgery and intraoperative BAEP changes in an attempt to define warning values.

RESULTS

Seventy-four patients (88%) had no hearing loss after surgery (Group 1). Eight patients (9.5%) had hearing impairment with a decrease in pure tone average of more than 20 dB (Group 2). Two patients (2.3%) experienced a definitive and complete hearing loss on the side operated on (Group 3). Among intraoperative BAEP changes, latency of Peak V was the most frequently observed and the most significant phenomenon, especially during cerebellar retraction and the decompression step of the microvascular decompression procedure. In the group of patients without hearing loss (Group 1), the mean delay in latency of Peak V was 0.61 millisecond (standard deviation, ±0.36 ms); in the group with hearing decrease (Group 2), the mean delay was 1.05 milliseconds (standard deviation, ±0.64 ms); and in the group with deafness (Group 3), Wave V was abolished.

CONCLUSION

From a practical standpoint, three warning values, based on delay in latency of Peak V, were established for use during surgery: an initial one at 0.4 millisecond (“watching” signal) at the safety limit; a second one at 0.6 millisecond (risk “warning” signal), which is the mean value corresponding to the group of patients without postoperative hearing loss; and an ultimate one at 1 millisecond (“critical” warning), before irreversibility. These warnings should help the surgeon to avoid or correct maneuvers that are dangerous for hearing function, which is mandatory in functional surgery.

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.

Prognostic significance of changes in the internal acoustic meatus caused by vestibular schwannoma

The prognostic significance of the variety of changes in the internal auditory meatus (IAM) caused by vestibular schwannoma was retrospectively analyzed in 69 consecutive patients with vestibular schwannoma. Preoperative bone-window computed tomography was used to classify IAM changes into extensive destruction (17%), widening (46%), and normal IAM (36%). Extensive destruction (47 +/- 19 years) and widening (48 +/- 13) occurred in significantly younger patients than normal IAM (59 +/- 9). Preoperative hearing was significantly more severely disturbed in patients with extensive destruction than in those with widening or normal IAM. IAM change was significantly related to the tumor consistency, as normal IAM was more common in patients with cystic tumor than in those with solid tumor. Postoperative hearing and facial function were worse in patients with severe IAM change, although the relationship between the IAM change and the surgical result was not significant. One patient with extensive destruction developed postoperative cerebrospinal fluid (CSF) leakage through the air cells around the IAM, and needed surgical repair. Severe IAM change occurs with solid tumor and causes severely disturbed preoperative hearing in younger patients, which reflects the tumor aggressiveness. Severe IAM change increases the technical difficulty of tumor removal and the risk of postoperative CSF leakage, and is associated with a poorer prognosis for patients with vestibular schwannoma.

Mechanisms of intraoperative brainstem auditory evoked potential changes

Brainstem auditory evoked potential (BAEP) changes during intraoperative monitoring may reflect damage to or potentially reversible dysfunction of the ear, the eighth nerve, or the brainstem auditory pathways up to the level of the mesencephalon. They may also be caused by other physiologic mechanisms such as anesthesia, hypothermia, and acoustic masking from drilling noise, or they may result from technical factors that prevent proper stimulus delivery or recording of an evoked potential that is actually present. Cochlear ischemia or infarction resulting from compromise of the internal auditory artery and inner ear damage during temporal bone drilling will affect all BAEP components, including wave I. Direct mechanical or thermal trauma to the eighth nerve will delay, attenuate, and possibly eliminate waves III and V, but wave I, which is generated at the cochlear end of the eighth nerve, may be preserved. During scraping of tumor off the eighth nerve, force applied in an ear-toward-brainstem direction can avulse the fragile fibers of the distal eighth nerve at the area cribrosa. Prolonging the I-to-III interpeak interval during retraction of the cerebellum and brainstem reflects stretching of the eighth nerve, and is often reversible. Vasospasm within the eighth nerve can cause similar, potentially reversible BAEP changes. Damage to the brainstem auditory pathways at or below the level of the mesencephalon will delay and attenuate or eliminate wave V. Wave III is affected similarly if the damage is at or caudal to the region of the superior olivary complex. These BAEP changes may reflect direct mechanical or thermal damage to the brainstem, brainstem compression, or ischemia or infarction resulting from vascular compromise. During BAEP monitoring, examination of the pattern of BAEP changes, analysis of their correlation with surgical maneuvers, and investigation for possible contributory technical factors can help to determine the cause of the BAEP changes and provide the appropriate information to the rest of the surgical team.

Correlation between auditory function and internal auditory canal pressure in patients with vestibular schwannomas

OBJECT

Hearing loss is the most common presenting symptom in patients who harbor a vestibular schwannoma (VS). Although mechanical injury to the cochlear nerve and vascular compromise of the auditory apparatus have been proposed, the exact mechanism of this hearing loss remains unclear. To test whether pressure on the cochlear nerve from tumor growth in the internal auditory canal (IAC) is responsible for this clinical finding, the authors prospectively evaluated intracanalicular pressure (ICaP) in patients with VS and correlated this with preoperative brainstem response.

METHODS

In 40 consecutive patients undergoing a retrosigmoid-transmeatal approach for tumor excision, ICaP was measured by inserting a pressure microsensor into the IAC before any tumor manipulation. Pressure recordings were correlated with tumor size and preoperative auditory evoked potential (AEP) recordings. The ICaP, which varied widely among patients (range 0-45 mm Hg), was significantly elevated in most patients (median 16 mm Hg). Although these pressure measurements directly correlated to the extension of tumor into the IAC (p = 0.001), they did not correlate to total tumor size (p = 0.2). In 20 patients in whom baseline AEP recordings were available, the ICaP directly correlated to wave V latency (p = 0.0001), suggesting that pressure from tumor growth in the IAC may be responsible for hearing loss in these patients.

CONCLUSIONS

Tumor growth into the IAC results in elevation of ICaP and may play a role in hearing loss in patients with VS. The relevance of these findings to the surgical treatment of these tumors is discussed.

Retrosigmoid approach to acoustic neuroma (vestibular schwannoma)

The retrosigmoid approach for the microsurgical removal of an acoustic neuroma (vestibular schwannoma) is described, and perioperative medical management of the patient is discussed. The techniques for monitoring facial and cochlear nerve function are presented. The supine-oblique position, skin incision, bone removal, dural opening, and initial exposure are outlined. Important points in the technique for removing acoustic neuromas and preserving hearing, when possible, are described and illustrated.

Current surgical results of retrosigmoid approach in extralarge vestibular schwannomas

BACKGROUND

Vestibular schwannomas (VS) are common tumors that can be cured; they are reported to comprise 6 approximately 8% of all intracranial tumors. The primary objective in the surgery of extralarge VS is total removal of the tumor mass while preserving the facial nerve. In extralarge tumors, complete excision of the tumor carries a significant risk of injuring the facial nerve and adjacent vital brain structures. The authors are reporting the techniques and results of operation on extralarge VS.

METHODS

The material consisted of 30 patients during the last 6 years with surgically treated VS that had a maximal extrameatal diameter exceeding 4 cm. Suboccipital craniotomy and tumor removal was performed with patients in the lateral position. Results and complications of the surgical technique will be reviewed.

RESULTS

Average age of patients was 45.2 years; there was a slight female predominance (1.5:1). Size of the mass ranged from 41 to 70 mm; all were removed by the retrosigmoid transmeatal approach. Peritumoral edema on MRI was seen in 50% (15/30). Total removal was achieved in 73.3% (22/30) with no significant relationship to peritumoral edema. In the cases of total removal, the facial nerve preservation rate was 86.4% (19/22). There was no mortality. Surgical complications were hemorrhage and CSF leakage in 1 case (3%) and 8 cases (26.7%), respectively, but in most of these cases, conservative treatment was adequate. In patients in whom anatomic preservation of the facial nerve was achieved, facial nerve function improved progressively within a year. In all cases except for one with gait disturbance, a good outcome was achieved.

CONCLUSIONS

Our surgical techniques, including the prediction of facial nerve displacement, not using retractors, and replacement of bone, contributed to good surgical results in a series of extralarge VS.

Selection of surgical approaches for small acoustic neurinomas

BACKGROUND

The purpose of this study was to evaluate the results of surgery for small acoustic neurinomas at our institute via the middle cranial fossa and retrosigmoid approaches, and to determine the indications for each approach.

METHODS

Fifty-three patients with unilateral tumors less than 2 cm in diameter were studied. Surgery was performed via the middle cranial fossa approach in 36 tumors and via the retrosigmoid approach in 17 tumors.

RESULTS

The hearing preservation rate was 68% (36/53) in all patients, 93% (14/15) in patients with intracanalicular tumors, 79% (15/19) in patients with tumors less than 1 cm in diameter, and 43% (7/19) in patients with tumors between 1 and 2 cm in diameter. The facial nerve function was excellent or good in 80% (42/53), 74% (11/15), 84% (16/19), and 78% (15/19), respectively. Among the 19 patients with tumors between 1 and 2 cm in diameter, the frequencies of hearing preservation and of excellent or good facial nerve function (47% and 87%, respectively) in the 15 patients approached via the retrosigmoid approach were higher than those (0% and 50%, respectively) in the four patients approached via the middle cranial fossa approach.

CONCLUSIONS

We conclude that tumors smaller than 2 cm should be removed because preservation of hearing as well as facial nerve function may be possible in almost all of these patients. Tumors larger than 1 cm should be surgically treated through the retrosigmoid approach.

The prognostic value of intraoperative BAEP patterns in acoustic neurinoma surgery

Based on a consecutive series of 70 hearing patients with unilateral acoustic neurinomas and intraoperative monitoring of brain-stem auditory evoked potentials (BAEP), 4 dynamic BAEP patterns could be characterized. These patterns correspond with early and late postoperative hearing outcome. All patients with stable wave V (pattern 1) showed definite hearing preservation, all patients with irreversible abrupt loss of BAEP (pattern 2) lost their hearing, despite early hearing preservation in two cases. All patients with irreversible progressive loss of either wave I or wave V (pattern 3) eventually suffered from definite postoperative hearing loss, despite early hearing preservation in two cases. Those cases with intraoperative reversible loss of BAEP (pattern 4) showed variable short and long term hearing outcome. In 34% hearing was preserved, 44% suffered from postoperative hearing loss, the remaining 22% showed postoperative hearing fluctuation, either as a delayed hearing loss or as reversible hearing loss. Postoperative hearing fluctuation indicates anatomical and functional preservation of the cochlear nerve during surgery and is suggestive of a pathophysiological mechanism initiated during the surgical procedure and continuing thereafter. Patients at risk for delayed hearing loss can be identified during surgery by a characteristic BAEP pattern and may benefit from vasoactive treatment.

Standardized format for depicting hearing preservation results in the management of acoustic neuroma

The Committee on Hearing and Equilibrium of the American Academy of Otolaryngology-Head and Neck Surgery recently published guidelines for reporting hearing preservation in the treatment of acoustic neuromas. These suggestions included pretreatment and posttreatment pure-tone hearing thresholds, word recognition scores, and hearing classification. We present a standardized reporting format that addresses the Committee's recommendations and displays individual patient audiologic data as a simple, concise plot of posttreatment hearing results. To illustrate the use of the recommended format, preoperative and postoperative hearing data from our institution are reported. Such reporting criteria will facilitate comparative reviews of studies of hearing preservation after surgical or radiotherapeutic management of acoustic neuromas, while providing specific data for individual patient outcome analysis.

Intraoperative monitoring during surgery for acoustic neuroma: benefits of an extratympanic intrameatal electrode

OBJECTIVES

To assess the utility of an extratympanic intrameatal electrode for intraoperative monitoring during acoustic neuroma and other cerebellopontine angle tumour surgery and to define the neurophysiological and surgical factors which influence hearing preservation.

METHODS

Twenty two patients, 18 with acoustic neuromas and four with other cerebellopontine angle tumours, underwent intraoperative monitoring during tumour excision. The extratympanic intrameatal electrode (IME) was used to record the electrocochleogram (ECoG) and surface electrodes to record the brainstem auditory evoked response (ABR).

RESULTS

The compound action potential (CAP) of the ECoG was two and a half times greater in amplitude than wave I of the ABR and was easily monitored. Virtually instant information was available as minimal averaging was required. Continuous monitoring was possible from the commencement of anaesthesia to skin closure. The IME was easy to place, non-invasive, and did not interfere with the operative field. Operative procedures which affected CAP or wave V latency or amplitude were drilling around the internal auditory meatus, tumour dissection, nerve section, and brainstem and cerebellar retraction. Hearing was achieved in 59% of patients.

CONCLUSIONS

The IME had significant benefits in comparison with other methods of monitoring. The technique provided information beneficial to preservation of hearing.

Facial nerve electromyography and other cranial nerve monitoring

Cranial nerves can be damaged during a variety of surgical procedures. The function of the cranial nerves can be monitored during surgery with electromyography (EMG), compound nerve and muscle action potentials (MAP), and auditory evoked potentials (AEP). We report important technical considerations that allow reliable recordings of these modalities during surgery, the criteria for determining a significant change in these electrophysiologic measures of function, the application of these techniques to specific surgical procedures, and the data that support the utility of cranial nerve monitoring.

Neurotopographic considerations in the microsurgical treatment of small acoustic neurinomas

OBJECT

The authors studied the relationships between tumor size, location, and topographic position relative to the intact facial nerve bundles in acoustic neurinomas to determine the influence of these factors on hearing preservation postoperatively. Consistent topographic relationships were found.

METHODS

Four hundred fifty-two patients with acoustic neurinoma treated via a retrosigmoid approach were analyzed with respect to hearing preservation and facial nerve function. One hundred fifteen tumors were identified as small and were categorized as Grades I and II. Patients with Grade I tumors, that is, purely intracanalicular lesions, all had good hearing preoperatively, defined by a less than 50-dB pure tone average and 50% speech discrimination score. All 14 Grade I tumors were removed, resulting in preservation of the patient's hearing by these criteria. There were no particular topographic anatomical relationships associated with these tumors that affected hearing preservation. Grade II tumors, defined as those protruding into the cerebellopontine angle without contacting the brainstem, were found in 101 patients and were divided by size into two grades: IIA (< 1 cm) and IIB (1-1.8 cm). In 90 patients with Grade IIA tumors, 72 (89%) of 81 who had preserved hearing preoperatively maintained it postoperatively, and in the 11 patients with Grade IIB tumors, six of whom had good hearing preoperatively, four (67%) had preserved hearing postoperatively. Six morphological types were identified based on their neurotopographic relationships to the elements of the vestibulocochlear nerve.

CONCLUSIONS

Hearing preservation postsurgery by tumor type was as follows: 1A, 92%; 1B, 88%; 1C, 100%; 2A, 83%; 2B, 92%; and 3, 57%. Combined, this represents a hearing preservation rate of 87% after surgical treatment of Grade II acoustic neurinomas. Full nerve function was maintained in 88% of patients with anatomically preserved facial nerves in both Grade I and II tumors. The remaining 12% of patients retained partial function of the facial nerve. Two patients in the series lost anatomical integrity of the nerve due to surgery.

ECochG, CNAP and ABR monitoring during vestibular Schwannoma surgery

Identification of the specific pathophysiological processes and correlation with post-operative hearing are the prerequisites for utilizing electrophysiological audio monitoring techniques in preventing damage to auditory structures during vestibular Schwannoma (VS) surgery. The present paper compares the value of auditory brainstem responses (ABRs), electrocochleography (ECochG) and directly recorded cochlear nerve action potentials (CNAPs) in detecting damage to auditory structures during VS surgery and predicting post-operative hearing. Eighteen consecutive patients operated on for VS, in an attempt at hearing preservation, participated in the investigation. The ipsilateral hearing level (pure tone average [PTA] 0.5-3 kHz) ranged from 10 to 50 dB HL (mean: 30.7 dB HL), with a speech discrimination score equal to or better than 50 per cent. CNAPs furnished the highest predictive score for post-operative hearing. In particular, when a permanent loss of CNAPs occurred the sensitivity and specificity were 100 per cent. The discrepancies between the ECochG and CNAP findings were attributable to high prevalence of cochlear nerve damage, capable of 'disconnecting' the ear from the central auditory pathways, causing persistence of peripheral auditory function and no propagation of the neural input. ABR monitoring was highly sensitive in detecting auditory damage but its prognostic utility was marred by its poor specificity.

Direct brainstem recording of auditory evoked potentials during vestibular schwannoma resection: nuclear BAEP recording. Technical note and preliminary results

The usefulness of intraoperative monitoring in cerebellopontine angle surgery should be improved by obtaining faster and stronger brainstem auditory evoked potential (BAEP) responses. A new technique of direct recording at the brainstem has been developed, which is applicable to all tumor sizes. By placing a retractor with electrodes attached to its tip at the cerebellomedullary junction, the authors have recorded BAEP amplitudes that are 10 times greater than those recorded using the conventional technique. Only small sampling numbers (64-256 recordings) are required and are obtained in 5 to 15 seconds. The technique has been applied successfully in 34 patients who underwent vestibular schwannoma resections. It has also been tested in patients with intrameatal-extrameatal meningiomas and in those with vascular compressive disorders; there have been no false results. The advantages of this new technique are: 1) identification of BAEP components is easier and faster; 2) reliable BAEP responses are obtained in some cases in which conventional BAEP responses are lost or severely deformed; and 3) BAEP response deterioration and improvement are recognized earlier than would occur using the conventional technique. This last advantage provides the surgeon with a useful warning at a stage of surgery at which BAEP changes are still temporary and can be reversed. This method is different from other trials of intradural BAEP recordings in three respects: its use is not limited to particular tumor sizes; there is no interference with the surgical process; and, most important, the obtained responses correlate well with those of conventional BAEP responses, probably because the recording site is in the vicinity of the anterior cochlear nucleus. In conclusion, the chances of useful monitoring feedback with adequate adaptation of the microsurgical strategy are improved considerably.

Management of vestibular schwannomas (acoustic neuromas): auditory and facial nerve function after resection of 120 vestibular schwannomas in patients with neurofibromatosis 2

OBJECTIVE

Vestibular schwannomas (VSs) affect young patients with Neurofibromatosis 2 (NF-2) and cause very serious problems for hearing, facial expression, and brain stem function. Our objective was to determine a therapy concept for the right timing and indication of neurosurgical therapy.

METHODS

In 1000 consecutive VS resections, 120 tumors in 82 patients with NF-2 were surgically treated by the same surgeon (MS) at the Department of Neurosurgery at Nordstadt Hospital from 1978 to 1993. The mean age of the patients was 27.5 years. Sixty tumors were surgically treated in 41 male patients, and 60 tumors were surgically treated in 41 female patients. Bilateral tumor resection was performed in 38 patients (76 operations, after previous partial surgery in 15 cases elsewhere), and unilateral operations were performed in 44 patients, 5 of whom had undergone ipsi- or contralateral surgery that was performed elsewhere. The operative and clinical findings are evaluated and compared with the data of patients without NF-2.

RESULTS

In 105 cases, complete tumor resections were achieved. In 15 cases, deliberate subtotal resections were performed. These were for brain stem decompression in 4 cases and for hearing preservation in the last hearing ear in 11 cases, with successful preservation in 8 of the 11. Pre- and postoperative hearing rates were higher in male than in female patients (70% in male versus 65% in female patients before surgery and 40.5 versus 31%, respectively, after surgery). Hearing was preserved in 29 of 81 ears (36%). The rate of preservation was 24% in cases of large tumors and 57% in cases of small tumors (<30 mm). Twenty-one of 82 patients (26%) were bilaterally deaf before surgery. Twenty-five patients had uni- or bilateral hearing after surgery (i.e., 41 % of those with preoperative hearing or 30.5% of the whole group). Anatomic facial nerve preservation was achieved in 85%. The facial nerve was reconstructed intracranially at the cerebellopontine angle by sural grafting in 17 cases and by hypoglossal-facial reanimation in 5. Two deaths occurred 1 and 3 months postsurgically as a result of malignant tumor growth with brain stem dysfunction and respiratory problems. In summary, for patients with NF-2, the presentation ages are lower, tumor progression is faster, the chances of anatomic and functional nerve preservation are lower, the chances of good outcomes are best when surgery is performed early and when there is good preoperative hearing function, and the danger of sudden hearing loss is higher. The chances and danger often differ from side to side among individual patients.

CONCLUSION

The indication and the timing of tumor resections are in some respects different from normal VS handling and are dependent on the tumor extension and related necessity of brain stem decompression and on the auditory function. As an optimal goal, completeness of resection with functional cochlear nerve preservation is formulated, and as an acceptable compromise, subtotal microsurgical resection with functional cochlear nerve preservation in the last hearing ear is suggested.

Management of 1000 vestibular schwannomas (acoustic neuromas): hearing function in 1000 tumor resections

OBJECTIVE

The realistic chances of hearing preservation and the comparability of international results on hearing preservation in complete microsurgical vestibular schwannoma resections were the focus of this study in a large patient population treated by uniform principles.

METHODS

One thousand vestibular schwannomas were operated on at Nordstadt Neurosurgical Department, from 1978 to 1993, by the senior surgeon (MS). There were 1000 tumors in 962 patients, i.e., 880 patients with unilateral tumors and 82 patients operated on for bilateral tumors in neurofibromatosis-2 (120 cases). Preservation of the cochlear nerve was attempted whenever possible. The audiometric data were analyzed by the Nordstadt classification system and graded in steps of 30 dB by audiometry and in steps of 10 to 30% by speech discrimination; for comparability, the data were also evaluated by the criteria of Gardner, Shelton, and House, and they were assessed in relation to the Hannover tumor extension grading system.

RESULTS

Anatomic cochlear nerve preservation was achieved in 682 of 1000 cases (68%), as well as in some preoperatively deaf patients, a very few of whom regained some hearing. Of a total of 732 cases with some preoperative hearing, anatomic cochlear nerve preservation was achieved in 580 cases (79%) and functional cochlear nerve preservation in 289 (39.5%); analysis over time revealed an actual preservation rate of 47% in the most recent 200 cases. Specific factors, such as gender, tumor extension, preoperative hearing quality, and symptom duration, were investigated for their predictive value for hearing preservation. Male gender, small to medium tumor size (mainly extending within the cerebellopontine cistern; Classes T2 and T3), good to moderate hearing (up to 40-dB loss), and short duration of hypoacusis (< 1.5 yr) or of vestibular disturbances (< 0.7 yr) were advantageous factors, with chances of hearing preservation between 47 and 88%.

CONCLUSION

Functional cochlear nerve preservation in complete microsurgical resection should belong to the contemporary standard of treatment goals.

Hearing preservation in vestibular schwannoma surgery: what factors influence outcome?

The goals in the management of patients with vestibular schwannomas have changed drastically over the past few decades, with preservation of useful hearing representing the newest challenge. The true incidence of preserved useful hearing, however, has become clouded by a lack of uniformity in reporting results. The authors have analyzed 56 consecutive cases, in which directed attempts were made to preserve hearing on the involved side, to understand what factors play a major role in postoperative hearing preservation. Of the 56 cases reviewed, there were 46 patients who had "good" preoperative hearing (pure tone average < 50 dB; speech discrimination score > 50%). We found that, in this group of patients, if the tumor was less than 2 cm in diameter from pons to petrous, there was a 52% (16 of 31 patients) chance of preserving good hearing and if the tumor was 1 cm or less, the chances increased to 83%. Factors such as preoperative hearing status, tumor size and location, tumor consistency, and preoperative and intraoperative brainstem auditory evoked potentials are discussed in detail as they relate to postoperative outcome.

Physiological identification of the auditory nerve during surgery for acoustic neuroma

We report the design and clinical use of an electrode which can locate the acoustic nerve fibres in the normal eighth nerve and also in eighth nerves deformed by acoustic neuromas. The improvement in facial nerve preservation during acoustic neuroma surgery is partly due to the use of a facial nerve stimulator to anatomically locate the fibres. Our new acoustic nerve detector has the capability of anatomical location of cochlear fibres which may help to improve hearing preservation in selected cases of acoustic neuroma. The device functions by detecting the compound action potential evoked by no frequency auditory simulation at 500 Hz. The 500 Hz compound action potential is detected with a bipolar probe and then amplified and filtered. This results in a 500 Hz tone when the probe contacts the auditory nerve. Detection is virtually instantaneous. The acoustic nerve detector (AND) is demonstrated in a normal eighth nerve complex and its use is then described in the total removal of an acoustic neuroma with a 1 cm extracanalicular extension in which useful hearing was saved post-operatively. The present prototype may not be sensitive enough to detect the very low signals that may result when cochlear fibres are widely distorted around a large tumour or in cases where slight contusion of the nerve occurs during dissection. In all other cases the real time anatomical information is extremely helpful in guiding acoustic nerve dissection and also in monitoring the effects of petrous bone drilling.

Hearing preservation in vestibular schwannoma surgery: fact or fantasy?

This study reviews 57 papers dealing with the issue of hearing preservation in vestibular schwannoma surgery published in otolaryngologic and neurosurgical literature between the years 1977 and 1994. The authors', in this review, have made an attempt to verify whether the claims of hearing preservation are real, whether there is a price to be paid in terms of morbidity and whether there are univocal criteria for reporting results. The review shows that there is a wide disarray in reporting hearing results and the claims of hearing preservation are often unreal and misleading. On retabulating the results of a few series according to the minimal prerequisites for normal hearing (PTA < or = 30 dB and SDS > or = 70 per cent) and according to other various commonly reported criteria, it became evident that rates of hearing preservation differed a lot depending upon criteria. While any measurable hearing could be preserved in many cases, only a few had normal hearing preserved.

Hearing preservation in acoustic tumour surgery

900 acoustic neurinomas were removed by the suboccipital approach at Nordstadt Neurosurgical Department from 1978 to 1992 by the same surgeon (M. S.). While 247 patients were deaf on the involved side before surgery, there were 653 patients ears with some preoperative hearing. Preservation of the cochlear nerve was always attempted, and the overall-rate of hearing preservation was 38% (249 of 653), regardless of pre- and postoperative quality of hearing or of tumour sizes. In small tumour sizes below 3 cm of diameter preservation rate was 51%, in large tumours above 3 cm of diameter it was 22%. A classification system of hearing quality was made up considering pure tone audiometric hearing losses (PTA HL) at 1 to 3 kHz, and individual maximum speech discrimination scores. The usefulness of the preserved hearing is further evaluated considering the quality of hearing in the contralateral ear, and by application of other classification schemes. Presentation of the surgical strategies and their refinements by personal experience provide the base for discussion questioning whether and how further progress may still be anticipated.

Neurophysiological monitoring of cranial nerves during posterior fossa surgery

Intraoperative neurophysiological monitoring of cranial nerve functions in surgery for microvascular decompression and tumors of the posterior fossa is important for minimizing risk of permanent damage to the nerves. In particular, intraoperative BAEP and the EMG function of muscle innervated by trigeminal and facial muscle have been found useful. We report here our experiences with intraoperative monitoring of brainstem auditory evoked potentials (BAEP) and EMG recorded from muscles supplied by the trigeminal and facial nerves.

Stereotactic radiosurgery for acoustic nerve tumors in patients with useful preoperative hearing: results at 2-year follow-up examination

Twenty patients with acoustic nerve tumors (mean diameter < or = 30 mm) and useful preoperative hearing were examined 2 years after stereotactic radiosurgery to determine the effectiveness of the surgery in the control of tumor growth and the preservation of cranial nerve function. Results showed tumor volume stabilization (12 cases) or reduction (seven cases) was achieved in a total of 19 patients (95%). Useful hearing (defined as Gardner and Robertson Class I or II) preservation was obtained in 100% of cases immediately postoperatively, 50% at 6 months, and 45% at both 1 and 2 years. Two years after stereotactic radiosurgery, facial nerve function was preserved in 90% of patients and 75% continued to have normal trigeminal nerve function. All patients returned to and maintained their preoperative functional status within 3 to 5 days after radiosurgery. These findings indicate that stereotactic radiosurgery with multiple isocenters and narrow radiation beams is a safe and effective management strategy for progressive acoustic nerve tumors. Auditory, facial, and trigeminal nerve function can be preserved in most patients. Prevention of further growth and preservation of cranial nerve function appear to be satisfactory goals in the current management of patients with acoustic neuromas.

Vulnerability of hearing function during acoustic neuroma surgery

Intraoperative monitoring of auditory evoked potentials reveals possible correlations between surgical maneuvers and the function of auditory structures and pathways in cerebellopontine angle surgery. Direct cochlear nerve action potentials (CNAPS) furnish realtime information on the function of the cochlea and cochlear nerve and were utilized in 14 patients undergoing removal of small (12-18 mm) unilateral acoustic tumors. The latency of the first negative component (N1) of the CNAPs proved extremely sensitive in detecting intraoperative auditory damage and was a good predictive index of postoperative auditory outcome. Evaluation of temporal and morphological CNAP patterns allowed identification of coagulation close to the cochlear nerve, drilling of the internal auditory canal and removal of the intrameatal portion of the tumor as the most critical steps in hearing preservation.

Facial and acoustic nerve preservation during excision of extracanalicular acoustic neuromas using the suboccipital approach

The results are presented from a consecutive operative series of 62 acoustic neuromas in 60 patients following the introduction of improved neurophysiological monitoring techniques. Twenty-two patients had usable preoperative hearing. Thirty tumours were less than 2.5 cm diameter and 32 greater in size. Operation was via a 3-4-cm diameter retromastoid craniectomy. The internal auditory meatus was opened by an ENT surgeon (RM) using a drill and the facial nerve identified by stimulation. The tumour was then centrally evacuated by a neurosurgeon (MT/HC) using an ultrasonic aspirator, and the thin exterior part of the tumour carefully dissected off the nerves in or around the capsule with constant stimulation and monitoring of facial EMG, BSAEP and electrocochleography. A new type of stimulation probe has been designed and coupled to a stimulator/integrator/tone burst generator (SB) so that continuous immediate direct feedback to the surgeon is possible. A variable amplitude discriminator rejects baseline EMG (> 50 microV) and a gating circuit prevents stimulus artefact (during monopolar stimulation) from causing interference. By these means the VII nerve could be identified even when translucent and undefinable as a nerve bundle. Anatomical preservation was possible in 98% of VII nerves. Full facial function was present in 20 cases immediately postoperatively. Full delayed recovery occurred in 23 cases giving an eventual total in House Grade I of 69%. Seven other cases recovered to House Grade II. There was therefore 81% satisfactory facial nerve function. This percentage is exactly the same for larger and for smaller tumours. Anatomical preservation of the VIII nerve was achieved in 24/62 (39%) of the whole series and 11/16 (69%) of those with a hearing loss of < 50 dB. Functional preservation of hearing described as usable by the patient (< 65 dB) was achieved in 7/22 cases (32%), 3/13 (23%) in tumours < 2.5 cm and 4/9 (44%) in those > 2.5 cm diameter. Hearing preservation of < 50 dB in patients with preoperative hearing threshold < 50 dB and tumours of < 2.5 cm was 3/11 (27%). Monitoring by BSAEP and ECochG was technically unsatisfactory because the responses were affected by drilling and stimulation. Acoustic nerve preservation should be attempted in all cases with measurable hearing, regardless of tumour size.