A 32-year-old woman presents with progressive bilateral conductive hearing loss over 2 years. Otoscopy shows a normal tympanic membrane. Weber test shows lateralization to the right ear, and Rinne test shows bone conduction better than air conduction bilaterally. Which investigation is most appropriate to confirm the diagnosis of otosclerosis?

Explanation

## Diagnosis of Otosclerosis: Role of Pure Tone Audiometry **Key Point:** Pure tone audiometry (PTA) is the **primary investigation of choice** to confirm the diagnosis of otosclerosis. It demonstrates the characteristic audiometric pattern including the pathognomonic **Carhart notch** at 2000 Hz, along with a conductive hearing loss pattern with a significant air-bone gap. ### Why Pure Tone Audiometry is the Investigation of Choice **High-Yield:** PTA findings in otosclerosis include: - **Conductive hearing loss** with a significant air-bone gap (typically >15 dB) - **Carhart notch** — a mechanical notch in bone conduction at 2000 Hz (pathognomonic of stapes fixation) - Normal or near-normal bone conduction thresholds (in early disease) - Bilateral involvement in ~70% of cases ### Audiometric Findings in Otosclerosis | Finding | Significance | |---------|-------------| | **Air-bone gap** | Confirms conductive hearing loss | | **Carhart notch at 2000 Hz** | Pathognomonic of stapes fixation (otosclerosis) | | **Normal tympanogram (As type)** | Reduced compliance due to stapes fixation | | **Absent stapedial reflexes** | Supports stapes fixation | **Clinical Pearl:** The Carhart notch (dip in bone conduction at 2000 Hz) is a mechanical artifact of stapes fixation and is not a true sensorineural component. It resolves after successful stapedectomy. This finding on PTA is considered pathognomonic for otosclerosis (Scott-Brown's Otorhinolaryngology, 8th ed.). ### Why Other Investigations Are Less Appropriate as the Confirmatory Test **HRCT temporal bone** — While HRCT can visualize lucent foci at the fissula ante fenestram and is valuable pre-operatively, it is **not the first-line confirmatory investigation**. HRCT has limited sensitivity (~70%) for early otosclerosis and is primarily used for surgical planning, not initial diagnosis confirmation. The diagnosis of otosclerosis is fundamentally a **clinical-audiological** diagnosis. **Tympanometry with acoustic reflexes** — Shows a shallow (As) tympanogram and absent stapedial reflexes due to stapes fixation, which supports the diagnosis but is non-specific and cannot confirm otosclerosis alone. **Electrocochleography** — Assesses cochlear (hair cell) function; not used in the diagnosis of otosclerosis. **Mnemonic:** **PTA-Carhart = Otosclerosis** — The Carhart notch on pure tone audiometry is the hallmark audiometric finding that confirms stapes fixation in otosclerosis. *Reference: Scott-Brown's Otorhinolaryngology, Head and Neck Surgery, 8th ed.; Dhingra's Diseases of Ear, Nose and Throat, 7th ed.*