Facial nerve schwannoma

Facial nerve schwannomas (also known as neuroma, or neurilemoma) are uncommon, and make up less than 1% of all temporal bone tumours. They can occur in any segment of the facial nerve and have a tendency to involve multiple segments. There is a predilection for the region of the geniculate ganglion.

As schwannomas elsewhere they originate from the surface of the nerve, and displace and splay the nerve fibres over their eccentric growth (this compares favorably to neurofibromas through which the axons are distributed). This allows for potential resection of the tumour while sparing the nerve.

Similar to acoustic schwannomas, they may grow to a quite sizable extent and exert positive mass effect with growith into the cerebellopontine angle. More frequently they present with gradual, and often incomplete, facial nerve palsy. Rapid onset mimicking a Bell's palsy is certainly also recognised. Mass effect on adjacent nerves may well cause sensory neural hearing loss (SNHL) or even conductive hearing loss if growth into the middle ear impairs the normal function of the ossicles. In a minority of cases ( ~10%) the tumour is extracranial, where it presents as an asymptomatic parotid mass.

Radiographic features

Imaging characteristics of the tumour itself is the same as that of schwannomas elsewhere : homogenous enhancement on both CT and MRI when small; heterogenous when large. The best clue that a schwannoma is of the facial nerve rather than the more common vestibulochoclear nerve (CN VIII) is expansion extenign along the facial nerve canal - especially along the labyrinthine segment.

In all cases, hi resolution thin section bone CT of the temporal bones is required to assess the otic capsule. Failure to preoperatively diagnose bony erosion can lead to postoperative fistula formation.