## Audiometric Findings in Otosclerosis **Key Point:** The **Carhart bone** (or Carhart notch) is the pathognomonic early audiometric finding in otosclerosis — a characteristic **bone conduction dip at 2 kHz** that appears before significant air-bone gap develops. ### The Carhart Bone Explained | Feature | Details | |---------|----------| | **Frequency** | 2 kHz (sometimes 1–4 kHz range) | | **Appearance** | Bone conduction threshold dips 5–10 dB below surrounding frequencies | | **Air-Bone Gap** | Initially minimal or absent | | **Timing** | Appears in **early otosclerosis** before stapes fixation is complete | | **Mechanism** | Thought to result from increased impedance and altered ossicular resonance due to early otosclerotic changes | ### Progression of Hearing Loss 1. **Early stage (Carhart bone present):** Bone conduction dip at 2 kHz, minimal air-bone gap 2. **Intermediate stage:** Progressive air-bone gap develops, Carhart bone may persist 3. **Advanced stage:** Large air-bone gap (30–50 dB), bone conduction may also decline if cochlear otosclerosis develops **High-Yield:** The presence of **Carhart bone at 2 kHz with a small air-bone gap** is highly suggestive of **early otosclerosis** and warrants imaging (HRCT temporal bone) and consideration of stapes surgery. **Clinical Pearl:** As stapes fixation progresses, the air-bone gap widens, but the Carhart bone may persist. In cochlear otosclerosis (sensorineural involvement), bone conduction thresholds also deteriorate. 
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