## Otosclerosis: Site of Involvement **Key Point:** The oval window niche and anterior footplate of the stapes is the most common site of otosclerotic bone involvement, accounting for >90% of clinical cases. ### Pathophysiology Otosclerosis is a primary disorder of the otic capsule bone characterized by abnormal bone remodeling. The disease process involves: 1. Osteoclastic resorption of normal otic capsule bone 2. Replacement with abnormal, highly vascular immature bone 3. Progressive fixation of the stapes footplate ### Why the Oval Window? The oval window niche is the site of predilection because: - It is the thinnest and most metabolically active part of the otic capsule - The anterior footplate is particularly vulnerable to otosclerotic involvement - This location directly impairs stapes oscillation, causing conductive hearing loss ### Clinical Correlation **Carhart's Notch:** A characteristic bone conduction dip at 2 kHz occurs due to: - Stapes fixation reducing ossicular chain compliance - Increased mass reactance at this frequency - This finding is pathognomonic for otosclerosis ### Stages of Stapes Fixation | Stage | Audiometric Finding | Stapes Status | |-------|-------------------|---------------| | Early | Air-bone gap 20-30 dB | Partial fixation | | Advanced | Air-bone gap >40 dB | Complete fixation | | Cochlear | Mixed/sensorineural component | Cochlear involvement | **High-Yield:** Oval window involvement → stapes footplate fixation → conductive hearing loss with Carhart's notch at 2 kHz. **Clinical Pearl:** While oval window is most common, otosclerosis can involve other sites (cochlear, round window) in advanced disease, potentially causing sensorineural hearing loss.
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