title: The bony crescent sign--a new sign of facial nerve schwannoma.
abstract: Schwannomas of the facial nerve are rare slowly growing lesions that have a predilection for the geniculate ganglion. Radiological evaluation is important in their diagnosis and in the assessment of their extent. In our series of 4 cases the facial nerve schwannoma was seen on high resolution CT as a soft tissue mass bounded anteriorly by a thin rim of bone. This bony crescent sign is a previously undescribed sign of facial nerve schwannoma which appears on the basis of this small series to be strongly indicative of the presence of this tumour. Schwannomas are relatively uncommon intracranial tumours. They most commonly involve the acoustic nerve followed in frequency by the trigeminal nerve. Other cranial nerves are rarely involved. Facial nerve schwannomas occurring within the petrous temporal bone are very rare. Their diagnosis may be missed prospectively even when appropriate CT scans are performed. Even in retrospect the site of abnormality may be difficult to identify, especially if there is an associated middle ear mass such as a cholesteatoma. Lesions occurring in the petrous area are all rare. The differential diagnosis includes cholesterol granuloma, epidermoid, carotid aneurysm and, very rarely, primary and secondary bone tumours. We describe a new sign associated with facial nerve schwannoma on CT, that of a bony crescent. Recognition of this sign makes those tumours arising in the region of the geniculate ganglion easy to diagnose prospectively.
authors.affiliation: Department of Radiology, St. Vincent's Hospital, Darlinghurst, NSW, Australia.
authors.forename: A
authors.initials: A
authors.lastname: Watts
authors.forename: P
authors.initials: P
authors.lastname: Fagan
mesh_terms: D000293:Adolescent; D000328:Adult; D002675:Child, Preschool; D005153:Facial Neoplasms / Q000175:diagnosis / Q000000981:diagnostic imaging* / Q000601:surgery; D005154:Facial Nerve / Q000000981:diagnostic imaging* / Q000503:physiopathology / Q000601:surgery; D006801:Humans; D008297:Male; D009442:Neurilemmoma / Q000175:diagnosis / Q000000981:diagnostic imaging* / Q000601:surgery; D014057:Tomography, X-Ray Computed