Pathologically Confirmed Cerebellopontine Angle Arachnoid Granulation: Case Report

Introduction: Arachnoid granulations (AG) allow normal cerebrospinal fluid (CSF) drainage from the subarachnoid space into the venous system. Rarely, they can become symptomatic if they grow large enough to affect surrounding structures. We report a rare case of a pathologically confirmed AG in the cerebellopontine angle (CPA).

Case: A 56-year-old female patient without significant medical history presented with a 3-year history of progressive right-sided hearing loss with tinnitus and recurrent vertigo. Repeat brain MRIs over a 10-month period showed a stable, multiloculated extra-axial cystic nonenhancing mass within the lateral aspect of the right CPA cistern. It was associated with extension through the posterior cortex of the right temporal bone. The lesion was isointense to CSF without restricted diffusion and CT scan confirmed bony erosion.

A right suboccipital craniotomy was performed and the radiographic bony defect was immediately appreciated within the CPA. The defect was superficial and inferior to the internal auditory canal with a small vascular bundle extending from the cerebellum into the bony defect, tethered by the dura. The lesion itself was completely empty of spinal fluid after having being drained inferiorly for opening of the CPA. The lesion appeared to be eroding into the endolymphatic sac. The lesion had the gross appearance of thickened arachnoid membrane resembling and arachnoid cyst. It was resected in piecemeal fashion. There were no complications following surgery and the patient discharged on postoperative day3.

Pathology confirmed a benign process comprised of several components including arachnoid cells, fibrous tissue, portions of bone, and neuroglia tissue. At 3-month follow-up, MRI showed complete resection and continued right-sided hearing loss.

Discussion: We believe this represents the first case of a pathologically confirmed giant AG in the posterior wall of the temporal bone associated with cerebellar herniation and subsequent strangulation. The lesion was likely an incidental finding in the setting of unilateral hearing loss workup.