A 35-year-old woman with progressive unilateral hearing loss and tinnitus undergoes audiometry. The audiogram shows a unilateral sensorineural hearing loss with a relatively flat configuration across frequencies. MRI reveals a mass in the internal acoustic meatus. Which is the most common histological type of this lesion?

Explanation

## Vestibular Schwannoma: The Most Common Retrocochlear Lesion ### Clinical Presentation & Diagnosis **Key Point:** A **unilateral sensorineural hearing loss** (SNHL) with **tinnitus** and a **mass in the internal acoustic meatus (IAM)** on imaging is **vestibular schwannoma (acoustic neuroma)** until proven otherwise. **High-Yield:** Vestibular schwannoma accounts for **80–90% of all IAM masses** and is the **most common retrocochlear cause of hearing loss**. ### Pathology & Epidemiology - **Benign tumor** arising from the vestibular division of CN VIII (Schwann cell origin) - Peak incidence: 40–60 years (but can occur at any age) - Usually **unilateral** (bilateral in neurofibromatosis type 2 [NF2]) - Slow growth; may remain asymptomatic for years - Accounts for ~8% of all intracranial tumors ### Audiometric Features **Clinical Pearl:** The audiogram in vestibular schwannoma is variable: - Often **unilateral SNHL** (distinguishes from presbycusis, which is bilateral) - May show **high-frequency loss** or **flat configuration** - **Asymmetric** hearing loss between ears is a red flag for retrocochlear pathology - Associated **poor speech discrimination** out of proportion to pure-tone threshold loss (a sensitive marker) ### Comparison of IAM Masses | Feature | Vestibular Schwannoma | Meningioma | Cholesteatoma | Hemangioma | |---------|----------------------|-----------|---------------|------------| | **Frequency** | 80–90% of IAM masses | 5–10% | Rare in IAM | <1% | | **Origin** | CN VIII (Schwann cell) | Dura/arachnoid | Epithelial; usually middle ear | Blood vessel | | **Laterality** | Usually unilateral (bilateral in NF2) | Usually unilateral | Usually unilateral | Unilateral | | **Age** | 40–60 years | 50–70 years | Any age | Any age | | **Imaging** | Well-demarcated, enhancing mass in IAM | Dural tail sign; broad base | Erosive, lytic lesion | Hypervascular | | **Hearing loss** | Unilateral SNHL | Variable | Conductive or mixed | Variable | | **Associated symptoms** | Vertigo, tinnitus, facial nerve involvement (late) | Headache, CN palsies | Otorrhea, conductive loss | Pulsatile tinnitus | ### Why Vestibular Schwannoma Is Most Common 1. **Schwann cells** are abundant along CN VIII 2. **Slow growth** allows long asymptomatic period before detection 3. **Benign nature** means patients survive to diagnosis 4. **Routine MRI** for hearing loss increases detection **Mnemonic: SCHWANNOMA** — **S**chwann cell origin, **C**N VIII (vestibular), **H**earing loss (unilateral), **W**ell-demarcated, **A**coustic neuroma (synonym), **N**eurofibromatosis association (NF2), **N**eoplasm (benign), **O**ften asymptomatic initially, **M**ass in IAM, **A**ccounting for 80–90% of IAM lesions. **Tip:** Any **unilateral SNHL with asymmetric audiogram** warrants **MRI of the IAM** to rule out vestibular schwannoma. This is a high-yield exam principle.