A 35-year-old woman presents with a 2-year history of vertigo triggered by loud sounds and autophony (hearing her own heartbeat and eye movements). Audiometry shows the pattern marked **A** in the diagram — low-frequency air-bone gap with bone-conduction thresholds better than 0 dB HL at 250–500 Hz. Stapedial reflexes are intact bilaterally. High-resolution CT temporal bone in Pöschl plane reveals absence of bone overlying the superior semicircular canal at the floor of the middle cranial fossa. Which of the following best explains the pathophysiology of the conductive-pattern hearing loss in this patient?

Explanation

## Why "Creation of a third mobile window..." is right Superior semicircular canal dehiscence (SCDS) creates a bony defect at the floor of the middle cranial fossa, establishing a third mobile window in the inner ear alongside the oval and round windows. Sound and pressure energy that should drive the cochlea is diverted through this dehiscent canal, resulting in a conductive-pattern low-frequency hearing loss. Simultaneously, the same energy stimulates the superior canal cupula, triggering the Tullio phenomenon (vertigo with loud sounds) and autophony. The paradoxical supranormal bone-conduction thresholds (negative dB values) occur because bone vibration can directly stimulate the cochlea via the third window, bypassing the normal ossicular pathway. This mechanism is pathognomonic for SCDS and is directly visualized on high-resolution CT in the Pöschl plane (perpendicular to the superior semicircular canal). ## Why each distractor is wrong - **Stapes footplate fixation at the oval window**: This is the mechanism of otosclerosis, which also produces low-frequency conductive loss with an air-bone gap. However, otosclerosis characteristically shows ABSENT stapedial reflexes and does NOT produce Tullio phenomenon, autophony, or supranormal bone-conduction thresholds. The intact reflexes in this patient exclude otosclerosis. - **Disruption of the ossicular chain continuity**: Ossicular discontinuity (marked **C** in the diagram) causes conductive hearing loss but does NOT explain the supranormal bone-conduction thresholds, Tullio phenomenon, or autophony. Bone conduction would be normal or reduced, not better than 0 dB HL. HRCT would show a gap in the ossicular chain, not a canal dehiscence. - **Perforation of the tympanic membrane**: Tympanic membrane perforation (marked **D** in the diagram) causes conductive hearing loss across all frequencies with reduced bone-conduction thresholds (not supranormal). It does not produce Tullio phenomenon or autophony and would be visible on otoscopy. **High-Yield:** SCDS = low-frequency conductive loss + INTACT stapedial reflex + supranormal bone-conduction thresholds + Tullio phenomenon/autophony → third mobile window shunting acoustic energy away from cochlea while stimulating superior canal cupula. Pöschl plane HRCT confirms bone defect over superior semicircular canal. [cite: Dhingra Diseases of Ear Nose & Throat 7e Ch 17; Cummings Otolaryngology 7e Ch 167]