A 65-year-old man, who was lost to follow-up after abdominal-perineal resection for rectal adenocarcinoma 9 months earlier, presents with progressively worsening neurological symptoms, including bilateral hearing loss, dizziness, gait disturbance, ataxia, and blindness in the right eye.
Audiometric evaluation demonstrates profound hearing loss with no identifiable thresholds in all frequencies, absent reflexes, and absent otoacoustic emissions.
Findings on a CT scan of the head are unremarkable. MRI scans of the brain with gadolinium reveal enhancement along the meningeal surfaces of the right optic nerve and to a lesser degree of the left optic nerve (A), and expanded cranial nerves VII and VIII bilaterally (B). No evidence of hydrocephalus, intraparenchymal mass, or stroke is present.
Based on the clinical picture and MRI findings, what would you include in the differential?