Facial nerve schwannoma
Facial nerve schwannoma, 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.
Facial nerve schwannoma 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 with 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.
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 vestibulocochlear 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 remodeling 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
Management options include nerve resection or tumor debulking, decompression of the facial nerve canal, radiosurgery, and observation 4. The choice depends on the level of residual facial nerve function, hearing function, comorbidities, and patient preference 4.
The differential depends on location of the lesion:
- 1. Robson C, 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, Shelton C, Kertesz TR, Glastonbury CM. The many faces of facial nerve schwannoma. (2006) AJNR. American journal of neuroradiology. 27 (3): 694-9. Pubmed
- 3. Shimizu K, Iwai H, Ikeda K, Sakaida N, Sawada S. Intraparotid facial nerve schwannoma: a report of five cases and an analysis of MR imaging results. (2005) AJNR. American Journal of Neuroradiology. 26 (6): 1328-30. Pubmed
- 4. Makadia L, Mowry SE. Management of intratemporal facial nerve schwannomas: The evolution of treatment paradigms from 2000-2015. (2016) World Journal of Otorhinolaryngology. 6(1): 13-18 Published online Feb 28, 2016. doi: 10.5319/wjo.v6.i1.13