Exophytic brain tumors mimicking primary lesions of the cerebellopontine angle

The vast majority of cerebellopontine angle (CPA) tumors are extraaxial masses arising from either the eighth nerve (acoustic neuroma) or meninges (meningioma). Rarely, a tumor that arises from the brain parenchyma may protrude laterally to present with a clinical and radiographic picture simulating that of the much more common extraaxial lesions. Three individuals with CPA lesions that ultimately proved at operation to be exophytic brain tumors (pontine medulloblastoma, cerebellar astrocytoma, and fourth ventricular ependymoma) are described. The clinical manifestations of these lesions, although not entirely typical, fell well within the possible range of presentation of benign tumors primary to the CPA. In two of these cases the tumor actually penetrated into the internal auditory canal. As the optimal management strategy for treating parenchymal tumors differs substantially from that for extraaxial lesions, it is essential that the surgeon have a preoperative awareness of the lesion's nature before embarking on a surgical endeavor. Critical to arriving at the correct diagnosis is the close examination of preoperative imaging studies. The clinician should be alerted to the possibility that a CPA tumor is of intraaxial origin when the preoperative magnetic resonance imaging scan shows 1) blurring of the margin between the tumor and brainstem or cerebellum; 2) a degree of peritumoral hyperintensity on Ta-weighted scans disproportionate to the size of the extraaxial mass; and 3) dilation of the lateral recess of the fourth ventricle.