A Facial Nerve Schwannoma Presenting as an External Auditory Canal Mass

A 46-year-old man presented with left hearing impairment, facial weakness, and otorrhea for 6 months. Otoscopy showed a reddish mass protruding through the tympanic membrane into the left external auditory canal (EAC; Figure 1). Physical examination revealed left-sided facial paralysis, House-Brackmann (H-B) grade IV/VI. A pure-tone audiogram showed mixed-type hearing loss with a 40 dB air–bone gap in the left ear. High-resolution computed tomography of the temporal bone showed a soft tissue mass in the left tympanomastoid cavity protruding into the EAC. Some bony erosion of the anterior wall of the tympanic cavity was also noted. Magnetic resonance imaging (MRI) showed an enhanced soft tissue lesion filling the left middle ear cavity with extension to the mastoid antrum and EAC. This lesion presented with hyperintense signal on T2 weighted image (T2WI) (Figure 2). A provisional diagnosis of a facial nerve tumor was made and it was excised using a transmastoid approach. During surgery, the tumor was found to originate from the facial nerve and involved the distal part of the geniculate ganglion and tympanic and mastoid segments. The tumor was completely removed from the facial nerve. Due to the difficulty preserving the integrity of the facial nerve following tumor debulking, the tumor was excised along with the involved facial nerve segments. End-to-end sural nerve graft anastomosis was performed between the residual geniculate ganglion and distal mastoid segment of the facial nerve. The pathology showed nuclear palisading with Verocay body formation (Antoni A) and a facial nerve schwannoma (FNS) was confirmed. There was no recurrence and the facial function had recovered to H-B grade III/VI at the 5-year follow-up.

Figure 1.

Otoscopy showed a reddish mass protruding through the tympanic membrane into the external auditory canal.

Figure 2.

Magnetic resonance imaging (MRI) displayed (A-C) an enhanced soft tissue lesion filling the left middle ear cavity with extension to the mastoid antrum and external auditory canal. D-F, This lesion presented with hyperintense signal on T2WI.

Discussion

An FNS is a benign slow-growing nerve sheath tumor derived from Schwann cells. ¹ Although it is the most common primary neoplasm of the facial nerve, it is a rare tumor and makes up only 0.15% to 0.8% of all intratemporal tumors. ² The most common presenting symptom of intratemporal FNS is asymmetric hearing loss (54%), followed by facial paresis (41%) and facial spasm (26%). ³

An FNS of the tympanic segment may extend to the middle ear cavity by eroding the lateral wall of the facial nerve canal and usually appears as a white mass behind an intact tympanic membrane on otoscopy. However, our FNS presented as an EAC mass that protruded through tympanic membrane, which is extremely rare. Clinically, the differential diagnosis should consider other middle ear tumors, such as cholesteatoma and glomus tumor.

Ideally, the diagnosis of FNS should be based on clinical symptoms and radiological data because histological confirmation with a biopsy can lead to facial nerve injury. The facial nerve fibers are often widely disseminated through the tumor or adhere intimately to its surface. ⁴ High-resolution computed tomography and MRI are useful for the diagnosis of FNS. An intratemporal FNS typically produces characteristic changes in the osseous canal along the course of the facial nerve in the tympanic region with homogeneous contrast enhancement in CT. Magnetic resonance imaging shows an iso- to hypointense lesion in T1 weighted images (T1WIs) that is hyperintense on T2WIs and shows good enhancement in gadolinium-enhanced T1WIs. ⁵ The typical “dumbbell” sign was present in our case.

There are no accepted guidelines for the management of FNS. Due to its slow-growing nature (2.0 mm/y), observation is the management of choice in patients with minimal symptoms and no brain stem compression. Recently, stereotactic radiosurgery has been added as a treatment option, but it is mostly reserved for small tumors causing H-B grade I to III paralysis. ⁶ Gross tumor resection with reinnervation via an interposition facial nerve graft or the nerve substitution technique is suggested in most patients with an enlarging tumor with poor facial function (H-B IV or worse) or concern regarding compressive symptoms, or stereotactic radiation failure. ⁷ However, nerve resection and anastomosis using a nerve graft gives an H-B grade III result at best postoperatively. ⁸

An intratemporal FNS presenting as an EAC mass is uncommon. Clinicians should bear this in mind when evaluating an EAC mass of uncertain origin. It is important to conduct imaging studies before performing a biopsy because a biopsy of FNS can result in facial nerve paralysis. An image survey with CT and MRI is useful for differentiating FNS from other middle ear tumors.