The differential diagnosis for an extrinsic intracranial mass in the cerebellopontine angle includes the vestibular Schwannoma, meningioma, and primary cholesteatoma (epidermoid inclusion cyst). Vestibular Schwannomas comprise 8-10% of cranial tumors and occur at a rate of 0.78-1.15/100,000 in the United States. They usually arise from the Obersteiner-Redlich transitional zone or line of the superior division of the vestibular nerve. This line occurs 8-12 mm distal to the brain, is the site where Schwann and connective cells give way to glia (oligodendrocytes), and marks the true boundary between the central and peripheral nervous system. They occur unilaterally in >95% of cases; bilateral masses are found in patients with neurofibromatosis Type 2, found on chromosome 22.

This mass is unusual in that one would expect extension of the mass to/from the IAC. Therefore, CPA meningioma was originally thought to be a strong possibility. However, the mass was confirmed by frozen section to exhibit pathology consistent with that of a vestibular Schwannoma.

Thin slice gadolinium-enhanced MRI is the study of choice with 98% sensitivity and a false-positive rate of ~0%. Computed tomography with intravenous contrast is the second line study.