giovedì 11 gennaio 2007
CT postcontrast reveals an unenhancing soft tissue mass centered at Meckel's cave on the left (Figure 1)
CT bone window (Figure 2) reveals a markedly enlarged foramen ovale on the left, with smooth, corticated margins.
Axial T1-WI (Figure 3) demonstrates a well-circumscribed extraaxial mass in the region of left Meckel’s cave that is mildly hypointense to brain parenchyma. The mass shows heterogeneous signal intensity on FLAIR (Figure 3). Postcontrast images show moderately intense enhancement, extending into the cisternal segment of cranial nerve V (Figure 4). The mass is seen to extend through a widened foramen ovale into left infratemporal fossa and present as masticator mass (Figure 4).
Diagnosis: Trigeminal schwannoma
Schwannomas account for 6% to 8% of intracranial neoplasms. Vestibular schwannomas are the most common cranial nerve schwannomas, followed by trigeminal and facial schwannomas.
The trigeminal nerve (CN V) exits the brainstem at the level of the mid pons, and its three divisions—the ophthalmic (CN V1), maxillary (CN V2), and mandibular (CN V3) branches—together proceed anteriorly toward the trigeminal ganglion in the Meckel’s cave. From here, the mandibular division exits inferiorly via the foramen ovale. The maxillary and ophthalmic divisions continue anteriorly along the lateral aspect of the cavernous sinus. Eventually, the ophthalmic division enters the orbit via the superior orbital fissure, while the maxillary division exits the cranial vault through the foramen rotundum.
Trigeminal schwannomas are encapsulated tumors of Schwann cells displace rather than infiltrate fascicles of CN V3.
Trigeminal schwannomas usually present in the third to fourth decade with sensory disturbances. Mastication muscles weakness or facial pain is less common.
Trigeminal schwannomas may arise along any segment of CN V, but the majority develop at the level of trigeminal ganglion. They may then extend posteriorly into the posterior fossa, or anteriorly through the skull base foramina, and often have a “dumbbell” configuration.
On nonenhanced CT scans, most schwannomas are isodense relative to brain parenchyma. Calcification or areas of hemorrhage are rare. On contrast-enhanced CT scans, the enhancement pattern is typically homogeneous. Foramen ovale is most commonly enlarged by CN V3 schwannoma.
Schwannomas typically are isointense or slightly hypointense relative to gray matter on T1-weighted images and hyperintense on T2-weighted images. Gadolinium enhancement typically is homogeneous, although larger schwannomas can show areas of cystic degeneration and heterogeneous signal intensity. Atrophy of masticator muscles may be seen as high signal fatty infiltration on T1-weighted images with volume loss.
Surgical resection is the treatment of choice.
Trigeminal schwannoma may be associated with Neurofibromatosis type 2, especially if it occurs in younger patients.