A 52-year-old man presents with a 3-year history of progressive unilateral hearing loss and tinnitus. Audiometry reveals asymmetric sensorineural hearing loss with disproportionately poor word recognition score. MRI with gadolinium shows the structure marked **A** in the diagram — a well-demarcated, avidly enhancing mass in the cerebellopontine angle with extension into the internal auditory canal, producing an "ice-cream cone" appearance. Which of the following best describes the cellular origin and pathological classification of this lesion?

Explanation

## Why option 1 is correct The structure marked **A** — a cerebellopontine angle mass with internal auditory canal extension producing the classic "ice-cream cone" appearance on contrast-enhanced MRI — is pathognomonic for vestibular schwannoma (acoustic neuroma). This is a benign, slow-growing tumor arising from Schwann cells of the vestibular division (typically the inferior vestibular nerve) of cranial nerve VIII. Vestibular schwannomas account for 80–90% of all cerebellopontine angle masses and 8% of intracranial tumors. The clinical presentation of unilateral sensorineural hearing loss with disproportionately poor word recognition (a retrocochlear sign) and tinnitus over years is classic for this diagnosis. The avidly enhancing, sharply demarcated lesion with intracanalicular extension is the gold-standard MRI appearance. (Harrison 21e Ch 32; Cummings Otolaryngology 7e) ## Why each distractor is wrong - **Option 2 (Meningioma)**: While meningiomas account for ~10% of cerebellopontine angle masses, they are dural-based, typically cause hyperostosis (not widening) of the internal auditory canal, and often show a broad dural tail. The "ice-cream cone" appearance with intracanalicular extension is characteristic of schwannoma, not meningioma. - **Option 3 (Malignant schwannoma from cochlear nerve)**: Vestibular schwannomas are benign, not malignant. Moreover, they arise from the vestibular division of CN VIII (typically the inferior vestibular nerve), not the cochlear division. Malignant transformation of acoustic neuromas is exceedingly rare. - **Option 4 (Epidermoid cyst)**: Epidermoid cysts are benign lesions of ectodermal origin that do show restricted diffusion on DWI sequences, but they account for only 5–10% of cerebellopontine angle masses and do not produce the characteristic avidly enhancing, well-demarcated "ice-cream cone" appearance seen here. They are typically non-enhancing or minimally enhancing. **High-Yield:** Unilateral sensorineural hearing loss with poor speech discrimination + "ice-cream cone" CPA mass on contrast MRI = vestibular schwannoma until proven otherwise; bilateral acoustic neuromas = neurofibromatosis type 2 (NF2). [cite: Harrison 21e Ch 32; Cummings Otolaryngology 7e]