A 52-year-old man presents with 6 months of progressive unilateral hearing loss and right-sided tinnitus. Pure-tone audiometry shows a moderate sensorineural hearing loss with PTA of 45 dB HL on the right. Speech audiometry reveals a Speech Reception Threshold (SRT) of 48 dB HL, which agrees with PTA. However, when monosyllabic phonetically balanced words are presented at 40 dB above SRT, the patient achieves only 32% correct identification. When intensity is further increased, the score paradoxically decreases to 18%. The finding marked **A** in the diagram — severely reduced SDS with high-intensity rollover — is demonstrated. Which of the following is the most appropriate next diagnostic step?

Explanation

## Why MRI of the internal auditory canal with gadolinium is correct The clinical presentation — unilateral asymmetric SNHL, unilateral tinnitus, and **disproportionately depressed SDS (32%) that is markedly out of proportion to the moderate PTA (45 dB HL)** — combined with the **rollover phenomenon (SDS decreasing from 32% to 18% as intensity increases)** is pathognomonic for retrocochlear pathology, most commonly vestibular schwannoma (acoustic neuroma). According to Dhingra ENT 7e and Cummings Otolaryngology 7e, a **PTA-SDS mismatch with rollover index >0.45** is a critical red flag requiring immediate imaging. MRI with gadolinium using thin-slice T1-weighted internal auditory canal (IAC) protocol is the **gold standard** for detecting vestibular schwannomas as small as 2–3 mm and is mandatory in any patient with asymmetric SNHL or asymmetric tinnitus to exclude retrocochlear lesions. This is the definitive diagnostic test and must be ordered urgently. ## Why each distractor is wrong - **Repeat pure-tone audiometry with bone conduction thresholds**: While bone conduction testing helps confirm sensorineural versus conductive loss, it does NOT address the specific red flag of disproportionately poor SDS with rollover. The diagnosis is already clear from the speech audiometry mismatch; repeating PTA delays the critical imaging needed to rule out a structural lesion. - **Auditory brainstem response (ABR) with interaural latency analysis**: Although ABR is a historical screening tool for retrocochlear pathology (showing prolonged I–V interpeak latency or interaural wave V latency difference >0.2 ms), it has ~85% sensitivity and may miss small intracanalicular tumors. ABR is NOT the gold standard; MRI is superior and can detect lesions as small as 2–3 mm. MRI should be ordered first, not ABR. - **Tympanometry and stapedial reflex testing**: These tests assess middle ear function and reflex arc integrity. Stapedial reflex decay was historically used as a retrocochlear screening tool but is now obsolete and has poor sensitivity. These tests do not directly address the need to image the internal auditory canal and brainstem to exclude vestibular schwannoma. **High-Yield:** **SDS <50% disproportionate to PTA + unilateral tinnitus + rollover phenomenon = MRI IAC with gadolinium to rule out vestibular schwannoma.** This is the PTA-SDS mismatch rule — the most important clinical pearl in retrocochlear diagnosis. [cite: Dhingra ENT 7e Ch 13; Cummings Otolaryngology 7e Ch 138]