Acoustic neuroma surgery: absent auditory brainstem response does not contraindicate attempted hearing preservation.

OBJECTIVE: Absence of auditory brainstem response (ABR) waveforms has been associated with a poor likelihood of hearing preservation following resection of acoustic neuromas. Our experience is reviewed for patients with absent preoperative ABR regarding hearing preservation, hearing improvement, and return of ABR. STUDY DESIGN: Retrospective review of 22 cases of acoustic neuroma resection. Nine patients with absent preoperative ABR were identified. All underwent tumor resection utilizing intraoperative cochlear nerve action potential (CNAP) monitoring. Postoperative hearing results and ABR waveforms were examined. METHODS: Charts were reviewed and tabulated for age, sex, tumor side, tumor size, preoperative and postoperative audiometric and ABR results, intraoperative monitoring results by ABR and CNAP, and surgical complications. RESULTS: Hearing preservation was achieved in seven of nine patients (78%) with absent preoperative ABR, as well as six of seven patients (86%) with tumors less than or equal to 20 mm in greatest dimension. Although intraoperative ABR monitoring was not possible in any of these patients, CNAP monitoring was successful in all. Return of ABR waveforms was observed in four of the six patients (67%) tested from 3 to 22 months postoperatively. Four of the seven patients (57%) enjoyed improvement in hearing class as defined by the guidelines of the American Academy of Otolaryngology-Head and Neck Surgery. CONCLUSIONS: Absent ABR waveforms have not been a negative prognostic sign regarding hearing preservation. CNAP monitoring is possible in these patients and likely helps to minimize iatrogenic cochlear nerve trauma. Patients with no ABR waveforms have hope of hearing preservation and even improvement following acoustic neuroma resection performed utilizing CNAP monitoring and hearing preservation surgical techniques.     

The efficacy of brainstem auditory evoked potentials in acoustic tumor surgery

As the identification of patients with small acoustic neuromas and salvageable hearing increases, intraoperative auditory nerve monitoring has been used increasingly in an attempt to improve the hearing preservation rate. Far-field recordings obtained by brainstem auditory evoked potentials (BAEP), at times enhanced by electrocochleography, have become a standard method of intraoperative auditory nerve assessment. To evaluate the usefulness of this monitoring technique, the hearing preservation results of a series of unmonitored acoustic tumor removals were compared to a series of patients monitored via the standard brainstem auditory evoked potentials. With comparable average tumor sizes, 4 of 7 unmonitored patients had hearing preserved at preoperative levels compared to 4 of 9 monitored patients. Neither preoperative BAEP assessments nor absolute tumor size were predictive of hearing preservation. This report brings into question the effectiveness of far-field intraoperative BAEP monitoring during acoustic tumor resection and suggests that direct auditory nerve monitoring may be more appropriate.     

Intra- and perioperative acoustic evoked brain stem potentials in cerebellopontile angle operations

From a series of 35 cases with intraoperative monitoring of acoustic evoked brainstem responses 4 cases with cerebellopontine angle surgery are presented in detail. The aim of neuromonitoring is to provide the surgeon with an additional instrument to assess the functional state of the hearing pathway with objective neurophysiological criteria during dissection near the eighth nerve and brainstem. Thus, a near total loss of potential during a neurovascular decompression of the trigeminal nerve could be reversed by modifying the surgical procedure. In 2 cases of acoustic neurinoma with preserved wave I this peak was obtainable throughout the procedure with a good postoperative result. Transient amplitude attenuation and latency increase was seen with a meningioma, which normalized before the end of operation with good postoperative hearing and brain-stem function. The relationship between intraoperative BAEP changes and postoperative brain-stem and eighth nerve function, and the value of neuromonitoring are discussed with regard to surgery around the internal auditory meatus and the eighth nerve.     

Function-saving microsurgery in suboccipital removal of large acoustic neuromas

Various successful approaches are available for acoustic neurinoma surgery, permitting total tumor removal and preservation of cranial nerve function. In smaller and medium sized tumors excellent results can be achieved with respect to facial and cochlear nerve function using the transtemporal approach. For larger tumors similar results can be achieved by the suboccipital approach. The results of 45 completely removed large acoustic neurinomas all operated upon via the suboccipito-lateral approach with microsurgical techniques and neurophysiological monitoring are presented. The average tumor size, excluding the portion within the internal auditory canal was 3 cm. Anatomical preservation of the VIIth cranial nerve was achieved in nearly all cases. Satisfactory to excellent facial nerve function was preserved in 70% of all cases. Initial hearing was preserved in 29%. However delayed postoperative hearing loss was encountered in 13%. Therefore definite hearing preservation was achieved in 16% of the cases. Intraoperative monitoring, especially of auditory evoked potentials, was very helpful in achieving these functional results.     

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

OBJECTIVES: To compare the hearing preservation results obtained with use of two intraoperative eighth nerve monitoring methods, cochlear nerve action potential (CNAP) and auditory brainstem response (ABR), during complete acoustic neuroma (AN) resection. STUDY DESIGN: Retrospective. SETTING: Tertiary referral center. PATIENTS: Thirty-three consecutive patients who underwent hearing preservation AN surgery. INTERVENTIONS: Intraoperative monitoring by CNAP and/or ABR during AN resection. MAIN OUTCOME MEASURE: Postoperative hearing. Hearing preservation was considered achieved for pure-tone average < or =50 dB and speech discrimination > or =50%. RESULTS: Thirty-two patients met inclusion criteria for the study. Monitoring was successfully performed in 23 of 25 patients (92%) who underwent attempted CNAP monitoring and 13 of 27 (48%) who underwent attempted ABR monitoring. When tumor size was < or =20 mm in greatest dimension, hearing preservation was achieved in 12 of 18 patients (67%) monitored with CNAP, versus 2 of 8 patients (25%) not monitored with CNAP (p = 0.05). Monitoring by ABR did not improve hearing preservation rates compared with those not monitored with ABR (40% vs. 63%). At the completion of surgery, the presence or absence of CNAP predicted the presence or absence of hearing preservation in 18 of 23 cases (p = 0.01), while ABR successfully predicted hearing results in 10 of 13 cases (p = 0.05). CONCLUSIONS: When CNAP and ABR monitoring techniques during AN surgery were compared, CNAP was more frequently obtainable. Monitoring by CNAP was significantly associated with a higher chance of hearing preservation. Monitoring by ABR did not have a positive influence on hearing preservation results. Both ABR and CNAP were useful for predicting postoperative hearing.     

两种听觉监护在听神经瘤术中的联合应用及评价

目的　探讨听神经瘤手术中听觉监护的意义。比较听性脑干反应（ABR）和蜗神经动作电位（cochlear nerve action potential,CNAP）在听觉监护应用中的价值。方法　回顾性分析我中心12例保留听力手术的听神经瘤患者资料及术中情况,患者术前均有实用听力（AAO-HNS分级A级、B级）,可引出ABR波形,V波分化可,所有手术均采取乙状窦后径路摘除肿瘤。所有患者均全程ABR监护,术中打开脑膜后联合监测CNAP。结果　手术过程中所有患者ABR波形较术前基线均有不同程度的变化,可记录到V波潜伏期均较基线延长（0.68±0.41）ms。CNAP波幅个体差异性较大,可记录到的P1潜伏期均较基线延长（0.25±0.16）ms。12例中有8例（66.7%）患者术后保留可用听力;另外4例（33.3%）术后达不到可用听力,此4例患者ABR表现为3例V波消失,1例V波潜伏期延长。CNAP表现为2例CNAP的P1消失,2例P1延长。电钻钻磨造成的震动和噪声、手术时神经被牵拉或热损伤后等急性损伤后ABR和CNAP波幅均出现短时间急剧下降,手术暂停5 min后,部分患者波幅可在一定程度恢复。结论　听神经瘤手术中ABR和CNAP对于听觉监护均有一定意义。ABR波形稳定可靠,但叠加时间相对较长;CNAP叠加快速,可提升监测灵敏度,但波形变异度较大。电钻钻磨造成的震动和噪声、操作过程中对神经组织的牵拉以及热损伤均被及时监控,两者联合应用可提升听神经瘤手术的听觉功能保留率。     

Hearing preservation using combined monitoring of extra-tympanic electrocochleography and auditory brainstem responses during acoustic neuroma surgery

The aim of this study was to evaluate the efficacy of an intraoperative monitoring hearing preservation strategy that includes simultaneous recordings of an auditory brainstem response (ABR) and non-invasive electrocochleography (ECochG). The combined ABR and tympanic membrane (TM) ECochG testing was performed in 74 patients undergoing acoustic neuroma (AN) surgery. In addition, EMG recordings were conducted to monitor the facial nerve function. Hearing was preserved in 19 of the 30 patients with residual hearing prior to surgery (63%), and facial nerve function was maintained in 89% of the patients. In most cases, the presence of both auditory brainstem and TM-ECochG responses at the end of surgery was associated with preservation of postoperative functional hearing; however, eight patients had a TM-ECochG response with a complete loss of the ABR, pointing to deafferentation of the auditory nerve. Tumour size and preoperative hearing thresholds significantly affected the postoperative hearing. The TM-ECochG response yielded large reproducible responses, which, in some patients, was the only way to monitor the auditory function. This auditory monitoring approach offers a valuable auditory tool that helps to improve the hearing preservation during AN surgery.     

Hearing preservation using combined monitoring of extra-tympanic electrocochleography and auditory brainstem responses during acoustic neuroma surgery

The aim of this study was to evaluate the efficacy of an intraoperative monitoring hearing preservation strategy that includes simultaneous recordings of an auditory brainstem response (ABR) and non-invasive electrocochleography (ECochG). The combined ABR and tympanic membrane (TM) ECochG testing was performed in 74 patients undergoing acoustic neuroma (AN) surgery. In addition, EMG recordings were conducted to monitor the facial nerve function. Hearing was preserved in 19 of the 30 patients with residual hearing prior to surgery (63%), and facial nerve function was maintained in 89% of the patients. In most cases, the presence of both auditory brainstem and TM-ECochG responses at the end of surgery was associated with preservation of postoperative functional hearing; however, eight patients had a TM-ECochG response with a complete loss of the ABR, pointing to deafferentation of the auditory nerve. Tumour size and preoperative hearing thresholds significantly affected the postoperative hearing. The TM-ECochG response yielded large reproducible responses, which, in some patients, was the only way to monitor the auditory function. This auditory monitoring approach offers a valuable auditory tool that helps to improve the hearing preservation during AN surgery.     

Intraoperative BAER monitoring and hearing preservation in the treatment of acoustic neuromas

We treated 20 cases of acoustic neuromas in the last 2 1/2 years using the suboccipital approach with intraoperative monitoring of eighth nerve function in an attempt to preserve hearing. Total tumor removal was accomplished in 18 cases. Three tumors were small (less than 2 cm in size); 3 tumors were moderate sized (2 to 3 cm) and 14 tumors were large (greater than 3 cm). In 15 cases, all BAER components were lost during surgery. These patients had no postoperative hearing. In five patients there was intraoperative preservation of some or all BAER components. These included the three patients with small tumors and two other patients with moderate sized tumors. One patient with a moderate size tumor had preservation of wave 1 only, and had no postoperative speech discrimination. One patient with a small tumor retained all 5 BAER components, but had no postoperative hearing. Three patients in our series retained functional hearing after surgery, including 3 of 5 patients with tumors 2 cm or smaller. Intraoperative BAER monitoring appears to be useful in predicting postoperative hearing. Tumor size seems to be the primary factor in preservation of hearing following acoustic neuroma surgery.     

Axonal injury in auditory nerve observed in reversible latency changes of brainstem auditory evoked potentials (BAEP) during cerebellopontine angle manipulations in rats

Intraoperative monitoring of brainstem auditory evoked potentials (BAEP) has been widely utilized to reduce the incidence of postoperative hearing disturbance due to cerebellopontine angle manipulations. The prolongation of wave V of BAEP is usually used as a criterion to warn the surgeons to modify their surgical maneuvers. However, it is not known whether all neuropathological changes are avoided if BAEP latency intraoperatively returns to the baseline level or some neuropathological changes 'silently' occur even if BAEP normalizes. The aim of this study was to experimentally clarify this point that would be important for the long-term prognosis of patients' hearing. The cerebellopontine angle portion of the auditory nerve was quantitatively compressed in the rats and reversible prolongation of BAEP latency was reproduced just as it occurs during surgery in humans. Twenty-four hours after the compression, the auditory nerve was removed for beta-APP immunostaining to investigate the degree of axonal injury. The results of the present study disclosed that axonal injury occurred even in the cases where the intraoperative normalization of prolonged wave IV (equivalent to wave V in humans) latency had been obtained. Therefore, the interpretation of BAEP changes based only on the prolongation of the latency of BAEP was not enough to prevent the auditory nerve from developing morphological changes. Changes in the amplitude of wave V of BAEP appears to be more sensitive than its latency change as an intraoperative indicator for axonal injury in the auditory nerve.     

Hearing preservation in acoustic neuroma surgery and postoperative audiological findings

One hundred fifty-three cases of acoustic neuroma were treated surgically by the middle cranial fossa approach or extended middle cranial fossa approach. Attempts to preserve hearing were made in 30 cases with tumours extending 2.0 cm or less into the posterior fossa; successful hearing preservation was achieved in 12 cases. Among the 15 patients with preoperative hearing levels (HL) of 50 dB or lower and speech discrimination scores (SDS) of 50% or higher, hearing was preserved in 9 (60%) patients. A similar rate of hearing preservation was achieved among the patients with normal or near-normal hearing. Compared with those patients in whom hearing could not be preserved, those with hearing preservation had better HL, higher SDS, and less abnormal ABR findings preoperatively. Postoperatively, the HL and SDS deteriorated slightly. In addition, there was a marked prolongation of the IT5, and the incidence of absence of the stapedius reflex increased. Compared with the preoperative HL, the postoperative HL was unchanged in 5 cases; deteriorated temporarily and then improved in 5 cases; and deteriorated, though with hearing preserved, in 2 cases. Intraoperative monitoring was conducted by recording the ABR and VIII nerve compound action potentials and by electrocochleography. However, postoperative hearing could not always be predicted from the findings obtained at the end of the operation.     

Hearing preservation following suboccipital removal of acoustic neuromas

Advances in the diagnosis and intraoperative management of acoustic neuromas have greatly reduced the incidence of neurologic deficits following their removal. Ninety-three patients underwent acoustic tumor removal during a 41/2-year period, and hearing preservation was attempted in 20 cases. Hearing was preserved in 65% of the entire series, and excellent results were obtained in tumors less than 1.5 cm. No patient with a tumor greater than 1.5 cm had serviceable hearing preserved when total tumor removal was performed. Two patients, one with neurofibromatosis and one with an acoustic neuroma in an only-hearing ear, had planned partial tumor removal with preservation of hearing. Preoperative auditory brainstem response results were not predictive of postoperative hearing preservation. Intraoperative auditory brainstem response monitoring demonstrated that loss of wave V consistently correlated with loss of hearing postoperatively, whereas persistence of wave V (with a latency prolongation not exceeding 3.00 ms) was predictive of successful hearing preservation regardless of latency increases.     

大型听神经瘤的显微手术技巧及面神经保护

目的探讨大型听神经瘤的显微手术技巧、效果及术中面神经的保护。方法回顾性分析解放军总医院耳鼻咽喉头颈外科2010年1月~2010年12月收治的采用显微外科手术治疗30例大型听神经瘤患者的临床资料。其中男性18例,女性12例;年龄19～71岁,平均39.6±4.2岁;病程3个月～2年。主要临床表现为桥小脑角综合征和颅内压增高征,首发症状表现为耳鸣、听力下降12例,头痛、恶心、呕吐10例,行走不稳4例,面部麻木7例,三叉神经痛2例,面瘫6例。30例术中均行面神经监测,显微镜下切除肿瘤,术毕刺激面神经的脑干端对术后面神经功能进行预测。结果本组30例大型听神经瘤全切除28例,次全切除1例,部分切除1例。术中面神经完整保留29例(96.67%),无死亡病例。肿瘤切除后,面神经刺激阈值的大小与术后面神经功能存在明显的相关性。刺激阈值越小,术后面神经功能越好。结论熟练地采用显微外科技术选择合适的手术入路可明显提高肿瘤的全切除率和面神经的解剖及功能保留率。手术入路的正确选择,娴熟的显微外科操作技术,术中应用面神经监测技术,能有效地保护桥小脑角周围的重要结构及面神经功能,并可预测术后面神经功能。     

Bone-conducted evoked myogenic potentials from the sternocleidomastoid muscle

The aim of this study was to show that bone-conducted clicks and short tone bursts (STBs) can evoke myogenic potentials from the sternocleidomastoid muscle (SCM) and that these responses are of vestibular origin. Evoked potential responses to bone-conducted auditory stimuli were recorded from the SCMs of 20 normal volunteers and from 12 patients with well-defined lesions of the middle or inner ear or the VIIIth cranial nerve. The subjects, who had various labyrinthine and retro-labyrinthine pathologies, included five patients with bilateral profound conductive hearing loss, two with bilateral acoustic neuroma post-total neurectomy and five with bilateral sensorineural hearing loss. Air- and bone-conducted evoked myogenic potentials in response to clicks and STBs were recorded with surface electrodes over each SCM of each subject. In normal subjects, bone- and air-conducted clicks and STBs evoked biphasic responses from the SCM ipsilateral to the stimulated ear. The bone-conducted clicks evoked short-latency vestibular-evoked myogenic potential (VEMP) responses only in young subjects or in subjects with conductive hearing loss. STBs evoked VEMPs with higher amplitude and better waveform morphology than clicks with the same acoustic intensity. Patients with total VIIIth cranial nerve neurectomy showed no responses to air- or bone-conducted click or STB stimuli. Clear VEMP responses were evoked from patients with conductive or sensorineural hearing loss. It is concluded that loud auditory stimuli delivered by bone- as well as air conduction can evoke myogenic potentials from the SCM. These responses seem to be of vestibular origin.     

听神经瘤术中听力监测及术后听力保留影响因素分析

目的探讨听神经瘤术中听力监测的应用及术后听力保留的可能影响因素。方法16例采用乙状窦后入路手术切除听神经瘤的成年患者,分为两组,术中采用听性脑干反应(auditory brainstem response,ABR)和耳蜗电图(electrocochleogram,ECochG)联合监测为监测组(8例),未监测者为未监测组(8例),比较两组患者术后听力保留情况,采用单因素分析,分析影响听力保留的可能因素,包括:年龄、病程、肿瘤大小、术前纯音听阈和言语识别率、术中是否行ABR和ECochG联合监测、内听道是否扩大、肿瘤和神经是否粘连等。结果前庭诱发肌源性电位(VEMP)提示16例患者肿瘤来源于前庭上神经,监测组中6例术中及术毕ABR波Ⅰ、Ⅴ和复合动作电位(CAP)持续存在,术后听力保留;1例术中ABR波Ⅰ、Ⅴ和ECochG CAP持续存在,但术后无可用听力;1例术中切除肿瘤时ECochG与基线重复性良好,ABR波V消失,手术结束波V仍未恢复;监测组术后听力保留率为75.0%(6/8),未监测组术后无一例保留听力,差异有统计学意义(P=0.007)。单因素分析显示,年龄、病程、肿瘤大小、术前纯音听阈以及内听道扩大与术后听力保留率无关(P>0.05),术前言语识别率、术中ABR和ECochG联合监测、肿瘤和神经粘连与否与术后听力保留率相关(P<0.05)。结论听神经瘤切除术中ABR和ECochG连续监测对指导手术和提高术后听力保留率有重要意义,肿瘤与神经粘连是术后听力保留的重要影响因素,手术技巧、术前听力、肿瘤大小、内听道扩大等是否是术后听力保留的影响因素需扩大样本进一步研究验证。     

Intraoperative monitoring of facial and cochlear nerves during acoustic neuroma surgery.

The likelihood of successful preservation of facial and cochlear nerve function during acoustic neuroma surgery has been improved by the advent of intraoperative monitoring techniques. The facial nerve is monitored by recording EMG from facial muscles, with no muscle relaxants used; mechanical irritation of the nerve during surgery causes increased EMG activity, which can be detected in real time using a loudspeaker. Brief episodes of activity associated with specific surgical maneuvers aid the surgeon in avoiding damage to the nerve, whereas prolonged tonic EMG activity may reflect significant neural injury. Electrical stimulation with a hand-held probe elicits evoked EMG responses, which can be used to locate and map the nerve in relation to the tumor. The threshold for eliciting evoked EMG responses provides a rough indicator of the functional status of the nerve. Different nerves in the posterior fossa (trigeminal, facial, spinal accessory) can be identified in multichannel recordings by the spatial distribution and latency of responses to electrical stimulation. The ability to elicit EMG responses from low amplitude stimulation of the facial nerve at the brain stem after tumor removal is a reasonable predictor of postoperative facial function. Cochlear nerve function is assessed by recording the ABR from ear canal and scalp electrodes or the CNAP with an electrode placed directly on the nerve at the brain stem root entry zone. The ABR is a well-known, noninvasive technique that can be adapted to intraoperative use relatively easily but is of limited utility owing to the delay inherent in signal averaging. Direct CNAP recordings require placement of an intracranial electrode in such a way as to contact the cochlear nerve without interfering with surgical access but have the distinct advantage of rapid feedback on changes in cochlear nerve status.     

Endoscope-Assisted Cerebellopontine Angle Surgery

Objective To report experiences with use of otoendoscopy in cerebellopontine angle (CPA) surgeries. Methods Twenty five cases of CPA surgeries performed between November 2002 and December 2008 in which microscope enabled otoendoscopy was used were reviewed.The 25 cases included 19 cases of acoustic neuroma, 3 cases of CPA facial nerve tumors, 1 case of trigeminal neurinoma, a case of glossopharyngeal neuralgia and 1 case of hemifacial spasm. Endoscopy was used in all cases together with monitoring of brainstem auditory responses and facial electromyography. Postoperative hearing and facial nerve function were evaluated and compared to pre-op-erative levels. Results Endoscopy provided improved visualization of local anatomy, revealed hidden lesions and reduced unnecessary anatomical distortions. Total resection was achieved in 18 of the 19 acoustic neuroma cases, Facial nerve anatomical integrity was preserved in all 19 cases. One week postoperative House-Brackmann grading was Ⅰ in 3 cases, Ⅱ in 10 cases and Ⅲ in 6 cases. Facial nerve function continued to improve in some cases at 3 months. Total tumor resection was achieved in all 3 patients with facial neurinoma. The facial nerve was sacrificed in 2 of the 3 cases with primary faciohypoglossal nerve anastomosis. Facial nerve function was Grade Ⅱ and Grade Ⅲ one year after surgery, respectively. In the case with anatomically preserved facial nerve, postoperative facial nerve function was initially Grade Ⅲ and improved to Ⅱ at 3 months. The tumor was completely resected in the trigeminal neurinoma patient with a Grade Ⅲ postoperative facial nerve function which improved Grade Ⅱ three months later. Seventeen of the 19 patients with acoustic neuroma retained hearing postoperatively, of these 12 maintained preoperative levels of hearing. Preoperative hearing capacity was preserved in 2 of the 3 patients with facial nerve tumors, but lost in patients with other tumor types. Glossopharyngeal neurotomy (n=1) and mi-crovascular deeompression(n=1) resulted in satisfactory symptom relief and no recurrence at 5- and 3-year follow up, respectively. Conclusions Otoendos aope-aided technique greatly helps surgical management of CPA and in-ternal auditory canal lesions and other disorders. This minimally invasive technique overcomes many shortcomings inherent to the traditional retrosigmoid approach.     

Electrical promontory stimulation in patients with intact cochlear nerve and anacusis following acoustic neuroma surgery.

Anacusis following hearing preservation surgery for acoustic neuroma removal in which the cochlear nerve was preserved has been explained on the basis of neural or vascular compromise. In the absence of pathologic evidence for either theory, a physiologic model was chosen. Electrical promontory stimulation with monitoring of subjective and electrically evoked auditory brainstem responses was undertaken. A positive response to stimulation suggests a vascular impairment of the cochlea sparing the cochlear nerve and spiral ganglion. The absence of response suggests loss of neural integrity at the level of the spiral ganglion or cochlear nerve. Six patients who suffered anacusis following hearing preservation surgery for acoustic neuroma were studied. Data regarding electrical promontory stimulation, auditory brainstem responses, and implications of the possible role of cochlear implantation are discussed.     

Acoustic tumor surgery. Prognostic factors in hearing conversation

Preoperative selection criteria for hearing conservation surgery in patients with acoustic tumors continues to be unresolved. The level of hearing that is worth saving is still debatable. However, most acoustic tumor surgeons agree that hearing preservation is less likely the larger the tumor. We reviewed the results of 106 middle fossa acoustic tumor removals with attempted hearing preservation. Measurable postoperative hearing remained in 59% of 97 cases analyzed for postoperative hearing results. Tumor size, preoperative auditory brain-stem response, and preoperative electronystagmography were found to be useful in predicting successful postoperative hearing preservation. We found no correlation between the level of preoperative hearing and our ability to preserve measurable postoperative hearing. A new classification system is proposed for reporting hearing results after acoustic tumor surgery. It is based on functional hearing results and we believe it is simpler than previously proposed systems.     

Intraoperative auditory monitoring in vestibular schwannoma surgery: new trends

OBJECTIVE: To investigate the efficiency of a new method of brainstem auditory-evoked potential (BAEP) monitoring during complete vestibular schwannoma (VS) resection with attempted hearing preservation. MATERIAL AND METHODS: Dedicated software providing near real-time recording was developed using a rejection strategy of artifacts based on spectral analysis. A small sample number (maximum 200) is required and results are obtained within 10 s. Fourteen consecutive patients with hearing class A operated on for VS, in an attempt to preserve hearing, participated in the investigation. Postoperatively, 7 patients (50%) had useful hearing (hearing class A, 4/14; hearing class B, 3/14) on the operated side. Seven patients (50%) were reduced to hearing class D. RESULTS: Drilling of the internal auditory canal (IAC) and tumor removal at the lateral end of the IAC were identified as the two most critical steps for achieving hearing preservation. Intraoperative BAEP monitoring was sensitive in detecting auditory damage with useful feedback but its effectiveness in preventing irreversible hearing impairment was not demonstrated in this study. CONCLUSION: Combined BAEP and direct auditory nerve monitoring using the same equipment will be performed in the future in an attempt to enhance the chances of preventing irreversible hearing damage, and possibly to improve the hearing outcome significantly.