Journal of Otology 2006 Vol. 1 No. 1

Corresponding author: Dr. Han Dongyi, Department of Otolar-yngology-Head and Neck Surgery, PLA General Hospital, Beijing100853. Tel: (010)66939512．E-mail：hdy301@263.com

Original Article

Hearing Preservation in Acoustic Neuroma SurgeryHAN Dongyi, YU Limei, YANG Shiming, YU Liming

Department of Otolaryngology-Head and Neck Surgery, PLA General Hospital, Beijing 100853

Abstract Objective To report the authors’experiences in hearing preservation during acoustic neuroma (AN)resection procedures. Methods Two cases of AN removal via retrosigmoid approach were reviewed. Hearing pres-ervation was attempted in the aid of endoscopic technique along with continuous monitoring of the compound ac-tion potential (CAP) and auditory brainstem response(ABR) during the surgery. Results The tumor in Case 1 was1.5 cm in diameter. The average pure-tone hearing threshold was 30 dB HL and ABR was normal. Waves I, III andV of ABR were present following tumor removal. At 7th month follow-up, audiometric thresholds and ABR in-ter-peak intervals had recovered to pre-operative levels, with normal facial nerve function. The patient in Case 2had bilateral AN. The tumors measured 4.0 cm(left) and 5.0 cm (right) on MRI scans. The AN on the right side wasremoved first, followed by removal of the left AN four months later. Intraoperative CAP monitoring was employedduring removal of the left AN. While efforts to preserve the cochlear nerve were not successful, CAPs were stillpresent after tumor removal. Conclusions Intraoperatively recorded CAPs are not reliable in predicting postopera-tive hearing outcomes. In contrast, ABRs are an indicator of function of the peripheral auditory pathway. Presenceof waves I, III and V following tumor removal may represent preservation of useful hearing.

Key words acoustic neuroma; cochlear nerve;hearing preservation; ABR; CAPIntroduction

An acoustic neuroma(AN) is a neurinoma arisingfrom the vestibular branch of the VIIIth cranial nerve,thereby also termed“vestibular schawnnomas (VS)”.The histo-pathologically benign nature of this tumorgives the possibility for neuro-otologists to preservethe cochlear nerve and hearing in tumor resection sur-geries. Advances in imaging technology have greatlyimproved early diagnosis of ANs with very small sizesand made tumor removal without significantly insult-ing hearing. Furthermore, intraoperative application ofendoscopy and auditory function monitoring has im-proved hearing preservation in AN resection proce-dures. As a result, the goals of current surgical manage-ment for AN have extended from merely resecting tu-mor for saving life to safely removing tumor removalassociated with preservation of facial and auditoryfunctions. However, hearing preservation remains achallenge to neuro-otologists(Samii，1997). The au-

thors have been applying intra-operative auditory nervefunction monitoring through recording compound ac-tion potentials (CAP) and auditory brainstem respons-es(ABR) during AN removal surgeries since 2003. Twocases are reviewed in this paper with a discussion onour experiences in respect to hearing preservation inAN resection surgeries.

Case Reports

Case 1. A 40 year-old female patient presented onJanuary 6, 2004 with a history of sudden hearing lossand tinnitus in the left ear for seven months. There wasno identifiable cause for the hearing loss. Her tinnituswas intermittent and of high pitches. Her hearing re-turned to“normal”after being treated with a routineprotocol for sudden hearing loss at a local medical fa-cility, but her tinnitus remained unresolved. CT scansshowed a small mass in the left cerebellopontine angle(CPA) area and careful observation was recommended.A follow-up CT study approximately six months latershowed continuing growth of the tumor and the patientwas referred to the authors for further evaluation andtreatment. MRI examination confirmed the presence ofthe neoplasm at the entrance of the internal auditory ca-nal(IAC) with a diameter of approximately 1.5 cm, as

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Fig3. Facial motor function seven days（left）and sevenmonths after surgery（right）

Before Surgery

7 Days after

7 Months after

The latency of wave Ⅰ（ms）

non-operated ear

5.52

5.48

5.72

operated ear

5.92

6.12

5.88

The intervals of waveⅠ-Ⅰ（ms）

non-operated ear

3.92

3.92

4.12

operated ear

4.32

4.20

4.12

Table1. Pre- and post-operative ABR latencies in Case 1

shown in Figure 1. Diagnosed with an acoustic neuro-ma, the patient underwent a surgery for AN removalvia retrosigmoid approach on January 12, 2004. Elec-trodes for facial nerve action potential, CAP and ABRrecording were placed and the system for the evokedpotential monitoring was set up before the operation.Upon exposure of the CPA, a pink-colored, ball-shapedencapsulated tumor was identified at the entrance ofthe IAC. The tumor was approximately 1.2 cm in diam-eter. Intracapsular tumor resection was performed, fol-lowed by dissection of the posterior portion of tumorcapsule from the cerebella and brainstem. Microdissec-tion of the rest of the tumor capsule from the facialnerve, cochlear nerve and blood vessels in the IAC wascarried out under a 30 degree endoscope. Careful exam-ination confirmed complete removal of the tumor andits capsule. Facial nerve function and ABRs were moni-tored continuously throughout the operation. Uponcompletion of the procedure, ABR responses similar topreoperative recordings were present(see Figure 2).Histological examination of tumor specimen confirmedthe diagnosis of schawnnoma. No facial palsy was no-ticed postoperatively (Figure 3). Postoperative recov-ery was uneventful. A follow-up MRI study showed nosings of residual tumor in either the CPA or IAC. Thefacial and auditory nerves appeared intact on these im-ages (Figure 1b). Follow-up pure tone audiogram andABRs were comparable to preoperative test results(Figure 4 and Table 1).

Case 2. A 40 year-old female complained of dis-comfort in the head on the left side with parietal head-

aches for one year. The symptoms became worse withprogressive tinnitus, hearing loss and imbalance duringthe six months prior to her appointment with the au-thors. She complained of no other concomitant symp-tomssuch as facial palsy, nausea, vomiting, eyesight de-cline, double vision, chocking or hoarseness. CT andMRI studies at a local medical facility suggested“bilat-eral acoustic neuroma”. The patient was admitted onFebruary 19, 2003. Physical examination was normal.Fig1. a. MRI before surgery b. MRI seven months after surgery

Ｆｉｇ２． CAPs and ABRs during surgery

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Fig7. Audiograms before and after surgery in Case 2

Fig5. Imaging studies in Case2. a. Axial MRI show-ing bilateral CPA tumors. c. CAT scan following bi-lateral CPA tumors removal. b. MRI Scan showingthe left CPA tumor after removal of the right side tu-mor. d. Facial nerve functions on both sides are pre-served following bilateral tumors resection.

On pure tone audiogram, hearing loss averaged at 35dB HL on the left and 40 dB HL on the right. Onlywave I was visible on ABR recordings. CT scanshowed substantial enlargement of the IAC with massshadow in the CPA on both sides. MRI confirmed ellip-tical shaped mass in the CPA area on both sides. Thesize of the tumor medial to the IAC was approximately5 cm in diameter on the right and 4 cm on the left (Fig-ure 5a). The patient underwent a right retrosigmoid ap-proach AN removal surgery on March 5, 2003. Lateralventricle drainage was placed prior to the surgery. Fa-cial never function was monitored throughout the sur-gery. A pink-colored, tenacious encapsulated tumorwas seen occupying the CPA, with extensive adhesionto the brainstem and adjacent nerves and blood vessels.Intracapsular debulking was performed first to reducetumor size, and then the tumor capsule was dissectedfrom the brainstem and surrounding nerves and bloodvessels. Following removal of the main portion of thetumor medial to the IAC, the posterior lip of the IACwas removed to expose the remaining tumor inside theIAC. A complete resection of the tumor was accom-plished. While facial nerve function preservation wasachieved, there was a total hearing loss in the operatedear. Postoperative recovery was otherwise smooth.X-ray knife treatment for the left AN was recommend-ed upon her discharge, but rejected by the radiologistout of the concern of risks from potential swelling oftumor tissue in reaction to irradiation. The patient wasadmitted again on July 30, 2003 and underwent leftside retrosigmoid approach AN removal (Figure 5b).Continuous monitoring of facial nerve function andCAP was performed during the surgery. The intraopera-tive findings and tumor removal process were similarto those on the right. CAPs were present during tumorremoval. However, dissection of tumor capsule fromthe cochlear nerve and blood vessels supplying to theinner ear was difficult and the cochlear never had to besacrificed for complete tumor removal. Histological

findings were consistent with schwannoma. Postopera-tive recovery was again uneventful with follow-up CTstudy showing no residual tumor (Figure 5c). Facial

nerve function was intact (Figure 5d) but hearing losswas profound (Figure 7).

Discussion

AN comprises 8-10% of intra-cranial and 70-90%of CPA tumors. It is currently a consensus that surgicalremoval is the first choice for AN treatment. Types of

Fig4. Pre- and post-operative pure tone thresholds in Case 1

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Fig6. CAPs during operation in case 2

AN removal procedures can be categorized as with andwithout hearing preservation. Retrosigmoid or middlecranial fossa approaches are used when hearing preser-vation is considered. The retrosigmoid approach wasused in both cases reported here. It has been reportedthat the rate of anatomic preservation of the cochlearnerve can be as high as 79%, although the rate of func-tional cochlear nerve preservation is only 39.5%(Samii，1997), suggesting that preservation of the co-chlear nerve does not guarantee preservation of itsfunction. Preservation of blood vessels supplying theinner ear is important in preserving hearing. Gormleyet al (Gormley, 1997) reported that functional hearingpreservation was achieved in 48% of small tumors (＜2 cm in diameter) and 25% of medium tumors(2-3.9cm in diameter). Hearing preservation is extremely dif-ficult in patients with large size tumors(＞= 4 cm in di-ameter). Most surgeons believe that it is very difficultto preserve hearing in patients with ANs larger than 2cm in diameter. The relative small size of the AN inCase 1(approximately 1.5 cm in diameter) allowedeasy identification of relations between the tumor andthe cochlear nerve as well as blood vessels. Preserva-tion of the cochlear nerve and blood vessels supplyingthe inner ear in this case was achieved without muchdifficulty, resulting in functional hearing preserva-tion(Figures 2, 4 and Table 1). In contrast, the patientin Case 2 had bilateral large ANs (＞4 cm in diameter).As a result, she ended up with total hearing loss as ef-forts in hearing preservation were unsuccessful. The co-chlear nerve in this case had to be sacrificed for total

tumor removal (Figure 3). The hearing outcomes re-ported in these two cases are consistent with those re-ported by others(Satar, 2003), i.e., the chance of hear-ing preservation decreases as the tumor size increases.Tumor size has hence been considered to be a prognos-tic factor for postoperative hearing outcomes in AN sur-gery (Satar, 2003).

Contemporary AN surgical management aims atpreserving facial nerve function and hearing as well assafe tumor removal. Improvement in the outcome ofsurgical AN treatment has come from advances in im-aging technology that enhances early diagnosis of AN,improvement in surgical techniques, and application ofintraoperative crania nerve function monitoring. Audi-tory brainstem response and cochlear compound actionpotential are most commonly used for hearing monitor-ing. It has been reported (Schmerber et al, 2004 ) thatthe auditory evoked potential monitoring is sensitive indetecting intraoperative damages to the auditory path-way from nerve retraction/compression or inner earischemia. The information gives the surgeon an oppor-tunity to adjust surgical maneuvers to avoid irrevers-ible damage to hearing. The latency, amplitude andmorphology of waves I through V in ABR are impor-tant parameters reflecting functioning status of the pe-ripheral auditory pathway. In Case 1, amplitude de-crease and latency delay of ABR waves were noticedwhen cochlear nerve retraction or inner ear blood ves-sel occlusion took place during the operation. Uponsuch findings, surgical maneuvers would be suspendedfor ABR to recover. Waves I, III and V were clearly vis-ible upon removal of the tumor, although their ampli-tudes were slightly decreased(Figure 2). An audiogramobtained seven days after the surgery showed an aver-age additional hearing loss of approximately 15 dBcompared to preoperative hearing level. Audiogramand ABRs were close to preoperative level at sevenmonth follow-up (Figure 4 and Table 1). These resultssuggest that ABR is a sensitive indicator for functionof auditory pathway below the brainstem level. Ifwaves I, III and V are distinguishable following tumorremoval, favorable hearing outcomes can be expected.

CAP is a near field potential which features largesignals, requires few numbers of averaging and allowsprompt information feed-back. CAP monitoring wasperformed in Case 2 as ABR were absence. Due to ex-tensive adhesion to adjacent blood vessels and nervesby the large tumors, these structures had to be sacrificedfor complete tumor removal. CAP was present with re-duced amplitudes, upon tumor removal(Figure 6).However, patient suffered total hearing loss postopera-

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Journal of Otology 2006 Vol. 1 No. 1

tively. This suggests that CAP is not a reliable predic-tor for postoperative hearing function as it comes fromresponses in the cochlea and from distal portion of thecochlear nerve.

Surgical endoscopy provides improved localillumination, wide angle view of the surgical field andvariable observation angles. It has been used in ANsurgeries in recent years. A surgical endoscope wasused in both cases reported here. The authors werepleased with improved view of the surgical field. Withthe endoscope, examination and removal of tumorsinside the ICA was possible with minimal removal ofits posterior wall(approximately 3 mm). It greatlyincreased safety of the procedure and facilitatedprotection of surrounding vessels and nerves byproviding the surgeon with improved view of thesurgical field and allowing visualization of contentinside the ICA and accurate assessment of relationsbetween the tumor and surrounding important

structures. The authors therefore recommend use ofendoscopy in AN resection procedures in addition tointraoperative auditory evoked potential monitoring asa measure to improve the odd of hearing preservation.

Reference

１ Samii M & Matthies C. Management of 1000 vestibularschwannomas(acoustic neuronas): hearing function in 1000tumor resection Neurosurgery. 1997, 40: 248-262.2 Gormley WB, Sekhar LN, Wright DC，et al. Acoustic neuro-mas:Resultsof current surgical management. Neurosurgery, 1997,41: 50-60.3 Satar B, Yetiser S, Ozkaptan Y . Impact of tumor size on hear-ing outcome and facial function with the middle fossa approachfor acoustic neuroma: a meta-analytic study. Acta Otolaryngol,2003, 123: 499-505.4 Schmerber S, Lavieille J-P, Dumas G, Herve T. Intraoperativeauditory monitoring in vestibular schwannoma surgery: newtrends. Acta Otolaryngol, 2004, 124: 53-61.

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