A 52-year-old woman presents with progressive unilateral hearing loss and tinnitus over 18 months. Pure tone audiometry shows asymmetric high-frequency sensorineural hearing loss with disproportionately poor speech discrimination. Gadolinium-enhanced MRI reveals the structure marked **A** in the diagram—a mass in the cerebellopontine angle with extension into the internal auditory canal, creating an "ice-cream cone" appearance. Which of the following best describes the origin and pathological nature of this lesion?

Question

Accepted Answer

A. Benign tumor arising from Schwann cells of the vestibular division of CN VIII at the Obersteiner-Redlich zone, accounting for 80-90% of cerebellopontine angle masses. ## Why option 1 is correct

The structure marked **A**—a CPA mass with IAC extension showing the classic "ice-cream cone" sign on gadolinium-enhanced MRI—is pathognomonic for vestibular schwannoma (formerly "acoustic neuroma"). This benign, slow-growing tumor arises from Schwann cells of the vestibular division of CN VIII at the Schwann cell–oligodendrocyte junction (Obersteiner-Redlich zone) near the porus acusticus. Vestibular schwannomas account for 80–90% of all cerebellopontine angle tumors and 8% of intracranial neoplasms. The clinical presentation—asymmetric high-frequency sensorineural hearing loss with disproportionately poor speech discrimination—is the classic audiological signature, reflecting the tumor's slow growth and location along the vestibular nerve. [Harrison's 21e Ch 449; Cummings Otolaryngology Ch 178]

## Why each distractor is wrong

- **Option 2 (Malignant meningioma)**: Meningiomas are typically dural-based, show different enhancement patterns, and present with facial nerve palsy as an early sign—not asymmetric hearing loss. They are also not the most common CPA mass.
- **Option 3 (Epidermoid cyst)**: Epidermoids are non-enhancing, show T2 hyperintensity without gadolinium uptake, and are usually incidental findings. They do not present with progressive hearing loss and poor speech discrimination.
- **Option 4 (Vascular malformation)**: AICA malformations cause mass effect and brainstem compression but do not produce the gradual asymmetric high-frequency hearing loss pattern seen here. They are not the most common CPA pathology.

**High-Yield:** Vestibular schwannoma = "ice-cream cone" on MRI + asymmetric high-frequency SNHL with disproportionate speech discrimination loss + 80–90% of CPA tumors; bilateral lesions = NF2 until proven otherwise.

[cite: Harrison's 21e Ch 449; Cummings Otolaryngology Ch 178]

Answer

B. Malignant meningioma arising from dura mater of the cerebellopontine angle, typically presenting with facial nerve palsy

Answer

C. Benign epidermoid cyst arising from ectodermal remnants in the internal auditory canal, usually asymptomatic until large

Answer

D. Vascular malformation of the anterior inferior cerebellar artery causing mass effect on the brainstem

Explanation

Why option 1 is correct

Why each distractor is wrong

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