A 52-year-old man presents with progressive unilateral high-frequency sensorineural hearing loss and tinnitus over 18 months. Audiometry shows asymmetric loss with disproportionately poor speech discrimination (rollover phenomenon). MRI reveals an isointense T1, hyperintense T2 lesion that enhances intensely post-gadolinium, with an intracanalicular component and a rounded cerebropontine angle component creating an "ice-cream-cone" appearance. The structure marked **A** in the diagram represents this lesion. Which of the following best describes the origin of this tumor?
A. Malignant nerve sheath tumor arising from the facial nerve (CN VII) within the internal acoustic meatus
B. Benign Schwann-cell tumor arising from the vestibular division of CN VIII at the Obersteiner-Redlich zone within the internal acoustic meatus
C. Benign epidermoid tumor with restricted diffusion and no post-contrast enhancement, arising from epithelial rests
D. Meningeal tumor with broad dural base and dural tail, arising from the dura of the cerebropontine angle
Explanation
Why option 1 is correct
The clinical presentation (progressive unilateral high-frequency SNHL with rollover phenomenon), imaging findings (ice-cream-cone sign with intracanalicular and CP angle components, intense post-gadolinium enhancement), and location at the internal acoustic meatus are pathognomonic for vestibular schwannoma. This is a benign Schwann-cell tumor arising from the superior or inferior vestibular division of CN VIII, typically at the glial-Schwann cell junction (Obersteiner-Redlich zone) within the IAM. This is the most common CP angle tumor (~80% of cases) per AAO-HNS guidelines.
Why each distractor is wrong
Option 2 (Meningioma): While meningioma is the second most common CP angle tumor, it presents with a broad dural base and dural tail on imaging. Meningiomas do not widen the internal acoustic meatus and do not produce the ice-cream-cone sign. The intracanalicular component is characteristic of vestibular schwannoma, not meningioma.
Option 3 (Epidermoid): Epidermoid cysts are benign but arise from epithelial rests, not from nerve tissue. They show restricted diffusion on DWI and do NOT enhance post-contrast—a key distinguishing feature. The intense post-gadolinium enhancement described in this case rules out epidermoid.
Option 4 (Facial nerve schwannoma): Although facial nerve schwannomas can occur within the IAM, they are rare compared to vestibular schwannomas and do not account for the typical presentation of asymmetric high-frequency SNHL with rollover. Vestibular schwannomas are ~20 times more common than facial nerve schwannomas in the IAM.
High-YieldNEET PG
Vestibular schwannoma = benign Schwann-cell tumor from CN VIII at Obersteiner-Redlich zone; ice-cream-cone sign on MRI; progressive asymmetric high-frequency SNHL with poor speech discrimination (rollover) is the classic presentation.
