A 28-year-old woman with a family history of hearing loss presents with bilateral progressive conductive hearing loss. Imaging shows bilateral stapes fixation with otosclerotic bone. Which finding best distinguishes cochlear otosclerosis from purely conductive otosclerosis?

Explanation

## Cochlear vs. Conductive Otosclerosis: The Bone Conduction Threshold ### Definition and Pathophysiology **Key Point:** Cochlear otosclerosis occurs when otosclerotic bone extends into the cochlear capsule (cochleitis), causing sensorineural hearing loss superimposed on conductive loss. The discriminating feature is **elevation of bone conduction thresholds** (sensorineural component) while the air-bone gap persists. ### Comparative Table: Conductive vs. Cochlear Otosclerosis | Feature | Conductive Otosclerosis | Cochlear Otosclerosis | |---------|------------------------|----------------------| | **Stapes Fixation** | Yes | Yes | | **Bone Conduction Thresholds** | Normal (0–15 dB HL) | Elevated (>20 dB HL) | | **Air-Bone Gap** | Present (30–50 dB) | Present (persists despite BC loss) | | **Audiometric Pattern** | Conductive only | Mixed (conductive + sensorineural) | | **Carhart's Notch** | Present | May be masked by SNHL | | **Cochlear Involvement** | No | Yes (otosclerotic bone in cochlear capsule) | | **Progression** | Slower, plateau phase | Faster, progressive | | **Imaging (CT)** | Stapes footplate involvement | Stapes + cochlear capsule involvement | ### Why Bone Conduction Elevation Matters **High-Yield:** In pure conductive otosclerosis, bone conduction remains normal because the cochlea is not affected—only the stapes is fixed. When bone conduction thresholds rise (sensorineural loss), it indicates cochlear involvement (cochleitis), defining cochlear otosclerosis. **Clinical Pearl:** Cochlear otosclerosis typically presents with faster progression and more severe hearing loss than conductive otosclerosis. The presence of both an air-bone gap AND elevated bone conduction is the hallmark. ### Mnemonic: **SNHL-in-Oto** = Sensorineural Hearing Loss appears in Otosclerosis when cochlea is involved (cochlear otosclerosis) **Warning:** Do not confuse cochlear otosclerosis with pure sensorineural otosclerosis (which has no air-bone gap). Cochlear otosclerosis retains the air-bone gap because stapes fixation persists, but bone conduction is also impaired.