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A Video Guide

Mustafa K. BaskayaG. Mark PyleJoseph P. Roche

Vestibular Schwannoma Surgery

Vestibular Schwannoma Surgery

Mustafa K. Baskaya • G. Mark Pyle Joseph P. Roche

Vestibular Schwannoma SurgeryA Video Guide

Mustafa K. BaskayaDepartment of Neurological SurgeryUniversity of Wisconsin School of Medicine and Public HealthMadison, WIUSA

Joseph P. RocheDepartment of Surgery, Division of Otolaryngology-Head and Neck Surgery Section of Otology/NeurotologyUniversity of Wisconsin School of Medicine and Public HealthMadison, WIUSA

G. Mark PyleDepartment of Surgery, Division of Otolaryngology-Head and Neck Surgery Section of Otology/NeurotologyUniversity of Wisconsin School of Medicine and Public HealthMadison, WIUSA

ISBN 978-3-319-99297-6 ISBN 978-3-319-99298-3 (eBook)https://doi.org/10.1007/978-3-319-99298-3

Library of Congress Control Number: 2018958876

© Springer Nature Switzerland AG 2019This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed.The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use.The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

This Springer imprint is published by the registered company Springer Nature Switzerland AGThe registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

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Foreword

Among the most challenging lesions and surgeries that neurosurgeons face throughout their training and early years of practice, some stand out as particularly formidable and daunting. Petroclival meningiomas, giant aneurysms, large arteriovenous malformations, and high-flow interposition bypasses are examples that come to mind. After all, these are the quintessential cases where drama is assured, whether in triumph or disaster. There is nothing more reinforcing to the learning process than the vivid memories of the surgical battlefield during residency. Perhaps most zealous residents look forward to scrubbing on these surgeries. They certainly may be eager to participate, particularly if cerebrovascular or skull base surgery is in their future, but I doubt very much that even the most overconfident among them would have the courage (or misguided desire!) to want to be a primary surgeon on any such cases.

I believe vestibular schwannomas have earned the right to join this elite group of intimidating lesions. I concede that, unless they are giant in size, they do not inspire quite the same emotions of fear and trepidation in the young surgeon that a large aneurysm might. After all, the dreaded complication of transecting the facial nerve during tumor dissection, while devastating to the patient, still happens in an instant, with a slip of a blade or an excess of traction. Yet it happens without much drama. It happens in silence, often with no gradual change in intraoperative monitoring, without a flood of blood or cardiorespiratory instability. It does not generate a sustained flurry of adrenaline-charged activity by everyone in the room. It easily goes unnoticed by all present, even the surgeon initially. Equally disappointing but less consequential is the sudden loss of auditory waves, and there evaporates the hope of hearing preservation. But it is precisely the anticlimactic nature of these complications, their instantaneous occurrence and their irreversible impact on patient satisfaction and quality of life, that should earn the vestibular schwannoma a deserved place in the pantheon of treacherous neurosurgical lesions. It simply should not be tackled by the young surgeon who has not spent enough apprenticeship time with “deep observation” of his/her mentor, followed by sequential and cumulative time acquiring the judgment and surgical skills required to tame this lesion without creating collateral injury. When I reflect on my own training and early years in practice, and I am asked which surgical procedure was associated with a substantial learning curve well beyond my fellowship time, I do not hesitate to respond: resecting a vestibular schwannoma.

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Unless the young surgeon performs the surgery for him/herself, over and over again, it is simply quite difficult for even the most skilled of surgical educators to convey in words or through demonstration to that young surgeon all the surgical subtleties required for surgical success. Indeed, the surgical pearls are not obvious; and there are many. Even discounting the entire complex topic of surgical indications, the balance of wait-and-scan versus radiosurgery versus surgical resection, the timing of treatment, and other popular debate-generating controversies, the steps involved in the surgical procedure itself are difficult to completely master quickly. A simple deviation from proper patient positioning, ignoring the importance of venous return, can result in cerebellar edema even before opening the dura. Placing the craniotomy a touch too high to access the lateral recess of the cisterna magna forces the surgeon to manipulate and retract the cerebellum to access CSF spaces, initiating a vicious cascade of cerebellar contusion and further worsening of tumor exposure. Tearing an out-of-field superior petrosal vein because one did not look for it early almost guarantees another cascade of bleeding, swelling, and venous contusions. The pitfalls abound at every step. But of all the skills needed for a great surgical outcome, the hardest one to teach is the mastery of the arachnoidal plane of dissection (a double plane in fact), which then leads to an intimate adhesion plane with the facial and/or cochlear nerve. It is the lack of confidence in recognizing, developing, manipulating, and maintaining the momentum in this plane that distin-guishes the novice from the master surgeon. It is these acquired skills—the intimate familiarity with this plane, the sixth sense of where the thinned out facial nerve fibers are heading, the prediction of how much traction is too much—that allow the experienced surgeon to resect in 30 min a tumor that easily takes a fully trained but relative novice surgeon 4 h to do. When I think of which tumor in neurosurgery exemplifies the greatest variability in surgical operative time among different opera-tors, vestibular schwannoma is the uncontested grand champion. A simple visit to different operating rooms around the globe will easily demonstrate these extremes. There are centers where masters run 3–4 simultaneous rooms all filled with sizeable vestibular schwannomas, and are done by 3 p.m. And there are centers where well-trained but younger surgeons will spend the whole day, evening, and sometimes the entire night removing one tumor. The learning curve is indeed long and steep.

It is precisely because of these and other considerations that I enthusiastically welcome the addition of this fantastic book to the list of rare, practical, truly educational, and surgically minded publications, which demonstrate and celebrate surgical skill and subtlety. Mustafa K. Baskaya is a master neurosurgeon who has accumulated extensive experience in cerebrovascular and skull base surgery. He has ridden the learning curve and has been coasting for some time. He has surrounded himself by a great team of collaborators in neurosurgery and neuro-otology at the University of Wisconsin in Madison and has distilled their years of tackling vestibular schwannomas into this remarkable publication. As the consummate student (and teacher) of neuroanatomy, and all-around accomplished surgeon and scholar, which he has always been, since his resident days in our program at the University of Miami, Dr. Baskaya includes video cadaveric demonstrations of all three basic surgical approaches: retrosigmoid, translabyrinthine, and middle fossa.

Foreword

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These accompany very well-written chapters and several surgical well-edited video examples of cases. A special chapter is included on clinical features and the physiology of intraoperative monitoring. Another chapter covers some complex lesions that required specific considerations.

Throughout the book (text and videos), mastery is on display. Subtleties in technique are constantly highlighted. Surgical moves are measured and carried out in confidence. Mastery in anatomy blends with and reinforces mastery in surgery. The technical steps demonstrate, over and over again, why knowledge well rooted in neuroanatomy is a prerequisite for proper surgical technique. This book will be of immense benefit to all neurosurgical learners interested in accelerating their curve in vestibular schwannoma surgery. I encourage the reader to watch the videos with “deep observation,” and re-watch with an analytical mind. Question yourself. Make sure you understand each move. You will indeed become a better surgeon, less prone to inadvertent moves, more cognizant of pitfalls that characterize this deceiving pathology.

For all this, I congratulate Drs. Baskaya, Pyle, and Roche and their team for a superb publication.

Department of Neurological Surgery University of Miami Miller School of Medicine Miami, FL, USA Jacques J. Morcos

Foreword

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Preface

The treatment of vestibular schwannomas has undergone numerous changes and advancements in the past century. Innovations in surgical and now microsurgical techniques allow for the removal of these lesions with the preservation of nearby cranial nerves in the majority of cases. In addition, the development of stereotactic radiation can provide disease control in a minimally invasive fashion. Further, improved understanding of the natural history of these tumors and the proliferation of cross-sectional imaging techniques allow the safe observation of selected lesions. Lastly, molecular and targeted therapies hold the promise to both treat these tumors and restore lost functions as a result of tumor growth and/or treatment. However, despite the increasing use of nonsurgical management strategies, surgery nonetheless remains an important tool in the treatment armamentarium for physicians and surgeons.

Advances in and the widespread use of audiovisual technology have made a significant impact in teaching surgical techniques in general and more specifically in teaching skull base surgery. Among all the surgeries that skull base surgeons perform on a daily basis, the surgery of vestibular schwannomas or so-called acoustic neuromas is among the most challenging for trainees, fellows, and young surgeons to learn and obtain experience with. This is because the hands-on training for these surgeries may be limited due to concerns for cosmetic outcome factors including hearing and facial function. Therefore, there is a need for improved materials to teach these surgeries. With audiovisual observation, trainees, fellows, and young surgeons can observe basic and complex surgeries. This operative audiovisual atlas, which we believe is the first of its kind, is intended to accomplish this pedagogical goal by enabling its viewers and readers to observe the step-by- step techniques of vestibular schwannoma surgery.

This book and video series was conceived and designed to provide both in-training and practicing neurosurgeons and neurotologists a complete educational reference on the surgical treatment of vestibular schwannomas. The cadaveric and human illustrations, in conjunction with video dissections, demonstrate the exposure and extirpative techniques utilized in contemporary skull base microsurgery. While other textbooks, articles, and videos are available as individual resources, this Video Guide to Vestibular Schwannoma Surgery provides a comprehensive collection of reference materials and video demonstrations involving all aspects of operative vestibular schwannoma management. It is the hope of the authors that readers will

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gain a detailed understanding of both the science and the techniques involved in the contemporary surgical management of vestibular schwannoma patients.

We thank our families for their continuous support and are grateful to our acous-tic neuroma patients and their families for their trust in us.

Madison, WI Mustafa K. Baskaya Madison, WI G. Mark Pyle Madison, WI Joseph P. Roche

Preface

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1 Acoustic Neuromas: General Considerations. . . . . . . . . . . . . . . . . . . . . 1Ihsan Dogan, Burak Ozaydin, Joseph P. Roche, and Mustafa K. Baskaya

2 Vestibular Schwannomas: Neurology, Neurophysiology and Anatomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Sima Sayyahmelli, Lucas Leonhard, Burak Ozaydin, and Joseph P. Roche

3 Middle Fossa Approach to Vestibular Schwannomas . . . . . . . . . . . . . . 59Ihsan Dogan, Omer S. Sahin, and Joseph P. Roche

4 Retrosigmoid Approach for Vestibular Schwannoma Surgery . . . . . . . 105Pınar Eser Ocak, Ihsan Dogan, Sima Sayyahmelli, and Mustafa K. Baskaya

5 Translabyrinthine Approach to Vestibular Schwannomas . . . . . . . . . . 135Burak Ozaydin, Melih Ucer, Ulas Cikla, and G. Mark Pyle

6 Complex Vestibular Schwannomas: Case Illustrations of Surgical Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151Mustafa K. Baskaya, Burak Ozaydin, Burcak Soylemez, and Cem Dinc

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

Contents

1© Springer Nature Switzerland AG 2019M. K. Baskaya et al., Vestibular Schwannoma Surgery, https://doi.org/10.1007/978-3-319-99298-3_1

1Acoustic Neuromas: General Considerations

Ihsan Dogan, Burak Ozaydin, Joseph P. Roche, and Mustafa K. Baskaya

AbstractVS account for approximately 6–8% of intracranial tumors in adults and are the most common tumor in the CPA ranging between 80% and 95%. Vestibular schwannomas constitute 6% of all intracranial neoplasms and are the most common benign lesions of the IAC and CPA cistern constituting between 60% and 90% of the entire lesions seen in this area. Growth rates in tumors that exhibit enlargement have been reported to be about 1–2 mm per year in linear growth in at least one dimension. While no formal consensus exists, most authors define normal growth rates as being between 1 and 2 mm per year and fast growth rates as 4 mm or greater per year. When considering common pre-senting manifestations, a helpful framework is to consider manifestations based on the size of the tumor, and its location; intracanalicular, cisternal, brainstem compressive, or hydrocephalic sizes. While any configuration is pos-sible, hearing loss associated with vestibular schwannomas of this size is typi-cally unilateral, progressive, and commonly affects high-frequency thresholds and speech perception. An occipital headache attributable to the tumor is a late

I. Dogan · B. Ozaydin · M. K. Baskaya (*)Department of Neurological Surgery, University of Wisconsin School of Medicine and Public Health, Madison, WI, USAe-mail: baskaya@neurosurgery.wisc.edu

J. P. RocheDepartment of Surgery, Division of Otolaryngology-Head and Neck Surgery, Section of Otology/Neurotology, University of Wisconsin School of Medicine and Public Health, Madison, WI, USA

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finding with 20% of patients with tumors between 1 and 3 cm, with 40% of patients with tumors larger than 3  cm verifying this symptom. Gross total resection is the goal of surgery while maintaining good facial function, in all sizes of tumors, and hearing preservation, in small to moderate size tumors in patients with serviceable preoperative hearing status.

KeywordsVestibular schwannoma · Acoustic neuroma · Cerebellopontine angle Retrosigmoid approach · Translabyrinthine approach · Middle fossa approach Internal acoustic canal

Introduction

Vestibular schwannomas (VS) are benign, extra-axial, encapsulated, Schwann cell derived neoplasms of the vestibular part of the eighth cranial nerve. Although some earlier reports indicated that the superior vestibular nerve is the more common ori-gin of VS, recent studies suggest otherwise. Sanna et al. found that the most com-mon origin for VS are the nerve sheath of the inferior vestibular nerve, and less frequently the superior vestibular nerve [1]. Eighth cranial nerve schwannomas rep-resent 90% of all intracranial nerve sheath tumors [2]. VS account for approxi-mately 6–8% of intracranial tumors in adults [3, 4] and are the most common tumor in the cerebellopontine angle (CPA) ranging between 80% and 95% [5, 6], followed by meningiomas ranging between 5% and 10% and epidermoid tumors ranging between 4% and 7% [7–9]. There are two types of VS; sporadic and familial. The sporadic form of VS is commonly seen in adults and is very rare in pediatric patients. The familial form of VS is most commonly seen in patients with neurofibromatosis type 2 (NF2), and is usually bilateral.

History

Various terminologies have been used to define the VS pathology, with “Acoustic Neuroma” the most common. Prior terminology has caused confusion regarding the origin, biological behavior, and natural history of VS. This confusion is now resolved by using the correct name of “Vestibular Schwannoma” (VS) to define this pathology. “Vestibular” clearly indicates the origin of the tumor as from the vestibular nerve, while “schwannoma” indicates that Schwann cells are the cell of origin.

In 1777, anatomist Eduard Sandifort was the first to recognize the VS as an acoustic neuroma at autopsy. It was observed as a fixed and rigid tumor adjacent to the cochlear nerve with extension into the internal acoustic canal (IAC) and that caused compression on the brainstem [10]. From that time to the early twentieth century, acoustic neuromas were used to the define all tumors located in the CPA.

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The first attempt to remove a VS in the CPA via the transcranial route was under-taken by Von Bergmann in 1890. The patient did not survive the surgery and histo-pathology confirmed a VS [11]. In 1894, Charles Balance performed the first successful removal of the CPA tumor at two separate stages [12]. Although the patient survived, surgery resulted in severe complications including complete facial palsy. Findings during this surgery which included a tight attachment to the dura of the petrous bone and the presence of a capsule, raised some questions about the origin of this tumor. At that time, it was thought that this tumor might have been a meningioma. Because of the lack of the definite diagnosis of this tumor, Thomas Annandale’s surgery in 1895 is considered as the first successful resection attempt to remove a VS [10, 11, 13]. In 1905, Victor Horsley performed a gross total resec-tion of a VS that unfortunately resulted in severe postoperative brain ischemia [14]. Since then, many cases have been reported with high mortality and morbidity. In the early twentieth century, morbidity and mortality following VS surgery were still unfavorable, ranging between 67% and 84% [15]. Paramedian suboccipital crani-otomy was the only approach to the VS, with manual resection of the tumor using fingers the common practice.

In 1936, Cushing introduced a new surgical technique. This consisted of a “T-shape” skin incision that included a horizontal incision between both mastoid notches, and a vertical incision from the midline to the middle level of the cervical spine. After this extensive skin incision, Cushing performed a large bilateral poste-rior fossa craniectomy that exposed both CPAs laterally, the cervicomedullary junc-tion and the cisterna magna inferiorly, and the venous sinuses superiorly. Cushing’s novel approach allowed a wider surgical working area, provided cerebellar relax-ation through CSF drainage from the cisterna magna, and mobilization of the cere-bellum and the brainstem. Thus, neuro-vascular structures became more mobile and were less affected by surgical maneuvers. Dandy modified Cushing’s technique, and hypothesized that total resection of the tumor would decrease the rates of recurrence and increase the long-term survival [16]. He performed more aggressive internal debulking to create free space and created a cleavage plane to pull the tumor capsule away from surrounding structures for circumferential dissection. Dandy also reported the first surgical series of VS in which the tumors were removed gross totally [16].

The next stage in the history of VS surgery was to achieve higher surgical resec-tion rates with lower morbidity and mortality rates. In 1949, Horrax and Poppen reported mortality rate of 10.8% in patients with VS after gross total resection [17]. Gradually, the size of the craniotomy/craniectomy was reduced, operative tech-niques were improved, neurophysiological monitoring was introduced, and special-ized new surgical instruments became available. However, none of these developments had the impact of the surgical microscope. This marked the beginning of the era of modern microneurosurgery. Additionally, the introduction of the high- speed drill was another significant milestone. With these new methods and tools, new surgical corridors and approaches were made possible. Through these advances, brain retraction was minimized, and direct anterior visualization of the VS was achieved. This lead in the 1960s to William House, utilizing high-speed drills and

1 Acoustic Neuromas: General Considerations

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surgical microscope, to introduce the translabyrinthine approach for VS surgery [18, 19].

Advances in microneurosurgical techniques due to new instruments, imaging systems, skull base techniques and neurophysiological monitoring have improved outcome parameters from just survival, to address quality of life and cosmetic results that include preservation of the facial nerve and hearing.

Epidemiology

Vestibular schwannomas constitute 6% of all intracranial neoplasms [4] and are the most common benign lesions of the IAC and CPA cistern constituting between 60% and 90% of the entire lesions seen in this area [20, 21]. VS are most commonly diagnosed in adults, and the median age of diagnosis is ranging between 52 and 55 years in different studies [22]. These tumors are located unilaterally in more than 90% of cases [23]. Both sides are affected with equal frequency. Bilateral vestibular schwannomas are seen in patients with neurofibromatosis type 2  in the pediatric population as well as in the adults [24].

According to recent population-based studies, the overall incidence of VS is 9–13 cases per million persons per year [25–27]. This translates to about 3000 cases per year within the United States, a number that is consistent with clinical experi-ence. However, these population-based studies likely underestimate the incidence, since in the pre and early magnetic resonance imaging (MRI) era, diagnostic cross-sectional imaging capable of detecting small lesions was unavailable or uncom-monly performed. Contemporary MRI technology is faster and less expensive than previously, and is capable of detecting small lesions. Recent reports have docu-mented the capability to identify small lesion without the use of paramagnetic con-trast material [28, 29]. Thus, it is expected that with this more sensitive diagnostic imaging, the incidence of VS, asymptomatic or symptomatic, will increase. Indeed, Anderson and colleagues found that a rate of asymptomatic VS was 0.7% per 10,000 MRI images obtained for reasons other than assessing for CPA lesions [30]. Similarly, Lin and colleagues found a rate of two incidental VS findings per 10,000 persons when they interpreted >46,000 MRI studies [31]. Likely related to the increasing incidence of VS over time (due to improved imaging techniques), the average size of the lesion at diagnosis appears to be decreasing. Stangerup and col-leagues reported that when assessing 30 years of data from a national population sample size, tumor size decreased from ~3 cm in the 1970s to ~1 cm in the mid-2000s [32]. Additionally, there was no lesions discovered in the 1970s that were limited to the IAC, but by the mid-2000s, 33% of lesions discovered were restricted to the IAC [32]. This suggests that reported incidence of VS is thus likely a function of increased diagnostic imaging sensitivity and the increased use of MRI for more indications.

Historical estimates for VS based on autopsy have placed the prevelance at 2.6% [33]. Through further review and reclassification of previous studies, the VS inci-dence was decreased to ~0.8% [34] by the mid twentieth century. According to

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Stangerup et al., the reported incidence of VS in Denmark was 3.1 per million per year in 1976. In 2004, there was an approximately sevenfold increase in the inci-dence of VS to 22.8 per million per year [35]. In our view, this is due to technical advances in diagnostic radiology, widespread use of radiological imaging methods, growing medical awareness, and knowledge about VS that facilitates timely diagnosis.

Growth and Natural History

Numerous studies have investigated VS tumor biology. This includes studies of in vitro and in vivo growth rates with the percentage of cells in replication (S-phase), as measured by methods that include immunohistochemical stains, preoperative infusion of 5-bromodeoxyuridine, and flow cytometric analysis. Tumor growth rates are variable but typically slow. Generally, only 0.1–3% of cells are in the S-Phase [36, 37]. While individual tumors grow at different rates, growth rates tend to be constant for a given tumor [38]. Correlation between growth and patient symp-toms are not perfect [39], and there is no evidence that VS have varying growth rates depending on patient age [40]. Some exceptions to typical growth patterns exist, including lesions with cystic components, and tumors that undergo intratumoral hemorrhage following injury or physical exertion.

When considering clinical natural history, the two most important factors for the treatment team are tumor growth and hearing changes. For lesions that are discovered and when up-front treatment is not undertaken, periodic (serial) cross-sectional imaging is used to monitor growth. The percentage of tumors demonstrating progressive growth after diagnosis has been reported to between 30% and 90% as reviewed by Stangerup and colleagues (2012), although, defini-tions of what constitutes growth vary. Growth rates in tumors that exhibit enlarge-ment have been reported to be about 1–2 mm per year in linear growth in at least one dimension. Nedzelski et al. found a mean growth rate of 1.1 mm per year (range −5 to 9.8 mm) in 50 patients [41]. Similarly, Selesnick and Johnson found an average tumor growth rate of 1.8 mm per year (with a range of 0.5–3 mm per year) in a meta-analysis of 508 patients [42]. Literature survey finds reported growth rates between 0.4 mm and 2.1 mm per year [43–49]. While no formal consensus exists, most authors define normal growth rates as being between 1 and 2 mm per year and fast growth rates as 4 mm or greater per year [32, 48, 49]. Thus, while the growth of individual tumors is variable, most lesions grow slowly. However, patients with NF2 exhibit higher tumor growth rates, particu-larly in young patients [50], which can result in more advanced symptoms at the time of diagnosis [51].

Conflicting evidence exists for whether the size of a lesion at the time of diag-nosis influences future tumor growth. Selesnick and Johnson were unable to find a relationship between tumor size progression and size at diagnosis [42]. Conversely, Stangerup and colleagues looked at a subset of all non-NF2 sporadic tumors (>1800 diagnoses) from 1975 to 2005 [52]. This group included 552 patients

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whose tumors were followed with at least one additional cross-sectional imaging study with a mean follow-up of 3.6 years. Growth was defined as either extension to the CPA in lesions confined to the IAC (i.e., small lesions) at diagnosis, or more than 2 mm in growth in any linear dimension for lesions with CPA involvement (i.e., large lesions) at the time of diagnosis. Tumors that were initially confined to the IAC demonstrated a 17% growth rate and lesions with initial CPA extension demonstrated a 28% growth rate. Interestingly, in both groups, the majority of growth was within the first 2 years of observation, and no tumor growth was seen in either group if growth had not occurred within 5 years of diagnosis [52]. Tschudi et al. also found that if tumor progression was found, this was demonstrated early during the follow-up period [47]. Battaglia et al. found that smaller tumors had lower incidences of tumor progression than larger lesions (39% vs. 61%), which is similar to, although higher than the rates reported by Stangerup et  al. [43, 52]. Other authors have demonstrated similar relationships of size at presentation and growth potential [44, 53]. What should be additionally noted from the previously presented data is that a substantial number of tumors do not demonstrate growth on repeat cross-sectional imaging. Lastly, tumor growth is not always consistent. Several authors have found that VS can undergo several types of changes including shrinkage, no growth, growth followed by no growth, no growth followed by growth, and continuous growth, although not every author reported all possible growth patterns [32, 45–47, 52]. In summary, while some trends exist, the only reliable tendency one should conclude is that if the growth of a lesion is observed, the majority enlarge slowly.

Hearing performance over time is also important. As will be discussed in other sections of this book, decisions for surgical intervention are at least par-tially based on hearing status at the time of diagnosis. Understanding how hear-ing in the index ear can be expected to change over time after diagnosis can be helpful in determining if a hearing preservation approach should be considered. Stangerup and colleagues demonstrated in a series of reports that there is a slow decline in hearing performance over time, both in pure tone detection (PTA) and speech discrimination [54–56]. Subjects with good hearing demonstrated better overall hearing preservation at the most recent follow-up point than those with worsened hearing performance at the time of diagnosis. In fact, patients with 100% word recognition showed an 89% chance of maintaining Word Recognition Scores (WRS) I-II hearing classification at their most recent follow-up, while those with 90–99% word recognition had a 54% chance of maintaining WRS I-II hearing classification [55]. Thus, once the VS starts to impact word recognition performance, the progression of hearing loss tends to become more likely over time. Elliott et al. demonstrated similar findings in that subjects with American Academy of Otolaryngology-Head and Neck Surgery Committee on Hearing and Equilibrium (AAO-HNS CHE) class A hearing at the time of diagnosis had better long-term hearing performance preservation when compared to those with AAO-HNS CHE class B hearing at the time of diagnosis [57]. In summary, while some patients maintain high levels of hearing, most subjects demonstrate a slow decline in both pure tone detection and speech discrimination performance. The

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best predictor of future hearing loss is the presence of measurable hearing loss at diagnosis.

Clinical Manifestations

The majority of lesions begin in the IAC and with progressive growth, extend medi-ally into the CPA cistern and eventually interact with surrounding neuroanatomical structures. When considering common presenting manifestations, a helpful frame-work is to consider manifestations based on the size of the tumor, and its location; intracanalicular, cisternal, brainstem compressive, or hydrocephalic sizes (please see Figs. 2.6a–d in Chap. 2), as discussed below:

Intracanalicular: These are defined as when the tumor is entirely within the IAC (please see Fig. 2.6a in Chap. 2). Symptoms of this stage typically include hearing loss, tinnitus, and vertigo or disequilibrium. Hearing loss is the most common pre-senting symptom of VS with roughly 95% of patients experiencing at least some level of hearing loss [58]. Compression and infiltration of the cochlear nerve fibers and/or impairment of the blood supply to the auditory nerve or cochlea are the most likely mechanisms of hearing loss, as discussed above. While any configuration is possible, hearing loss associated with vestibular schwannomas of this size is typi-cally unilateral, progressive, and commonly affects high-frequency thresholds and speech perception [59]. Classically, speech perception is worse than would be expected based on pure tone averages (PTA). Variations include different hearing loss frequency patterns, sudden hearing loss possibly caused by sudden vascular occlusion, and rarely, normal hearing performance. With high-resolution imaging becoming more universally available, observations of normal hearing are becoming more prevalent since diagnosis precedes tumor-induced damage to the auditory nerve or inner ear structures [60].

Cisternal: These are defined as when the tumor is outside the boundaries of the IAC and enters the CPA cistern (please see Fig. 2.6b in Chap. 2). Cisternal tumors can displace cranial nerves VII and VIII, and the anterior inferior cerebellar artery. Hearing loss may occur due to compression and infiltration of the auditory nerve and/or compression of the labyrinthine vessels [58]. Episodes of vertigo tend to be less frequent with cisternal tumors, but symptoms of disequilibrium tend to be more prevalent. With progressive injury to the vestibular nerves and end organs, more substantial shifts in peripheral signaling to the central vestibular system that result in vertigo become less frequent. However, a small persistent decline in peripheral vestibular function still occurs slowly over time and prevents central compensation. This lack of compensation can result in the perception of disequilibrium.

Brainstem compressive: The tumor comes in contact and may displace the brain-stem (Please see Fig. 2.6c in Chap. 2). Symptoms may include trigeminal aberra-tions, occipital headaches, intention tremors, and gait ataxia. Compression of the cisternal portion of the trigeminal nerve (cranial nerve V) or the Gasserian ganglion can lead to decreased sensation or paresthesias of the midface. With further tumor

1 Acoustic Neuromas: General Considerations

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enlargement, this progresses to involving the lower and upper divisions of cranial nerve V. Also, the corneal reflex may become decreased or absent. Conversely, tri-geminal neuralgia (pain in the distribution of the trigeminal nerves) can be a pre-senting symptom of VS. Typically, tumor displacement of a nearby vessel into the root entry zone of the trigeminal nerve is thought to be the cause of this painful condition. The offending vessel is most commonly the superior cerebellar artery or the petrosal vein. Non-surgical, pharmacological treatment (e.g. carbamazepine) can help control symptoms similar to traditional trigeminal neuralgia. An occipital headache attributable to the tumor is a late finding with 20% of patients with tumors between 1 and 3 cm, with 40% of patients with tumors larger than 3 cm verifying this symptom [61]. Intentional tremor and gait ataxia are two symptoms caused by progressive compression of the middle cerebellar peduncle and cerebellum when the ipsilateral output from the cerebellum has been compromised. As brainstem compression occurs with significant enlargement of the tumor, these symptoms tend to occur late in the natural history of vestibular schwannomas.

Hydrocephalic: These tumors cause profound brainstem and fourth ventricle compression (please see Fig. 2.6d in Chap. 2). Previously discussed manifestations including trigeminal symptoms, gait deterioration, and worsening headaches. Tumor growth can progress to cause a complete lack of facial sensation. Additionally, atrophy of the masticatory muscles, thus causing a crossbite, may be a late sign of cranial nerve V dysfunction. Gait deteriorates as ipsilateral cerebellar tracts con-tinue to become involved. Finally, generalized headache may be present if hydro-cephalus is present. However, not all patients with hydrocephalus secondary to vestibular schwannoma develop a headache. Likewise, in patients with VS experi-encing headache, hydrocephalus is the cause only in a minority of cases [62]. New symptoms may also develop in hydrocephalic stage, including visual loss, lower cranial nerve dysfunction, long tract signs, and even death. Visual loss is either caused by increased intracranial pressure leading to papilledema, or by communi-cating hydrocephalus leading to optic atrophy, and thus loss of peripheral vision, development of tunnel vision, and eventually, blindness. Lower cranial nerve dys-function, such as dysphonia, dysphagia, aspiration, shoulder or arm weakness, and tongue weakness or disarticulation are very rare in VS. If these are found, evaluation for a concomitant schwannoma of the jugular foramen or other lesions is warranted. Long tract signs, such as contralateral hemiparesis, are likewise very uncommon. Motor and sensory functions of the extremities are typically intact even with brain-stem compression, hydrocephalus, and worsening balance. Finally, death due to tonsillar herniation and respiratory failure is possible, but this is unlikely in VS patients.

Exceptions

While classifying the tumor as intracanalicular, cisternal, brainstem compressive, and hydrocephalic has clinical use, exceptions to this system exist. There are vari-ations in IAC involvement, with the tumor occasionally not involving the IAC at

I. Dogan et al.