Large Facial Nerve Schwannoma With Extensive Temporal Bone Destruction

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A 41-year-old man presented with 4 years of left sided progressive mixed hearing loss and 3 years of facial nerve palsy. He complained of otalgia, tinnitus, aural fullness, and otorrhea. His physical examination was notable for a left House–Brackmann V/VI facial nerve palsy along with a mass extruding through his external auditory canal. He denied vertigo, dysphagia, voice changes, headaches, or vision problems. A computed tomography (CT) examination showed extensive destruction of left temporal bone (Fig. 1) with erosion of the left stylomastoid foramen, medial, and posterior mastoid air cells, posterior wall of the external auditory canal (EAC), middle ear, and lateral wall of the jugular foramen. Based on the CT appearance, the differential diagnosis included aggressive schwannoma, paraganglioma, metastasis, primary malignant neoplasm of the external auditory canal, and less likely, an endoylymphatic sac tumor. Magnetic resonance imaging (MRI) confirmed the presence of an enhancing mass corresponding to the area of bony destruction on the CT (Fig. 2). The enhancing lesion had no intracranial involvement but extended through the stylomastoid foramen into the parotid gland. Furthermore, no prominent flow voids were observed within or surrounding this lesion. Lack of intracranial extension and dura invasion in a mass this large seemed inconsistent with a metastatic process. Absence of inherent T1 shortening within the lesion rebutted an endolymphatic sac tumor. Lack of internal flow void and extension along portions of the normal facial nerve trajectory favored facial nerve schwannoma over paraganglioma (glomus jugulare). Paragangliomas of the temporal bone cause erosion of the surrounding bony matrix in a moth-eaten pattern on CT. On an MRI, paragangliomas have a heterogeneous “salt-and-pepper” appearance due to flow voids caused by extreme vascularity and intratumoral hemorrhages. A glomus jugulare would be more likely to cause erosion of both the medial and lateral wall of the jugular foramen with the epicenter of the tumor around the dome of the jugular bulb (1,2).
The patient was taken for surgery, and the mass was resected through a translabyrinthine craniotomy. The mass was found to intimately involve the facial nerve, which was resected en bloc with the tumor. The final histopathology revealed a facial nerve schwannoma without malignant transformation.
Facial schwannomas often involve multiple segments of the facial nerve most commonly the geniculate and labyrinthine segments (3). In one retrospective study, the mastoid segment was involved in 21.6% of the cases and had led to erosion into the EAC similar to the current case. Additionally, 9.1% of tumors extended through the stylomastoid foramen and into the parotid gland (3,4). On CT imaging schwannomas typically cause remodeling of the temporal bone leading to a scalloped appearance rather than destructive changes of the adjacent bony matrix. The unusual imaging findings of this tumor were the amount of bone destruction rather than remodeling which is commonly associated with intratemporal schwannomas. However, when eroding into pneumatized structures they may infiltrate into nearby air cells. This is particularly seen in vestibular schwannomas extending into air cells surrounding the internal auditory canal as well as facial schwannomas arising from the descending mastoid segment (5). On MRI, schwannomas tend to avidly enhance with contrast but may have homogenous, heterogenous, or even a cystic appearance. They are usually isointense to the brain on T1 and may have areas of T2 signal hyperintensity. Facial nerve and vestibular schwannomas are rarely difficult to differentiate on imaging due to their different growth pattern. Facial nerve schwannomas extend along the path of the facial nerve with classic dumbbell appearance on either side of a widened labyrinthine segment (1,4).

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