Carhart's notch in otosclerosis is a depression in bone conduction thresholds at 2000 Hz. This finding is best described as:

• A True cochlear damage caused by diffusion of toxic metabolites from the otosclerotic focus into the scala tympani
• B Neural degeneration caused by cochlear nerve compression by expanding otosclerotic bone
• C Outer hair cell dysfunction secondary to endolymphatic hydrops from otosclerotic oval window obstruction
• D A mechanical artefact caused by stapes fixation eliminating the ossicular resonance contribution to bone conduction at 2000 Hz ✓

Explanation

Carhart's notch is a MECHANICAL artefact, not true sensorineural damage. The ossicular chain has a natural resonant frequency of approximately 2000 Hz; stapes fixation eliminates the inertial contribution of ossicular movement to bone conduction particularly at this frequency. The notch measures approximately 5 dB at 500 Hz, 10 dB at 1000 Hz, 15 dB at 2000 Hz, and 5 dB at 4000 Hz on the bone conduction curve. Clinically critical: after successful stapedectomy/stapedotomy, the Carhart's notch characteristically disappears ('overclosure'), confirming its mechanical origin and distinguishing it from genuine cochlear SNHL.

Reference: Dhingra Diseases of Ear, Nose and Throat, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.