Secondary Intracanalicular Extension of a Primary Intracochlear Schwannoma

Abstract

Vestibular schwannomas represent 6 to 8% of all intracranial tumors. Verocay, along with Antoni, suggested the idea of the Schwann cell as the cell of origin, a theory that has since been confirmed by electron microscopy and immunohistochemical studies. Until recently, the commonly accepted teaching was that these tumors originate from the Schwann-glial cell junction along the nerve, an area also known as the Obersteiner-Redlich zone. This transition zone is located along the internal auditory canal (IAC) portion of the nerve, approximately 7 to 13 mm from the brainstem (1). More recently, however, numerous cadaveric and clinical studies have described tumors arising from the myelinated portion of the nerve, lateral to the transition zone (2). With recent improvements in imaging techniques, there have been an increasing number of reports describing exclusively intralabyrinthine schwannomas, not involving the more medial segments of the nerve (3). More than 50 such cases have been reported in the literature to date, the first one by Mayer as early as 1917 (3,4). While most of the earlier cases were reported at autopsy or during surgery for severe vertigo, improved imaging resolution has resulted in earlier identification of these tumors (5). Intralabyrinthine schwannomas are currently considered a separate clinical entity from those involving the IAC and cerebellopontine angle (CPA). Although initially thought to arise more commonly from the cochlear division of the nerve, a recent review of all published cases by Neff et al. (3) reveals an equal incidence between the vestibular and the cochlear varieties. Depending on the involved portion of the labyrinth, these tumors present with sensorineural or mixed hearing loss, tinnitus, and/or vestibular symptoms. The vestibular symptoms related to labyrinthine schwannomas differ from those of patients with IAC and CPA tumors in that true vertigo exists in a higher percentage of patients (6). Of interest, all of the cases of labyrinthine schwannomas reported to date have been restricted to the osseous labyrinth without involvement of the IAC. Although tumors involving both the IAC and the labyrinthine compartment have been described, most have been attributed to secondary labyrinthine extension by a primary IAC tumor rather than the converse (6). This conclusion likely reflects the fact that the IAC-CPA schwannomas are relatively common, whereas the intralabyrinthine tumors are rare. In fact, serial imaging studies that document progressive, direct tumor extension either from or into the inner ear over time are lacking. The images in Figure 1, A to D, show the axial cuts of a gadolinium-enhanced magnetic resonance imaging scan of the same patient taken during a 5-year period. They reveal a clear progression of the tumor, with medial extension from the labyrinth into the IAC.