The many faces of facial nerve schwannoma

Abstract

Background and purpose: The imaging appearance of facial nerve schwannomas (FNSs) has been described as an enhancing tubular mass (using T1-enhanced MR) within an enlarged facial nerve canal (using CT). The purpose of this study is to identify how often the FNS imaging findings conform to this description and determine whether there are underlying anatomic explanations for the discrepant imaging appearances identified.

Materials and methods: The clinical, pathologic, and radiologic records of 24 FNS in 23 patients were retrospectively reviewed. Each FNS was evaluated for location along the facial nerve. The lesions were cataloged by facial nerve segment with the imaging characteristics of each segment described.

Results: The average age at time of first imaging was 39 years (age range, 10-70 years). Eighteen (71%) of the 24 FNSs were pathologically confirmed, while the others were determined intraoperatively or diagnostically by the presence of both enlargement of the facial nerve canal and enhancement on contrast-enhanced T1 MR examination. The most common location was in the geniculate fossa (83%), followed by the labyrinthine and tympanic segments of the facial nerve (both 54%). The most common clinical presentation was facial neuropathy (42%).

Conclusion: The classic description of FNS on enhanced T1 MR is that of a well-circumscribed fusiform enhancing mass along the course of the intratemporal facial nerve with bone algorithm CT showing sharply defined bony canal enlargement. Modern imaging techniques, however, demonstrate the importance of the surrounding anatomic landscape, leading to various imaging appearances. Lesions traversing the labyrinthine segment can demonstrate a dumbbell appearance. When FNSs track along the greater superficial petrosal nerve, they may present as a round mass projecting up into the middle cranial fossa. FNS of the tympanic segment of the facial nerve preferentially pedunculate into the middle ear cavity, clinically presenting as a middle ear mass. When the mastoid segment of the facial nerve is involved, irregular and "invasive" tumor margins seen on MR can be explained on CT as tumor breaking into surrounding mastoid air cells.

Fig 1.

Axial T2-weighted MR imaging at the level of the internal auditory canals, demonstrating a large, homogeneous mass filling the right internal auditory canal and extending into the cerebellopontine angle (between white arrows). From Wiggins and Harnsberger (2001). Used with permission.

Fig 2.

Axial images demonstrating focal enlargement of the labyrinthine segment of the facial nerve on the axial bone algorithm CT (left), and a homogeneously enhancing mass filling the internal auditory canal with extension into the cerebellopontine angle and labyrinthine segment on the axial T1-weighted postcontrast-enhanced MR image from a similar level (right). These images are from the same case as in Fig 1, and the patient was misdiagnosed preoperatively because of failure to note the enhancement and enlargement along the labyrinthine segment into the geniculate fossa (arrows). From Wiggins and Harnsberger (2001). Used with permission.

Fig 3.

Axial bone algorithm CT image demonstrating focal enlargement of the labyrinthine segment of the facial nerve canal from a facial nerve schwannoma (between arrows). From Wiggins and Harnsberger (2001). Used with permission.

Fig 4.

Axial images from the same case of a facial nerve schwannoma, demonstrating homogeneous enhancement of the labyrinthine segment and geniculate fossa on the axial T1-weighted postcontrast-enhanced MR imaging (left), and focal enlargement of the corresponding geniculate fossa and the labyrinthine segment of the facial nerve on the bone algorithm CT (right), at a similar level (between arrows). From Wiggins and Harnsberger (2001). Used with permission.

Fig 5.

MR images demonstrating a large left middle cranial fossa mass. The axial T2-weighted image (left), and sagittal T1-weighted postcontrast image (right) show an extra-axial lesion, with a visible CSF/vascular cleft and associated buckling of the gray/white junction. The right image demonstrates a focal bulbous portion of the large mass extending to the geniculate fossa (between arrows), which was the origin of this FNS. From Wiggins and Harnsberger (2001). Used with permission.

Fig 6.

Bone algorithm CT images from the same case, demonstrating focal enlargement of the right tympanic segment, in the axial (left) and coronal (right) planes (at arrows). From Wiggins and Harnsberger (2001). Used with permission.

Fig 7.

Two different cases of FNS, both coronal plane bone algorithm CT images show significant enlargement of the tympanic segment of the facial nerve with the case on the left demonstrating extension of the lesion primarily superiorly and medially, as well as extension into the cochlea, whereas the lesion on the right shows extension primarily inferiorly and laterally, displacing the ossicles, explaining this patient’s presentation with conductive hearing loss and a middle ear mass on clinical examination (at arrows). From Wiggins and Harnsberger (2001). Used with permission.

Fig 8.

Two axial images from the same case of a facial nerve schwannoma involving the right mastoid segment of the facial nerve canal. The axial T1-weighted postcontrast MR image (left) shows homogeneous enhancement of the mass (between arrows). The bone algorithm CT (right) at the same level shows focal enlargement of the descending segment with extension toward the external auditory canal (between arrows). From Wiggins and Harnsberger (2001). Used with permission.

Fig 9.

Two coronal views of the same case, demonstrating the aggressive appearing imaging features of mastoid segment FNSs. The left coronal T1-weighted postcontrast-enhanced image shows homogeneous enhancement. The right coronal bone algorithm CT study demonstrated an aggressive appearing lesion, as the FNS erupts into the surrounding thin-walled mastoid air cells of the same lesion (between arrows). From Wiggins and Harnsberger (2001). Used with permission.