Outgrowing schwannomas arising from tympanic segments of the facial nerve.

PURPOSE: Although there have been numerous reports of clinical cases of facial nerve schwannoma, histopathologic studies of its early stages of development are rare. The purpose of this study was to describe the incidence, origin, and pathology of small, asymptomatic schwannomas of the facial nerve.

MATERIALS AND METHODS: Fifteen hundred twenty-six ears from the human temporal bone collection of the Otitis Media Research Center Otopathology Laboratory at the University of Minnesota were studied under light microscopy.

RESULTS: Twelve bones with morphologic indications of type A Antoni tissue that mimics facial schwannoma in the tympanic cavity were selected. The 12 ears were from nine individuals with asymptomatic histories. The twelve schwannomas developed from perineurium at dehiscences in the facial canal near the oval window, forming defects in the epineurium by growth of Schwann cells downward and outward toward the stapes. No obvious compressions or lesions within fibers in the facial nerve were observed.

CONCLUSION: Because these tumors grow slowly without hearing loss or facial nerve symptoms, they may be mistaken for granulation tissue during surgery for chronic otitis media.