Vestibular schwannomas make up 80% to 90% of the masses found in the cerebellopontine angle (Lalwani 1992; Smirniotopoulos et al 1993; McKenzie 1994). The differential diagnosis of a mass in the cerebellopontine angle includes meningioma, epidermoid cyst, arachnoid cyst, exophytic glioma, cerebellar hemangioblastoma, metastasis, choroid plexus papillomas, glomus tumor, and primary bone tumors. Meningioma is the second most common cerebellopontine angle tumor (10% to 15% of cerebellopontine angle lesions). Distinguishing clinical features of meningiomas include their propensity to cause facial nerve dysfunction or trigeminal neuralgia rather than isolated hearing loss. Radiologically, meningiomas often lack extension into and expansion of the internal auditory canal, have a sessile base, cause underlying hyperostosis of the petrous bone, tend to be more homogeneously enhancing, have a "dural tail," and sometimes allow visualization of the seventh and eighth nerve complex, even in large tumors (Mulkens et al 1993).
The third most common mass of the cerebellopontine angle is the epidermoid inclusion cyst, making up 5% to 9% of all lesions (Lalwani 1992; Smirniotopoulos et al 1993). Patients often have a long history of hearing loss and tinnitus. On CT, epidermoid cysts are usually hypodense compared to brain (similar to water or CSF). Calcification of the cyst rim is noted in 25% of cases. Enhancement is negligible within the cyst but may occur along the rim. On MRI, epidermoid cysts are usually isointense to CSF (ie, low signal intensity on T1 images and high signal intensity on T2 images) and may have a lamellated appearance. Similar to CT, the cyst does not enhance with gadolinium, except for the rim.
Arachnoid cysts can also occur in the cerebellopontine angle. They have signal intensity and attenuation characteristics identical to CSF and do not enhance with contrast. Many other lesions can arise in the cerebellopontine angle, each of which accounts for less than 1% of all cases. Tumors that may extend into, but not arise from, the cerebellopontine angle include exophytic brainstem gliomas, ependymomas, choroid plexus papillomas, schwannomas of cranial nerves V, VII, IX, X, and XI, jugular foramen paragangliomas, lipomas, and metastases (Lalwani 1992; Smirniotopoulos et al 1993). Vascular processes such as aneurysm, malformations, and aberrant loops of normal blood vessels can develop in the cerebellopontine angle. Enhancement on CT and a flow void on MRI are diagnostic of a vascular process. Rarely, infectious processes can develop in the cerebellopontine angle, such as tuberculomas and cysticercosis. Benign vestibular conditions (eg, Meniere disease) can suggest vestibular schwannoma with symptoms and signs of hearing loss, tinnitus, nystagmus, and vertigo (Mafee 1995). However, tumor can be ruled out by a careful history (ie, fluctuating symptoms that occur in attacks, bilateral involvement in approximately 20%) and a normal enhanced MRI scan.