Hearing preservation in solitary vestibular schwannoma surgery using the retrosigmoid approach

INTERNATIONAL ORIGINAL ARTICLES EUGENE N. MYERS, MD International Editor

Hearing preservation in solitary vestibular schwannoma surgery using the retrosigmoid approach DAVID A. MOFFAT, FRCS, BSc, MELVILLE J. DA CRUZ, FRACS, MSc, DAVID M. BAGULEY, MSc, MBA, GRAHAM J. BEYNON, MSc, and DAVID G. HARDY, FRCS, Cambridge, United Kingdom

The results of 50 cases of vestibular schwannoma surgery with hearing preservation performed by the retrosigmoid approach at Addenbrooke’s Hospital, Cambridge, during a 10-year period are presented. The hearing-preservation rate, using audiometric criteria set by others as “serviceable hearing” (Wade PJ, House W. Otolaryngol Head Neck Surg 1984;92:1184-93; Silverstein H, et al. Otolaryngol Head Neck Surg 1986;95:285-91; Cohen NL, et al. Am J Otol 1993;14:423-33) was 8% (4 of 50 cases). When the more stringent selection criteria of nearnormal hearing and reporting criteria of socially useful hearing preservation (pure-tone average < 30 dB/speech discrimination score > 70%) is used, the hearing-preservation rate is 4.8% (1 of 21 cases). The only preoperative factor that may predict a favorable hearing-preservation outcome is normal auditory brain stem response morphology (Fisher’s exact 2-tailed test, P < 0.001). The number of suitable candidates for hearing-preservation surgery are few. Reasonable indications for attempted vestibular schwannoma surgery with hearing preservation are discussed. (Otolaryngol Head Neck Surg 1999;121:781-8.)

Increasing

refinement in contemporary vestibular schwannoma surgery has led to fulfilled expectations of low mortality, complete tumor removal, and improved facial nerve outcomes. In contrast, attempts to preserve useful hearing have been less successful.1,2 This article presents the results of the Cambridge hearing-preserva-

From the Department of Otoneurosurgical and Skull Base Surgery, Addenbrooke’s Hospital. Reprint requests: M. J. da Cruz, FRACS, Box 48, Clinic 10, Addenbrooke’s Hospital, Hill’s Rd, Cambridge, CB2 2QQ, UK. Copyright © 1999 by the American Academy of Otolaryngology– Head and Neck Surgery Foundation, Inc. 0194-5998/99/$8.00 + 0 23/77/91263

tion series. The postoperative hearing outcomes are reported in terms of criteria described by previous authors3-6 such that reasonable comparison can be made with their results. A critical appraisal of the preoperative factors that may predict a successful hearing-preservation outcome is made. Finally, conclusions regarding the social usefulness of preserved hearing are discussed, and the socially useful hearing-preservation rate is calculated. METHODS AND MATERIAL The notes of 50 cases of solitary, nonfamilial vestibular schwannoma in which hearing preservation was attempted were examined. The surgery was performed solely through the retrosigmoid transmeatal approach by the senior surgeons (D.A.M., D.G.H.) using techniques that are widely practiced and published.7,8 The only selection criterion for entry into the study was a preoperative hearing level of greater than 50 dB pure-tone average (PTA; 0.5 to 4 kHz) and speech discrimination score (SDS) greater than 50% (Table 1). Pure-tone audiometry was performed according to the guidelines of the British Society of Audiology.9 Speech audiometry was performed by use of Boothroyd single word lists.10 Data pertaining to maximum tumor diameter, radiologic findings, and auditory brain stem responses (ABRs), although recorded and used in the postoperative analysis, were not used to select or exclude cases from the study. Patients were followed up 3 and 12 months after surgery, with all 50 cases being followed up for 12 months or more at the completion of the study. At each postoperative visit the patient was assessed clinically, the facial nerve was graded,11 and pure-tone and speech audiometry were recorded. A follow-up MRI scan looking for residual tumor12 was performed at 2 years. RESULTS Age

The age distribution of the studied cases is shown in Fig 1. There was a wide range of ages (range 13 to 68 years, mean 45 years, standard deviation 11 years). The majority of cases were clustered around the middle ages 781

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Fig 1. Age distribution.

Fig 2. Tumor size.

Table 1. Selection criteria Sporadic vestibular schwannoma Retrosigmoid approach PTA (0.5-4 kHz) < 50 dB SDS > 50% Hearing equal to or better than PTA 50 dB and SDS of 50% were the sole selection criteria. No case was excluded on the basis of tumor size, patient age, or preoperative ABR character.

Tumor Size

Tumor size was measured as the distance from the most lateral to the most medial portion of the tumor on preoperative imaging (MRI or CT). This measure includes both intracanalicular and cerebellopontine angle components of the tumor. Tumor size ranged from <1 to 4.5 cm (mean 2.4 cm) (Fig 2). Preoperative Hearing Levels

(30 to 60 years), with a few cases at either extreme. For patients older than 60 years, the small number is influenced by the increasing tendency to observe slow-growing tumors in the elderly age group, rather than recommend surgery,13 and the small number in the age groups younger than 20 years reflects the relative rarity of vestibular schwannoma at this period of life.

The PTA was calculated with the pure-tone thresholds recorded at 500 Hz, 1 kHz, 2 kHz, and 4 kHz. The distribution of the PTA thresholds at presentation are shown in Fig 3 (range 48 dB, mean 31 dB, standard deviation 12 dB). The results of speech audiometry were broadly divided into several groups. These speech discrimination results are displayed Fig 4. The majority of cases had high SDSs, with 84% of cases having an SDS of 90% or greater.

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Fig 3. Hearing threshold.

Fig 4. Speech discrimination.

Preoperative ABR

Preoperative click-evoked ABR (90 dB SPL, filter 100 to 3000 Hz at 8.1 clicks/second, masking when required) was recorded in 49 of the 50 cases. Normal waveform morphology is defined by the clear presence of the evoked waveform and its individual components, and the shape, amplitude, and interpeak intervals within accepted normal limits.14,15 Results were separated into 1 of 3 groups according to the morphology of the evoked waveform: normal waveform patterns, a wave IV delay of greater than 0.2 msec when compared with the contralateral side, and those with poor waveform morphology. The results are displayed in Fig 5. In the majority of cases (86%) the ABR was abnormal, consisting of a wave I-V delay of greater than 2.0 msec or a poor waveform morphology. Normal waveform morphology and latencies were seen in 14% of cases.

Table 2. Hearing-preservation rate of the Cambridge series tabulated against postoperative criteria proposed by others

Study

Normal Serviceable hearing hearing Any (PTA < 30 dB/ (PTA < 50 dB/ hearing SDS > 70%) SDS > 50%) (PTA < 110 dB)

Wade and House4 (n = 20) 25% Sterkers et al13 (n = 135) 3% Silverstein et al5 (n = 16) 12.5% Kemink et al18 (n = 20) 35% Cohen et al3 (n = 131) 10% Cambridge series (n = 50) 4.8%

30% 7.4% 12.5% 50% 37% 8%

35% 23% 37% 65% — 18%

Postoperative Results

The hearing outcome 12 months after surgery was analyzed by use of previously defined criteria3-5 into the

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Fig 5. ABR waveform.

Fig 6. Interaural PTA 12 months after surgery.

following categories: normal hearing (PTA < 30 dB/SDS > 70%), serviceable hearing (PTA < 50 dB/SDS > 50%), and any measurable hearing (PTA > 100 dB). Results are shown in Table 2. For a more complete analysis of the hearing-preservation results and a discussion of socially useful preserved hearing, hearing thresholds in the nonoperated ear were taken into account by calculating the interaural pure-tone difference (IAD) 12 months after surgery. Seven cases (14%) had a postoperative IAD of 50 dB or less, and only 1 case (2%) had an IAD of less than 30 dB (Fig 6). To examine for residual tumor, we subjected all cases that had been followed up for 2 years or more after surgery (44 cases) to MRI scanning of the poste-

rior fossa using a postgadolinium fat-suppression sequence. 12 Residual tumor was seen in 1 case (2.3%). Facial nerve grading was recorded at 12 months according to widely accepted grading schemes.11 Results are summarized in Table 3. A satisfactory result (House-Brackmann grade I-II) was seen in 80% of cases (31 of 50 cases). Raw Data

In keeping with the recommendations of the American Academy of Otolaryngology–Head and Neck Surgery,16 a summary of the raw audiometric data, tumor size, and facial nerve grade at 12 months for each case are tabulated in Appendix 1.

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Table 3. General results 50 hearing-preservation attempts Residual tumor: 1 case (2%) Facial nerve results (12 mo) Grade I-II Grade III Grade IV-VI

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Table 4. Preoperative predictive factors

31 (80%) 6 (11%) 4 (10%)

Facial nerve grading is by the House-Brackmann criteria.11 One case of residual tumor detected on 2-year follow-up MRI is being observed.

SIGNIFICANCE OF PREOPERATIVE FACTORS IN PREDICTING HEARING RESULTS

To determine the significance of preoperative factors in predicting a successful hearing-preservation result, Fisher’s exact 2-tailed test17 was applied to the data. For the sake of simplicity and ease of analysis, the preoperative patient and tumor factors were grouped as shown in Table 4, and any measurable hearing 12 months after surgery was considered a hearing-preservation success. On the basis of this broad criterion for successful hearing preservation, the success rate was 18% (9 of 50 cases). With Fisher’s method of analysis the only preoperative factor that correlated with successful hearing preservation was ABR morphology. Recognizable ABR waveform with or without interpeak delay is more likely to be associated with hearing preservation than any other preoperative factor, and poor ABR morphology is strongly predictive of a poor outcome (P < 0.001). No other preoperative patient or tumor factor (PTA, tumor size and speech discrimination, age) carried a significant predictive value with regard to hearing preservation (P = 1.0, P = 0.66, P = 0.48, P = 0.56, respectively). These findings are at variance to those of other investigators who found that preoperative ABR morphology was not predictive of preserved hearing,18 smaller tumor size was related to a better outcome,19,20 and preserved preoperative hearing levels were correlated with better hearing outcomes.19-21

Tumor size <2.5 cm >2.5 cm Preoperative PTA <30 dB >30 dB SDS 100% <100% ABR morphology* Normal Abnormal Sex Male Female Age <30 yr >30 yr *The

Hearing preserved (n)

Dead ear (n)

Fisher’s exact test (P)

8 1

31 10

0.66

4 5

17 24

1.0

5 4

17 24

0.48

5 3

2 39

<0.001

7 2

24 17

0.45

0 12

4 34

0.56

only factor that has predictive value.

Table 5. Hearing-preservation rates of the Cambridge series Preoperative hearing

Postoperative hearing

Normal hearing (PTA < 30 dB/ SDS > 70%) Serviceable hearing (PTA < 50 dB/ SDS > 50%) Any hearing Total no.

Normal Serviceable hearing hearing (PTA < 30 dB/ (PTA < 50 dB SDS > 70%) SDS > 50%)

4.8%

—

14%

8%

19% 21

18% 50

The different rates have been calculated with preoperative and postoperative hearing criteria set by previous authors. The more stringent the preoperative hearing criteria applied to the series, the smaller the number of patients suitable for hearing-preservation surgery.

DISCUSSION

The aims of contemporary vestibular schwannoma surgery—low mortality rate (0%), complete tumor removal (98%), and preservation of near-normal facial nerve function (80% House-Brackmann grade I-II)— have been achieved in this series of patients. However, the hearing-preservation rate has been disappointing. With postoperative hearing criteria set by others for serviceable hearing, PTA < 50 dB/SDS > 50%,1,6 the hearing-preservation success rate is 8% (4 of 50 attempts). If more stringent reporting criteria are applied (PTA < 30 dB/SDS > 70 dB) the hearing-preser-

vation rate is 4.8% (1 of 21 cases). If broadened criteria are applied (any measurable hearing) the results rise to 18% (9 of 50 cases, Table 5). These success rates in broad terms, when uncontrolled for factors like tumor size, patient age, and preoperative hearing thresholds, were lower than the reported 25% to 58% hearing-preservation rates measured by the same criteria for surgery performed through the posterior fossa18,19 and middle fossa.22,23 One possible reason for the discrepancy in results is that the tumor size was small in the selection of cases in the

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series cited above. However, when tumors more than 2.5 cm in diameter are excluded from analysis in the Cambridge series, the serviceable hearing-preservation rate is still only 8% (3 of 39 cases with tumor diameter < 2.5 cm). Unlike facial nerve outcome, which is clearly related to tumor size,19,24 the relationship of tumor size and hearing preservation is controversial.25-27 Successful hearing preservation in moderate and large tumors, although less likely, is possible, as is illustrated by our series in which 1 case with a tumor diameter of 3.5 cm had serviceable postoperative hearing. In predictions of a favorable hearing result from preoperative factors, only ABR morphology correlates with hearing outcome, albeit the relationship is weak. One possible explanation for this relationship is the ability of ABR to assess the normal physiology of the cochlear nerve fibers. Pressure exerted by compact tumor in the IAC or stretching of the cochlear nerve in the cerebellopontine angle may reversibly affect neural function and result in increased latencies but preserved waveform morphology. This physiologic interruption to nerve conduction may correlate with better hearing outcomes once the tumor is removed. In contrast, tumor infiltration of the cochlear nerve fibers28-30 results in irreversible loss of neural fibers and is represented with poor ABR morphology and consequently a poor hearing-preservation outcome. This supports the observation that hearing-preservation rates in neurofibromatosis cases are poorer than in solitary vestibular schwannoma surgery where the tumors are known to diffusely infiltrate the auditory nerve.31,32 Surprisingly, maintained preoperative hearing thresholds and SDSs do not predict a favorable hearingpreservation outcome (Fisher’s exact 2-tailed test P = 1 and P = 0.48, respectively). This finding may reflect the complex interactions of factors involved in auditory pathway function, which are disrupted by the presence of vestibular-cochlear nerve schwannoma and surgery. However, because hearing thresholds rarely improve after surgery and there is a decline in thresholds after successful hearing-preservation surgery,33,34 maintained preoperative hearing levels are more likely to be associated with socially preserved hearing. IS PRESERVED HEARING SOCIALLY USEFUL?

The contribution a unilateral retrocochlear hearing loss makes to overall audition has yet to be evaluated. It has been suggested that in the setting of unilateral vestibular schwannoma surgery an IAD of 25 dB or less is required for effective binaural audition.35 Alternatively, an estimate of the effect of unilateral hearing loss on overall hearing can be developed from the experiences

gained from treating conductive hearing loss where an IAD of greater that 10 to 15 dB36 will impair effective binaural hearing. If these guidelines are followed and a postoperative IAD of 25 dB is used as a measure of successful socially useful hearing preservation, the effective hearing-preservation rate is 2% (1 of 50 cases). Audition is a complicated process, and this simple parameter of IAD does not take into consideration the contribution residual hearing makes to sound localization37 and to speech discrimination in noise,38 problems patients commonly report after severe unilateral hearing loss. Further issues such as the long-term preservation of initially successful hearing33,34 and the slight risk of residual tumor (2%) when compared with translabyrinthine approaches to tumor removal need to be taken into account when hearing-preservation surgery is planned. Under these conditions hearing-preservation surgery through the posterior fossa approach has little to recommend it. However, “star” hearing-preservation patients do occur, and it seems that hearing, on rare occasions, can improve after tumor removal,13,39,40 although predicting these results before surgery is problematic. Given these limitations, what then constitutes reasonable indications for attempting hearing-preservation surgery? Absolute indications include clinical situations in which any measurable hearing would be useful: surgery in an only hearing ear or in the situation of bilateral acoustic tumors where total hearing loss in time seems inevitable, although the reported hearingpreservation results with neurofibromatosis are worse than with sporadic vestibular schwannoma surgery.31,32 Relative indications include preserved preoperative hearing (PTA < 30 dB/SDS > 70%) particularly when accompanied by recognizable ABR morphology and small tumor size. The emergence results of hearing preservation in middle fossa surgery for small, laterally placed vestibular schwannoma appear encouraging.41-43 CONCLUSION

The success rate of hearing-preservation vestibular schwannoma surgery when performed through the retrosigmoid approach has been calculated with audiometric criteria set by others.3-5 The rate of serviceable hearing (PTA < 50 dB/SDS > 50%) is 8%, which is lower than that of other series that have used the same reporting criteria. In the context of socially useful hearing (PTA < 30 dB/SDS > 70%) the hearing-preservation rate is 4.8%. Preoperative ABR morphology is the only significant factor that correlates with hearing outcome. There was a 2% residual tumor rate in this series, which needs to be borne in mind when planning posterior fossa hearing-preservation surgery. These results suggest that there are limited indications for hearing-

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preservation surgery through the posterior fossa approach at its present state of refinement. Hearing preservation still remains an elusive goal after surgery for vestibular schwannoma in its present state of refinement. Few patients are suitable candidates for attempted hearing-preservation surgery through the posterior fossa, and predicting a favorable outcome is problematical. The issues highlighted in this discussion need to be borne in mind when treating patients with vestibular schwannoma and discussed openly when counseling patients before recommending hearingpreservation surgery. REFERENCES 1. Wade PJ, House W. Hearing preservation in patients with acoustic neuromas via the middle fossa approach. Otolaryngol Head Neck Surg 1984;92:184-93. 2. Silverstein H, McDaniel A, Norrell H, et al. Hearing preservation after acoustic neuroma surgery with intraoperative direct eighth cranial nerve monitoring: part II. A classification of results. Otolaryngol Head Neck Surg 1986;95:285-91. 3. Cohen NL, Lewis WS, Ransohoff J. Hearing preservation in cerebellopontine angle tumor surgery: the NYU experience 19741991. Am J Otol 1993;14:423-33. 4. Gardner G, Robertson JH. Hearing preservation in unilateral acoustic neuroma surgery. Ann Otol Rhinol Laryngol 1988;97: 55-66. 5. Sanna M, Karmarkar S, Landolfi M. Hearing preservation in vestibular schwannoma surgery: fact or fantasy? J Laryngol Otol 1995;109:374-80. 6. Haines SJ, Levine SC. Intracanalicular acoustic neuroma: early surgery for preservation of hearing. J Neurosurg 1993;79:51520. 7. Cohen NL. Retrosigmoid approach for acoustic tumor removal. Otolaryngol Clin North Am 1992;25:295-310. 8. Kartush JM, Telian SA, Graham MD, et al. Anatomic basis for labyrinthine preservation during posterior fossa acoustic tumor surgery. Laryngoscope 1986;96:1024-8. 9. Recommended procedures for pure-tone audiometry using a manually operated instrument. Br J Audiol 1981;15:213-6. 10. Boothroyd A. Developments in speech audiometry. Sound 1968; 2:3-10. 11. House JW, Brackmann DE. Facial nerve grading system. Otolaryngol Head Neck Surg 1985;93:146-7. 12. Moffat DA, Phelps PD, Hardy DG. Magnetic resonance imaging following vestibular schwannoma excision. In: Sterkers JM, Charachon R, Sterkers O, editors. Acoustic neuroma and skull base surgery. Amsterdam: Kugler Publications; 1996. p. 75-80. 13. Sterkers JM, Sterkers O, Maudelonde C, et al. Preservation of hearing by the retrosigmoid approach in acoustic neuroma surgery. Adv Otorhinolaryngol 1984;34:187-92. 14. Katz J. Handbook of clinical audiology. Baltimore: Williams and Wilkins; 1978. 15. Hall JW III. Handbook of auditory evoked responses. Needham Height (MA): Allyn and Bacon; 1992. 16. Committee on Hearing and Equilibrium. Committee on Hearing and Equilibrium guidelines for the evaluation of hearing preservation in acoustic neuroma (vestibular schwannoma). Otolaryngol Head Neck Surg 1995;113:179-80. 17. Altman DG. Practical statistics for medical research. London: Chapman and Hall; 1996. 18. Kemink JL, LaRouere MJ, Kileny PR, et al. Hearing preservation following suboccipital removal of acoustic neuromas. Laryngoscope 1990;100:597-602. 19. Nadol JB Jr, Levine R, Ojemann RG, et al. Preservation of hearing in surgical removal of acoustic neuromas of the internal audi-

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Appendix 1. Raw data Case no.

45 56 58 62 87 101 117 134 159 160 161 171 176 184 188 198 203 204 214 215 219 229 238 239 247 249 258 262 270 271 273 283 300 325 334 338 342 346 347 363 374 375 403 409 416 420 443 444 445 446

Age at operation (yr)

Tumor size (cm)

49 36 33 48 48 62 40 68 13 56 32 40 39 56 53 40 52 65 38 48 47 59 40 42 65 42 51 32 48 55 40 20 58 45 45 50 47 53 47 44 61 34 30 48 23 46 48 53 47 37

<1.5 <1.5 2.5-3.4 <1.5 1.5-2.4 <1.5 <1.5 1.5-2.4 <1.5 <1.5 <1.5 1.5-2.4 <1.5 >4.5 3.5-4.4 <1.5 <1.5 1.5-2.4 <1.5 <1.5 <1.5 1.5-2.4 2.5-3.4 2.5-3.4 <1.5 3.5-4.4 1.5-2.4 <1.5 1.5-2.4 1.5-2.4 1.5-2.4 3.5-4.4 1.5-2.4 2.5-3.4 2.5-3.4 2.5-3.4 <1.5 1.5-2.4 1.5-2.4 1.5-2.4 <1.5 3.5-4.4 <1.5 1.5-2.4 <1.5 1.5-2.4 1.5-2.4 1.5-2.4 1.5-2.4 1.5-2.4

PTA at SDS at presentation (dB) presentation (%)

23 44 1 40 10 49 23 24 40 46 21 21 35 20 41 31 43 31 48 50 46 31 46 13 29 20 29 8 21 44 40 33 33 34 19 23 36 33 21 35 30 10 21 46 20 33 39 40 36 34

95 100 100 100 100 85 100 90 92 80 100 85 90 100 100 100 80 100 95 95 100 95 90 100 90 100 80 100 100 90 100 95 75 95 100 100 85 100 100 95 95 100 95 80 95 100 95 95 90 90

PTA at 12 mo (dB)

SDS at 12 mo (%)

Facial nerve grade at 12 mo

140 56.25 36.25 140 140 140 140 140 140 140 36.25 140 140 140 140 56.25 140 140 140 140 140 140 140 140 63.75 140 140 18.75 140 140 140 140 140 140 140 140 140 140 140 43.75 140 140 140 140 140 140 63.75 92.5 140 140

— 90 100 — — — — — — — 100 — — — — 80 — — — — — — — — 40 — — 100 — — — — — — — — — — — 65 — — — — — — 70 20 — —

1 1 1 2 4 1 1 3 1 1 1 2 2 3 3 1 6 1 2 1 1 2 2 2 1 3 3 1 4 1 1 1 1 3 2 1 2 1 1 1 1 6 1 1 1 1 1 1 1 1

Tumors size is the greatest diameter seen on axial CT or MRI. PTAs were calculated by use of the 4 frequencies 500 Hz, 1 kHz, 2 kHz, and 4 kHz. SDSs were measured with standard word lists (Boothroyd6). Facial nerve outcomes were documented with the House-Brackmann grading system.