A 52-year-old man presents with 2 years of progressive right-sided hearing loss, high-pitched tinnitus, and recent right-sided facial numbness (V1 distribution). Otoscopy is normal. Weber test lateralizes to the left ear; Rinne is positive bilaterally. Pure-tone audiometry shows the pattern marked **C** in the diagram — high-frequency SNHL on the right (mild at 1 kHz, moderate-severe at 4–8 kHz) with normal left ear. Critically, speech discrimination score is only 30% on the right, markedly worse than predicted from the pure-tone average. Stapedial reflexes show decay on the right. Which of the following is the GOLD STANDARD investigation to confirm the diagnosis suggested by the audiometric pattern marked **C**?
A. High-resolution CT temporal bone — best for detecting ossicular erosion and canal enlargement
B. MRI internal auditory canal with gadolinium contrast — shows T1 hypointense, T2 hyperintense enhancing mass
C. Electrocochleography — measures summating potential and action potential to localize retrocochlear lesion
D. Auditory brainstem response (ABR) — shows prolonged latency and interaural latency difference
Explanation
Why MRI internal auditory canal with gadolinium contrast is right
The audiometric pattern marked C — asymmetric high-frequency SNHL with disproportionately poor speech discrimination (roll-over phenomenon) — is the classic retrocochlear signature of vestibular schwannoma (acoustic neuroma). The combination of unilateral high-frequency SNHL, tinnitus, facial numbness (CN V involvement), and decay of stapedial reflexes all point to a mass in the internal auditory canal (IAC) or cerebellopontine angle (CPA). MRI with gadolinium is the gold standard investigation (per Cummings Otolaryngology — Vestibular Schwannoma). The tumor appears T1 hypointense, T2 hyperintense, and shows homogeneous gadolinium enhancement. When a CPA component is present, the classic "ice cream on cone" sign is seen. This is the definitive imaging modality for diagnosis and staging.
Why each distractor is wrong
High-resolution CT temporal bone: While HRCT is useful for assessing bony anatomy, ossicular chain, and canal enlargement, it is NOT the gold standard for soft-tissue tumor detection. CT has poor soft-tissue contrast and cannot reliably visualize small intracanalicular tumors or characterize the lesion as well as MRI.
Electrocochleography: Although ECoG can help localize retrocochlear pathology by measuring summating potential and action potential, it is a functional test, not an imaging modality. It cannot identify the specific tumor, its size, location, or extent — essential for management decisions.
Auditory brainstem response (ABR): ABR is sensitive for retrocochlear lesions (showing prolonged latency, interaural latency difference, or absent waves) but, like ECoG, is a functional test. It cannot visualize the tumor, determine its size, or guide surgical planning. It is used as a screening tool, not for definitive diagnosis.
High-YieldNEET PG
Asymmetric SNHL with speech discrimination worse than pure-tone average (roll-over phenomenon) + unilateral tinnitus + facial nerve signs = vestibular schwannoma until proven otherwise. MRI with gadolinium is mandatory.
