51.1 Introduction

Over the last century, advances in treatment of vestibular schwannoma (VS) have paralleled refinements in diagnostic capability.s. Literatur ^,​ s. Literatur In particular, improvements in audiometric screening protocols and the widespread availability of contrast-enhanced magnetic resonance imaging (MRI) have led to an increasing number of small to medium size tumors being diagnosed in patients with minimal symptoms.s. Literatur ^,​ s. Literatur Less tolerance of treatment-associated morbidity and a better understanding of VS growth patterns—particularly the realization that many tumors remain stable or may even involute—has led to greater acceptance of observation as an initial management strategy.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur In fact, according to recent national trends in the United States, it is estimated that by 2026, half of all cases of VS will be managed initially with observation.s. Literatur

Ultimately, most patients who elect initial observation of their VS will experience gradual deterioration of hearing and vestibular function over time.s. Literatur ^,​ s. Literatur ^,​ s. Literatur However, there is substantial variability in audiovestibular loss between patients at the time of diagnosis, and the rate of decline is inconsistent. Unfortunately, the exact mechanism(s) and factors that drive loss of inner ear function remain unclear. With the recent paradigm shift toward conservative observation, understanding the natural history of hearing and vestibular function has become ever more relevant. Such information may be used to guide patient counseling and provides a context to compare the advantages and drawbacks of radiation therapy and microsurgery.

51.2 Mechanisms of Hearing Loss and Vestibular Dysfunction

Potential mechanisms of hearing loss and vestibular dysfunction in sporadic VS include neural compression, vascular compression, VIIIth nerve infiltration, invasion of the inner ear, and accumulation of toxic metabolites or proteins in the fundus and inner ear from poor CSF recirculation. Our knowledge of vestibular dysfunction in observed VS patients is very limited compared with the rather extensive knowledge gathered on hearing deterioration and tumor growth,s. Literatur but presumably many of these factors that influence hearing decline will also result in vestibular loss.

VS-associated hearing loss is thought to occur due to both cochlear and retrocochlear degenerative changes. Supporting evidence for retrocochlear dysfunction include abnormalities on auditory brainstem response measurementss. Literatur ^,​ s. Literatur and histopathologic data demonstrating atrophy of the cochlear nerve.s. Literatur ^,​ s. Literatur Evidence for cochlear dysfunction comes from clinical data showing abnormalities in otoacoustic emissions in VSs. Literatur ^,​ s. Literatur ^,​ s. Literatur and histopathologic data in patients with VS showing pathologic changes within the cochlea such as degeneration of the organ of Corti and the stria vascularis, occurrence of endolymphatic hydrops, and an acidophilic-staining precipitate.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur This inner ear precipitate reflects higher protein content present in ears with VSs. Literatur ^,​ s. Literatur that may interfere with labyrinthine biochemical homeostasis.s. Literatur ^,​ s. Literatur Proposed mechanisms of hearing loss attributed to this protein include blockage of cochlear nerve protein transport, an immune response to VS, or an alteration in the blood–labyrinthine barrier.

Perilymphatic protein accumulation is also being evaluated using fluid-attenuated inversion recovery (FLAIR) MRI sequences. Increased inner ear signal on FLAIR sequences is thought to represent elevated perilymphatic protein, and several authors have found a positive association between FLAIR intensity within the cochlea and hearing deterioration in patients with VS.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur

In cases of cochlear nerve atrophy or neuron loss, speech discrimination is often disproportionately affected over pure-tone hearing thresholds—a pattern classically termed “retrocochlear pattern loss.”s. Literatur ^,​ s. Literatur ^,​ s. Literatur Studies have shown that pure-tone threshold shifts occur only after losses of more than 80 to 90% of cochlear nerve fiberss. Literatur; however, clinical data have also demonstrated preserved speech discrimination with mild-to-moderate sensorineural hearing loss (SNHL) in many cases.s. Literatur If retrocochlear mechanisms alone lead to hearing loss in VS, then pure-tone shifts should always be accompanied by severe decrement in speech discrimination, but this is clearly not always the case. Therefore, hearing loss in VS is multifactorial and likely results from a combination of cochlear and retrocochlear mechanisms with pure-tone hearing most affected by cochlear pathology and speech discrimination most affected by retrocochlear mechanisms.

It has also been shown that labyrinthine as well as “retrolabyrinthine” pathology contributes to tumor-related vestibular dysfunction. Hizli et al discovered that findings of labyrinthine pathology, including hair cell loss, hydrops, and inner ear fluid precipitate, are analogous to findings in the cochleae of ears with VS.s. Literatur ^,​ s. Literatur ^,​ s. Literatur Moller et al demonstrated peripheral nerve atrophy and degeneration of Scarpa’s ganglion and vestibular neuroepithelia in cases of observed VS.s. Literatur Histopathologic changes were localized to the nerve of tumor origin, and the degree of peripheral nerve fiber and vestibular end-organ degeneration was related to the degree of vestibular ganglion degeneration. The authors described a tumor-induced anterograde progression of vestibular nerve atrophy, and degeneration of Scarpa’s ganglion and vestibular neuroepithelia.

51.3 Interpreting the Literature

When reviewing the relevant VS literature, there are several significant biases and limitations that should be acknowledged. Today, common indications for electing initial observation include small to medium tumor size (generally ≤1.5 cm in the cerebellopontine angle [CPA]), the presence of serviceable hearing, minimal attributable symptoms at the time of diagnosis, poor surgical candidacy, advanced age or limited life expectancy, and patient preference. Despite trends toward greater use of observation in recent years, it is still important to realize that many patients who could be safely observed may ultimately undergo upfront treatment due to patient or treating physician preference. Thus, the pools of subjects analyzed in natural history studies often represent biased heterogeneous populations at baseline.

In addition to selection bias, disparities in outcome reporting render pooled data analyses and comparison between studies challenging. Hearing loss may be reported using several classification schemes including the 1995 American Academy of Otolaryngology–Head and Neck Surgery (AAO–HNS) guidelines,s. Literatur the Gardner–Robertson classification,s. Literatur the word recognition score (WRS),s. Literatur the modified word recognition score (mWRS)s. Literatur systems, and the more recent 2012 AAO–HNS hearing reporting guidelines.s. Literatur Even with a broad range of grading systems available, many publications merely report the presence of any detectable hearing or patient subjective report (refer to Chapter 50 for further discussion). Lastly, because hearing loss is expected to decline over time, it is critical that similar time points are examined between studies and, ideally, loss of serviceable hearing should be examined using time-to-event analysis, such as the Kaplan–Meier survival curve.

51.4 Pattern(s) of Sensorineural Hearing Loss in Untreated Sporadic Vestibular Schwannoma

Asymmetric SNHL is the most common symptom experienced by patients with sporadic VS, present in over 90% of subjects at the time of diagnosis. In most patients, ipsilateral SNHL occurs at a slow rate, and is often only brought to the patient’s attention after experiencing difficulty with telephone use, for example. The published rate of pure-tone threshold and WRS deterioration varies between studies. Fayad et al found that after 4.8 years of follow-up, patients experienced a mean pure-tone average (PTA) deterioration of 20.7 dB HL to a mean of 58.1 dB and an average WRS decrease of 23.3% to a mean of 55.5%.s. Literatur

Among VS patients electing initial wait-and-scan management, unilateral SNHL usually occurs most rapidly over the first 4 to 5 years of observation after which a more indolent loss is typically observed (Fig. 51‑1 ).s. Literatur Authors have reported a wide range of hearing preservation in conservatively managed VS over time. The results of recent large published series are summarized in Table 51‑1 and select publications are discussed further here.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur

To date, the largest published series of VS managed with a wait-and-scan approach comes from Denmark, where centralized VS care is performed, and a national prospective database has been maintained since 1976.s. Literatur In a large series of 932 patients with audiometric follow-up data, 491 patients (53%) presented with good hearing with speech discrimination better than 70%. After a mean of 4.7 years of observation, 59% of these patients maintained good hearing. In the subgroup of 159 patients (17%) with 100% speech discrimination at diagnosis (mWRS class 0), 138 (87%) maintained good hearing. In the 116 patients with even a minor loss in speech discrimination at diagnosis (1–10%), 63 (54%) maintained good hearing. In the group with a loss of speech discrimination of 21 to 30% at diagnosis, only 28 (33%) of 84 patients maintained good hearing.s. Literatur

Overall, it is estimated that between 1 and 4% of patients with VS experience sudden SNHL as a presenting symptom or during the course of observation.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur Curiously, several reports had documented that in some instances these patients may experience partial or complete recovery of their hearing loss either spontaneously or after steroid therapy.s. Literatur ^,​ s. Literatur Thus, successful treatment of “idiopathic sudden SNHL” with glucocorticoids does not exclude the possibility of an underlying VS—all patients experiencing this symptom should be evaluated with imaging. This also suggests that patients with VS who are being observed and experience sudden SNHL may potentially gain benefit from steroid therapy.s. Literatur ^,​ s. Literatur ^,​ s. Literatur