Vestibular Schwannoma Surgery: A Video Guide

By Mustafa K. Baskaya, G. Mark Pyle and Joseph P. Roche

This video guide increases the surgeon's understanding of all types of surgical approaches for vestibular schwannoma surgery: retrosigmoid, translabyrinthine, middle cranial fossa and combined approaches.  2D and 3D videos are included to increase the readers’ understanding of these complex surgical techniques. These are accompanied by step-by-step narrated cadaveric dissection videos showing the crucial steps of each approach.

This book is a learning tool and video reference for those training to perform the procedure and enhances the readers understanding of neuroanatomy. A detailed review of all surgical options and their risks, along with tips, tenets and pitfalls is included. The authors provide an unbiased discussion of all options with balanced comparison between surgical approaches and algorithms for patient selection.

• Language: English
• Publisher: Springer
• Release date: Jan 1, 2019
• ISBN: 9783319992983

Book preview

© Springer Nature Switzerland AG 2019

Mustafa K. Baskaya, G. Mark Pyle and Joseph P. RocheVestibular Schwannoma Surgeryhttps://doi.org/10.1007/978-3-319-99298-3_1

1. Acoustic Neuromas: General Considerations

Ihsan Dogan¹, Burak Ozaydin¹, Joseph P. Roche² and Mustafa K. Baskaya¹  

(1)

Department of Neurological Surgery, University of Wisconsin School of Medicine and Public Health, Madison, WI, USA

(2)

Department 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

Mustafa K. Baskaya

Email: [e-mail]

Abstract

VS 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 presenting 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 possible, hearing loss associated with vestibular schwannomas of this size is typically unilateral, progressive, and commonly affects high-frequency thresholds and speech perception. 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. 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.

Keywords

Vestibular schwannomaAcoustic neuromaCerebellopontine angleRetrosigmoid approachTranslabyrinthine approachMiddle fossa approachInternal 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 origin of VS, recent studies suggest otherwise. Sanna et al. found that the most common origin for VS are the nerve sheath of the inferior vestibular nerve, and less frequently the superior vestibular nerve [1]. Eighth cranial nerve schwannomas represent 90% of all intracranial nerve sheath tumors [2]. VS account for approximately 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.

The first attempt to remove a VS in the CPA via the transcranial route was undertaken by Von Bergmann in 1890. The patient did not survive the surgery and histopathology 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 resection 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 craniotomy 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 posterior fossa craniectomy that exposed both CPAs laterally, the cervicomedullary junction and the cisterna magna inferiorly, and the venous sinuses superiorly. Cushing’s novel approach allowed a wider surgical working area, provided cerebellar relaxation through CSF drainage from the cisterna magna, and mobilization of the cerebellum 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 resection 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 techniques were improved, neurophysiological monitoring was introduced, and specialized 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 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 experience. 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 uncommonly performed. Contemporary MRI technology is faster and less expensive than previously, and is capable of detecting small lesions. Recent reports have documented the capability to identify small lesion without the use of paramagnetic contrast 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 colleagues 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 incidence was decreased to ~0.8% [34] by the mid twentieth century. According to 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 incidence 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 symptoms 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, definitions of what constitutes growth vary. 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. 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, particularly 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 diagnosis 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 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 partially based on hearing status at the time of diagnosis. Understanding how hearing 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 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 medially into the CPA cistern and eventually interact with surrounding neuroanatomical structures. When considering common presenting 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 (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 presenting 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 typically 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 brainstem (Please see Fig. 2.​6c in Chap. 2). Symptoms may include trigeminal aberrations, 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 enlargement, this progresses to involving the lower and upper divisions of cranial nerve V. Also, the corneal reflex may become decreased or absent. Conversely, trigeminal neuralgia (pain in the distribution of the trigeminal nerves) can be a presenting symptom of VS. Typically, tumor displacement of a nearby vessel