A 52-year-old man presents with a 6-month history of unilateral hearing loss and tinnitus. Pure-tone audiometry reveals a 20 dB inter-aural difference at 2 kHz and 3 kHz, with speech discrimination 35% in the affected ear despite a PTA of 40 dB HL. The audiogram is consistent with asymmetric sensorineural hearing loss. The step marked **D** in the diagnostic algorithm is recommended as the gold standard investigation. Which of the following best describes why this investigation is essential in this clinical scenario?

Explanation

## Why MRI brain and internal auditory canals with contrast is right Asymmetric sensorineural hearing loss (>15 dB inter-aural difference at 2 consecutive frequencies) is a red flag for retrocochlear pathology, particularly vestibular schwannoma (acoustic neuroma). The combination of asymmetric SNHL, unilateral tinnitus, and disproportionately reduced speech discrimination (>15% worse than expected from PTA) significantly raises suspicion for vestibular schwannoma. MRI brain with dedicated thin-slice internal auditory canal (IAC) sequences and gadolinium contrast is the gold standard investigation because it detects even small vestibular schwannomas (which may be missed on routine brain MRI without dedicated IAC sequences). This imaging is essential to rule out a Schwann cell tumor arising from the vestibular division of CN VIII, which is the primary concern in this clinical presentation (Dhingra ENT 7e; AAO-HNS Guidelines). ## Why each distractor is wrong - **ABR with increased latency**: While ABR can show abnormal waveforms (increased latency, interpeak intervals) in retrocochlear pathology, it is less sensitive than MRI for detecting small vestibular schwannomas. ABR is a historical test and is no longer the primary investigation for suspected VS. MRI is superior for definitive diagnosis. - **Serial audiometry over 6 months**: Serial audiometry is useful for monitoring hearing trends and confirming consistency of asymmetric loss, but it cannot visualize or definitively diagnose a structural lesion such as vestibular schwannoma. Imaging is mandatory when asymmetric SNHL with red flag features is identified; delaying MRI risks missing a growing tumor. - **High-resolution CT of temporal bones**: CT is excellent for assessing bony anatomy and is useful in post-operative evaluation or when MRI is contraindicated, but it has poor soft-tissue resolution and cannot reliably detect small vestibular schwannomas. MRI with contrast is far superior for detecting intracanalicular and small extracanalicular tumors. **High-Yield:** Asymmetric SNHL (>15 dB inter-aural difference) + disproportionate speech discrimination loss = MRI IAC with contrast mandatory to exclude vestibular schwannoma. [cite: Dhingra ENT 7e; AAO-HNS Guidelines on Asymmetric Sensorineural Hearing Loss]