Facial nerve schwannoma (FNS), also known as facial nerve neuroma/neurilemoma, is a schwannoma that arises from the facial nerve. They are generally uncommon, and when involving the temporal bone, make up less than 1% of all temporal bone tumors.
FNS is a rare tumor 2.
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 tumor is extra-cranial, where it presents as an asymptomatic parotid mass.
As schwannomas elsewhere they originate from the surface of the nerve, and displace and splay the nerve fibers over their eccentric growth (this compares favourably to neurofibromas through which the axons are distributed). This allows for potential resection of the tumor while sparing the nerve.
Typically positive for S100 neuronal staining.
Imaging characteristics of the tumor itself is the same as that of schwannomas elsewhere: homogeneous enhancement on both CT and MRI when small; heterogeneous when large. The best clue that a schwannoma is of the facial nerve rather than the more common vestibulochoclear nerve (CN VIII) is expansion extending along the facial nerve canal - especially along the labyrinthine segment.
In all cases, high resolution thin section bone CT of the temporal bone is required to assess the otic capsule. Bony scalloping and remodelling may be present. Failure to preoperatively diagnose bony erosion can lead to postoperative fistula formation.
Signal characteristics include 2:
- T1: typically iso- to hypointense relative to gray matter
- T2: typically hyperintense; large lesion may show heterogeneous signal
- T1 C+ (Gd): usually demonstrate homogeneous enhancement with larger lesions with cystic degeneration seen as focal intramural low signal intensity
Treatment and prognosis
- content pending
Depends on location on lesion along the facial nerve:
- 1.MBChB CDR, Koch BL, Harnsberger HR. Specialty Imaging: Temporal Bone: Published by Amirsys. Lippincott Williams & Wilkins. ISBN:1931884692. Read it at Google Books - Find it at Amazon
- 2. Wiggins RH, Harnsberger HR, Salzman KL et-al. The many faces of facial nerve schwannoma. AJNR Am J Neuroradiol. 2006;27 (3): 694-9. AJNR Am J Neuroradiol (full text) - Pubmed citation
- 3. Shimizu K, Iwai H, Ikeda K et-al. Intraparotid facial nerve schwannoma: a report of five cases and an analysis of MR imaging results. AJNR Am J Neuroradiol. 26 (6): 1328-30. AJNR Am J Neuroradiol (full text) - Pubmed citation