A 72-year-old woman presents with gradual onset bilateral hearing loss affecting both low and high frequencies equally over 10 years. Audiometry shows air-bone gap of 35 dB across all frequencies, with normal speech discrimination. Tympanometry is normal (Type A). Bone conduction thresholds are normal. What is the most likely diagnosis?

Explanation

## Audiogram Interpretation: Otosclerosis ### Critical Audiometric Findings | Feature | Finding | Diagnostic Significance | |---------|---------|------------------------| | Air-bone gap | 35 dB across all frequencies | Conductive component | | Bone conduction | Normal | Rules out sensorineural loss | | Tympanometry | Normal (Type A) | Ossicular chain intact | | Frequency pattern | Flat (all frequencies affected equally) | Characteristic of otosclerosis | | Speech discrimination | Normal | Confirms conductive mechanism | **Key Point:** The **normal bone conduction with elevated air conduction thresholds** (air-bone gap) indicates **pure conductive hearing loss**. The normal tympanogram excludes middle ear effusion or TM perforation. ### Pathophysiology of Otosclerosis Otosclerosis is abnormal bone remodeling of the otic capsule, most commonly affecting the **stapes footplate at the oval window**. This causes: 1. Stapes fixation → reduced ossicular chain mobility 2. Conductive hearing loss (air-bone gap widens) 3. Bone conduction remains normal (cochlea unaffected initially) 4. **Carhart notch** at 2 kHz may appear (bone conduction dip due to resonance changes) ### Why This Is Otosclerosis, Not Presbycusis **Presbycusis** (age-related sensorineural loss): - Produces **sensorineural pattern** (air-bone gap absent) - Affects **high frequencies first** (sloping audiogram) - Bone conduction also elevated - This patient has normal bone conduction → rules out presbycusis **High-Yield:** The **flat conductive audiogram with normal bone conduction** in a patient >60 years is otosclerosis until proven otherwise. Otosclerosis can present late in life ("late-onset otosclerosis"). ### Clinical Correlation - Female predominance (2:1) - Progressive conductive loss over years - Tinnitus common - Vertigo less common than in Ménière's disease - Family history in 50% of cases **Clinical Pearl:** **Carhart notch** (bone conduction dip at 2 kHz) is a secondary finding in otosclerosis and helps confirm the diagnosis, though not always present. Its presence increases specificity for otosclerosis.