76.1 Introduction

Cystic vestibular schwannomas (CVSs) are a subtype of vestibular schwannoma (VS) that contain solid portions of tumor mixed with single or multiple, xanthochromic, fluid-filled cysts making up the other part of the tumor. CVSs represent anywhere from 5.7 to 48% of all VSs, with most recent study estimates reporting approximately 10 to 20%. The true incidence varies because there is not a universally accepted definition of what constitutes a CVS.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur

CVS patients are often characterized as presenting with larger tumors, having faster tumor growth rates, shorter symptomatic periods before presentation, worse outcomes from surgical intervention, and poorer stereotactic radiosurgery (SRS) results. However, all of these generalizations are disputable, and the literature covering CVS outcomes is limited to retrospective case series and retrospective analysis of prospectively collected databases. This chapter will cover patient presentation, radiologic differential classification, pathophysiology, and treatment results for CVS.

76.2 Patient Presentation

Many authors have reported that CVSs have a more aggressive growth rate and present as larger tumors than their solid vestibular schwannoma (SVS) counterparts.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur Recent retrospective series disagree on the percentage of CVSs that present at a large size with 17 to 77.6% of tumors presenting greater than 4.0 cm at diagnosis.s. Literatur ^,​ s. Literatur In the presence of a cyst, there can be rapid expansion of the lesion, intratumoral hemorrhage, brainstem compression, and hydrocephalus associated with elevated intracranial pressure, papilledema, cranial nerve paresis, and facial paresthesias.s. Literatur ^,​ s. Literatur Up to 17 to 27% of CVS patients can present with acute worsening of symptoms due to rapid cyst enlargement.s. Literatur ^,​ s. Literatur In VS greater than 3 cm in diameter, hydrocephalus was seen in 21 of the 22 cystic tumors (95.5%) compared to 33 of the 40 solid tumors (82.5%).s. Literatur

In contrast to the aforementioned points, a literature review of nine studies containing 428 CVS and 1,287 SVS showed no difference in age or sex distribution. The average size at presentation was not significantly different between CVS at 3.9 ± 0.84 cm and SVS at 3.7 ± 1.2 cm (p = 0.7).s. Literatur In compiling data from three studies, the difference in symptom duration was not statistically significant (18.7 vs. 23.3 months, respectively, p = 0.3).s. Literatur

76.3 Radiology and Differential Diagnosis

CVSs are best characterized by magnetic resonance imaging (MRI) studies. The solid portions of CVS appear identical to SVS and are isointense or hypointense with the brain on T1-weighted imaging.s. Literatur Additionally, the solid tumor component and the cystic wall enhance with the addition of gadolinium contrast, and thus, this imaging characteristic can be used to differentiate CVS from arachnoid cysts and epidermoids.s. Literatur Conversely, the cystic portions of CVS appear as fluid-filled compartments that are hypointense on T1-weighted and hyperintense on T2-weighted imagess. Literatur (Fig. 76‑1 ). The radiographic differential diagnosis of cystic lesions of the cerebellopontine angle (CPA) is summarized in Table 76‑1 .

76.4 Definition and Classification

There is wide variability in the reported prevalence of CVS that is in large part due to a lack of consensus regarding the definition of what constitutes a cystic tumor. Several classification schemes have been proposed over the last few decades; however, a single classification system has yet to be universally adopted. The simplest and earliest definitions relied on radiographic demonstration of cyst formation and the presence of “significant” cysts at the time of surgery.s. Literatur ^,​ s. Literatur However, it was suggested that there were a lot of radiologic false positives; so, definitions were revised to include cystic radiologic appearance, perioperative identification of cystic elements, and histological verification of S100 positive membranes.s. Literatur The definition of CVS has since evolved to attempt to quantify the amount of tumor that is cystic and to classify the tumors by cyst location: central versus peripheral. For example, “cystic VS” has been defined as all VSs for which the MRI exhibits evidence of cyst formation and for which the pathologic specimens are larger than 3 mm.s. Literatur Other retrospective case series have defined “cystic VS” as tumors with cysts making up anywhere from one-third to two-thirds of the mean CPA tumor diameter or the estimated whole tumor volume.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur Most of these definitions include both intramural and extramural/arachnoid cysts.

Other classification systems emphasized the importance of multicystic tumors and the cyst location. Tumors were first classified based on overall cyst location and cyst wall thickness: central and thick-walled (type A) or peripheral and thin-walled (type B). Type A lesions were then subdivided by cyst characteristics (polycystic or monocystic) and cyst size or volume (small/medium/large). Type B lesions were classified according to cyst orientation with respect to the internal auditory canal (anterior, medial, posterior, or a combination of these).s. Literatur However, determination of the gross total volume (GTV) of cystic tumors is sometimes difficult when the cyst is extramural. If the extramural cyst is a pathologically pure arachnoid cyst, some authors argue that they should not be factored into the tumor volume if they are broadly based against the dura without any enhancement of the cyst wall.s. Literatur As a final thought on the classification of CVS, a VS consensus group defined CVS to only include tumors with extramural cysts, and central cystic tumors should not be labeled CVS.s. Literatur This definition has largely not been adhered to in most subsequent reports on CVS which often include intratumoral cystic tumors.

76.5 Pathophysiology

A precise understanding of cyst formation and growth in CVS remains elusive; nonetheless, various hypotheses have been created. Originally, it was thought that cyst formation was due to an increased cell growth rate that outpaced the tumor’s blood supply, thus leading to tumor degeneration and cyst formation.s. Literatur ^,​ s. Literatur However, later studies did not find any difference in the Ki-67 proliferative index of CVS compared with SVS.s. Literatur Now it is believed that the unpredictable and rapid growth in CVS is due to the expansion of the cysts themselves.s. Literatur ^,​ s. Literatur

CVS may expand due to the production of a myxomatous material in small cystic areas within Antoni type B tissue in which the smaller cysts coalesce into larger cysts and compress the surrounding Antoni type A cells. This is consistent with the reported finding that the cyst portions of the CVS were predominantly observed in the Antoni B areas.s. Literatur ^,​ s. Literatur More recent studies have shown that the cyst wall is histologically more complex consisting of an outer capsular portion, showing S100 and vimentin-positive fibrils, and an inner luminal portion, which has an epithelium-lined appearance. Therefore, the cyst wall consists of a compressed outer type A tissue and an inner layer with type B tissue.s. Literatur In contrast to this view, other studies have not found any relationship between the distribution of cysts and the Antoni A and B tissues.s. Literatur ^,​ s. Literatur

Microhemorrhages within the tumor have been hypothesized to cause the initial presence of microcysts that may coalesce to form larger cysts.s. Literatur ^,​ s. Literatur Park et al documented the presence of significantly increased histological evidence of microhemorrhage, such as hemosiderin-laden macrophages, hemosiderin deposits, thrombotic vessels, and abnormal vessel proliferation in CVS compared with SVS.s. Literatur ^,​ s. Literatur However, the factor that triggers this hemorrhagic tendency in CVS still needs to be elucidated. Once formed, an increase in cyst size is promoted by fluid accumulation due to a direct osmotic effect or extravasation of serum proteins from an impaired blood–tumor barrier.s. Literatur ^,​ s. Literatur ^,​ s. Literatur

A final hypothesis on cyst formation explains that extramural tumor cysts associated with CVS have a higher protein and/or colloid content than that observed in usual arachnoid cysts as a result of secretions from the tumors. Most extramural cysts have epicenters between the tumor and brain, suggesting that the most likely mechanism of formation is peritumoral adhesions. These adhesions lead to the development of pseudoduplication due to the trapping of fluid between the leptomeninges and the mass, resulting in an acquired type of arachnoid cyst that differs slightly from a primary arachnoid cyst not associated with VS.s. Literatur ^,​ s. Literatur