53.1 Introduction

The three main benchmarks for evaluating treatment outcomes for vestibular schwannoma (VS) are facial nerve function, tumor control, and hearing preservation. The first two outcomes take precedence and are discussed elsewhere in this book. Of the three classic approaches to VS microsurgery, the retrosigmoid/suboccipital (RS/SO) and middle cranial fossa approaches (MCF) are the two that confer the possibility of hearing preservation while the translabyrinthine approach inherently eliminates this possibility. The widespread use of magnetic resonance imaging (MRI) has resulted in the diagnosis of VS much earlier than in the past, and now patients with excellent hearing routinely present for treatment. In these cases—patients with smaller tumors and good hearing—hearing preservation has become a realistic treatment goal.

A critical analysis of the hearing preservation literature is challenged by inconsistent reporting of pre- and postoperative hearing levels, tumor size, variable inclusion criteria, treatment selection biases, heterogeneous study populations, and limited or inconsistent follow-up. The Committee on Hearing and Equilibrium from the American Academy of Otolaryngology—Head and Neck Surgery (AAO–HNS) adopted a standard for reporting hearing outcomes in 1995, but there were obvious shortcomings even with this original consensus guideline which ultimately led to the latest modification by the academy in 2012.s. Literatur ^,​ s. Literatur Although this recently modified standard for reporting should help provide uniformity in future clinical studies, it cannot be readily applied to the older literature since the raw data on individual subjects are typically unavailable in most of these studies. Thus, for the purpose of comparing literature in this chapter, the AAO–HNS classification scheme from 1995 will be utilized as often as possible when discussing the results of prior studies.

The primary objective of the current chapter is to review hearing preservation outcomes—short-term and long-term—in patients who have undergone attempted hearing preservation microsurgery with either the RS/SO or MCF approach for sporadic VS. The secondary objective is to examine preoperative and intraoperative factors that influence hearing outcomes in hearing preservation surgery. Hearing outcomes with conservative management and radiation therapy are described separately, in Chapters 51 and 52, respectively.

53.2 Terminology in Hearing Outcome Data

Patients may have unchanged, worse, or, rarely, better hearing after microsurgical resection. One of the confusing aspects in the hearing preservation literature is the usage of different phrases to describe the hearing status of patients. It is not uncommon in the literature for hearing outcomes to be described by ambiguous terms, such as “good hearing,” “useful hearing,” or “functional hearing,” with definitions that vary according to different authors. In 1984, Wade and House described “serviceable hearing” as a pure-tone average (PTA) less than or equal to 50 dB and a word recognition score (WRS) as greater than or equal to 50%.s. Literatur This became known as the “50/50 rule” when assessing the hearing status of patients prior to pursuing a hearing preservation surgical intervention. As the name “serviceable hearing” suggests, patients within this category could potentially benefit from amplification. In 1995, the AAO–HNS took this into account when creating a four-tiered classification scheme for evaluating hearing status in patients with VSs.s. Literatur Specifically, the 1995 AAO-HNS VS reporting guidelines divided hearing status into the following four classes:

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  Class A: PTA ≤ 30 dB and WRS ≥ 70%.

  
  
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  Class B: PTA > 30 dB and ≤ 50 dB and WRS ≥ 50%.

  
  
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  Class C: PTA > 50 dB and WRS ≥ 50%.

  
  
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  Class D: PTA any level and WRS < 50%.

Thus, patients with class A or class B hearing are considered to have “serviceable hearing” and patients in classes C and D have “nonserviceable hearing.” However, it is conceivable that patients in class C could also benefit from amplification if their word recognition scores are high enough. Clearly, the value of any retained hearing is also strongly influenced by the status of the hearing in the contralateral ear.

The phrase “measurable hearing” is also vague and is defined differently by different authors.s. Literatur ^,​ s. Literatur In most settings, “measurable hearing” entails nonserviceable hearing with PTA scores that are quantifiable according to the upper output limits of commercial audiometers (i.e., <125 dB HL), along with a word recognition score of any value. In particular situations, “measurable hearing” that is not serviceable may still be very valuable to the patient, for example, preservation of class D hearing in the only hearing ear.

The degree of hearing preservation is exceedingly important to decipher from each study. Some studies are cleverly worded to embellish results and conclusions. For example, the percentage of patients who obtained “serviceable hearing” postoperatively reflects a different degree of hearing preservation when compared to a study that examined “measurable hearing” as its criterion for hearing preservation. Thus, the phrase “hearing preservation” must be analyzed carefully before drawing conclusions. In this chapter, the focus is on preserving serviceable hearing, which is categorized as class A or B hearing according to the 1995 AAO–HNS scale. This method of categorization was used to summarize the existent literature, since the 2012 AAO–HNS guidelines have only been in place for the last 6 years. This cut point is essentially equivalent to grade I or II hearing in the Gardner–Robertson scale. Further discussion regarding hearing outcome reporting guidelines can be found in Chapter 50.

53.3 Short-Term Hearing Results after the Middle Cranial Fossa Approach

In 1961, William F. House presented the middle fossa craniotomy technique to expose the contents of the internal auditory canal as his thesis to the Triologic Society.s. Literatur Since then, multiple surgical groups have employed the MCF approach to resect VSs and preserve hearing. Successful hearing preservation using this approach has varied widely from 20 to 85%.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur However, inconsistent study parameters in these reports make it incredibly challenging to compare hearing outcome data from one study to that of the next. The relevant studies along with key study criteria have been listed in Table 53‑1 . Some of these studies have been discussed further below.

`Wade and House published their experience with the MCF technique from 1975 to 1979.s. Literatur Out of approximately 300 patients operated for VS resection, 20 patients underwent a MCF approach with the intent of hearing preservation. In this article, the authors proposed the “50/50 rule” as the definition of serviceable hearing. From their experience, they also identified the ideal candidate as one who has a PTA of ≤ 30 dB and a WRS ≥70% (i.e., AAO–HNS class A) along with a tumor that is less than 1.5 cm in its maximal dimension. All 20 patients in their study had “serviceable hearing” (class A or B) preoperatively and 17 of the 20 met the proposed ideal criteria. The average tumor size was 13 mm. Of these 20 patients, 6 patients retained serviceable hearing postoperatively (30%). One patient had class C hearing postoperatively and the other 13 patients had no measurable hearing postoperatively.

In 1998, Irving et al compared the hearing results for patients who underwent either the MCF approach or RS approach for similar size VSs over a 9-year period of time.s. Literatur In addition to being divided by surgical approach, hearing outcome data were stratified by tumor size into the following categories: intracanalicular, 1 to 10 mm of cerebellopontine angle (CPA) extension, and 11 to 20 mm of CPA extension. The CPA component was determined from the axial MRI that demonstrated the tumor’s maximum diameter parallel to the petrous ridge and the maximum diameter perpendicular to the petrous ridge. Most of the patients in both surgical groups had serviceable hearing (class A or B) preoperatively. In the intracanalicular group, 46% of the MCF patients preserved serviceable hearing, while only 13% of the RS patients preserved serviceable hearing. In the group with minimal CPA extension (1–10 mm), 70% of the MCF patients preserved serviceable hearing, while only 27% of the RS cohort preserved serviceable hearing. In the group with larger CPA extension (11–20 mm), 20% of the MCF patients preserved serviceable hearing and 13% of the RS patients preserved serviceable hearing. However, the sample size in this “larger CPA extension” group was too small to make meaningful conclusions. The authors supported using the MCF approach over the RS approach for intracanalicular tumors and for tumors with a CPA component measuring 10 mm or less. Of note, facial nerve function was noticeably worse in the MCF group immediately after surgery. However, after 1 year, the facial nerve function results were similar for both groups.

Brackmann et al published a more extensive dataset in 2000 that reviewed 333 patients operated via a MCF approach at the House Ear Institute from 1992 to 1998.s. Literatur In this study, patients with neurofibromatosis type 2 (NF2) were excluded. The mean tumor size was 11.2 mm. One important classification they clarified in their study was the difference between “measurable hearing” and “no measurable hearing.” “Measurable hearing” was defined as a PTA ≤ 90 dB and any WRS. “No measurable hearing” was defined as a PTA greater than 90 dB and any WRS. Three-hundred of the 333 patients had class A or B hearing preoperatively. Postoperatively, 188 patients preserved class A or B hearing, providing a 63% overall rate of serviceable hearing preservation. Fifty percent of patients preserved their hearing at or near their preoperative baseline levels, which was defined as a change within 15 dB of their preoperative PTA and within 15% of their preoperative WRS. Eight patients improved from class C or D to class A or B. Overall, 5% improved their hearing classification, which included patients who improved from classes D to C and those who improved from classes B to A. Seventy-six percent of patients had some degree of measurable hearing postoperatively.

In 2001, Gjurić et al from Germany published their experience with the MCF approach over a 23-year period (1975–1998).s. Literatur Seven hundred thirty-five patients were included in this study. Twenty-two percent had intracanalicular tumors, 42% had tumors with less than 1 cm of extension into the CPA, 29% had tumors with 1 to 1.9 cm of extension into the CPA, and 7% had greater than 2 cm of extension into the CPA. Of the patients with preoperative serviceable hearing and intracanalicular tumors, 61% maintained serviceable hearing after surgery. Two patients improved from class C or D to class A or B. The authors demonstrated that as tumor size increases, the rate of serviceable hearing preservation decreases (see Table 53‑1 ). In addition, none of their patients with tumors that extended 1.0 cm or greater into the CPA demonstrated improvement of hearing from the class C or D stage to class A or B. The authors advocated for early diagnosis and treatment because the best outcomes are seen in patients with smaller tumors and better preoperative hearing status.

In 2006, Meyer et al published their results on 162 patients who underwent the MCF approach for VS resection with the intent of hearing preservation.s. Literatur These surgeries were performed over the course of 11 years (1993–2004); the latter portion of their study involved direct cochlear nerve action potential (CAP) monitoring. Patients with NF2 were excluded. The majority of their patients had serviceable hearing preoperatively, and after surgery 57% maintained serviceable hearing. One of the important features of this study was the analysis of hearing outcome data as a function of tumor size, as shown in Table 53‑1 . Their data demonstrated improved hearing preservation results with CAP monitoring and smaller tumors.

In 2006, Arts et al reported their results on 73 consecutive patients who underwent the MCF approach for VS resection over a span of 6 years (1999–2005), including three patients with NF2.s. Literatur The mean tumor size was 8.9 mm with a range between 3 and 18 mm and the vast majority of patients had serviceable hearing preoperatively. Postoperatively, 73% of patients maintained serviceable hearing. Interestingly, none of their patients improved their hearing status from class C or D to class A or B and as tumor size increased beyond 8 mm, hearing outcomes noticeably declined.

53.4 Short-Term Hearing Results after the Retrosigmoid Approach

In 1954, Elliott and McKissock were the first to report successful hearing preservation after VS surgery in three patients, all performed via a RS/SO approach.s. Literatur Although they did not describe the level of hearing pre- or postoperatively, this marked the inauguration of hearing preservation surgery. Since this time, reported hearing preservation rates utilizing the retrosigmoid approach have ranged between 0 and 58%.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur However, as with the MCF literature, there is great variability in outcome reporting and in what constitutes hearing preservation. Table 53‑2 summarizes the pertinent studies that investigated hearing preservation via the RS approach. A few of these studies have been selected for more detailed discussion.

In 1984, Harner et al reported their results on 149 patients (151 ears) with the RS approach including 119 patients who had at least measurable hearing preoperatively.s. Literatur Postoperatively, 7% of their patients had serviceable hearing. Their study does not allow one to make a comment on hearing preservation success, as the preoperative hearing status for the 119 patients was not detailed. In 1984, Ojemann et al examined 22 patients who underwent the RS approach for tumor resection and hearing preservation. Of the 20 patients with serviceable hearing preoperatively, 6 retained serviceable hearing following surgery, providing a 30% rate of serviceable hearing preservation. Of note, this study utilized only WRS and did not include PTA in evaluating hearing outcomes.s. Literatur Gardner and Robertson performed a thorough literature search in the late 1980s and discovered 17 articles that reviewed a total of 621 operated cases.s. Literatur However, there were only 394 patients for whom pre- and postoperative audiometry was documented. Of these, 131 patients had preserved hearing after surgery, providing a 33% rate of measurable hearing preservation.

In 1990, Kemink et al reported their results on 20 patients who underwent hearing preservation RS surgery. One of their patients underwent planned subtotal resection, one had nonserviceable hearing prior to surgery, and three other patients had NF2. Of the remaining 15 sporadic VS with preoperative serviceable, 9 retained serviceable hearing postoperatively providing a 60% rate of serviceable hearing preservation. They noticed that neither serviceable nor measurable hearing was preserved in patients with tumors 1.5 cm or larger.s. Literatur

In 1995, Post et al reported their results on 56 patients who underwent the RS approach with the intention of total tumor resection and hearing preservation.s. Literatur Tumors were measured in terms of their extrameatal extension into the CPA and the intracanalicular portion of the tumor was intentionally not measured. Forty-six patients had serviceable hearing preoperatively and 41% retained serviceable hearing after surgery. Notably, in patients who had serviceable hearing and tumors smaller than 1 cm, the rate of serviceable hearing preservation was 83% (5 out of 6 patients). If the tumor was less than 2 cm, the authors maintained serviceable hearing in 52% of patients (16 out of 31 patients). Finally, when the extrameatal component was greater than 2 cm, the rate of serviceable hearing preservation was 21% (3 out of 14 patients).

In 1997, Samii and Matthies published their results using the RS approach in 1,000 VS resections including 880 patients with sporadic VSs and 82 with NF2.s. Literatur Of these 1,000 tumors, 268 ears were profoundly deaf prior to surgery and 732 ears had at least some measurable hearing preoperatively. Of these 732 ears, 289 had some degree of measurable hearing preserved postoperatively (40% rate of measurable hearing preservation). Of these 289 ears with measurable hearing postoperatively, 140 had enough hearing data to be classified by the 1995 AAO–HNS criteria. Within this subgroup, 56 retained class A or B postoperatively, conferring a 40% rate of serviceable hearing. Although this study has one of the largest patient populations in the hearing preservation literature, its lack of thoroughly evaluating pre- and postoperative hearing status of patients makes it very difficult to utilize and compare their results to those of other studies. In particular, the single most obvious deficiency is the lack of adequate WRS data. Only 76% of patients had preoperative WRS reported. Postoperatively, WRS data were available in only 50% of patients who had “some hearing” after surgery.

To investigate the chance of preserving hearing in patients who have serviceable hearing but also have medium to large tumors, Yates et al performed at retrospective study in 2003. They examined 64 of their patients who had serviceable hearing and underwent the RS approach to resect tumors that were at least 15 mm in size in the CPA. Overall, they had a 6% rate of serviceable hearing preservation. Tumors in which the CPA component measured 15 to 19 mm in size had a 17.6% chance of serviceable hearing preservation. As tumor size increased, the rate of serviceable hearing preservation declined and was 0% for patients who had tumors 25 mm and larger.s. Literatur