In otosclerosis, the Carhart notch on pure tone audiogram is a MECHANICAL (not cochlear) artefact. It is most pronounced at which frequency, and what is the physiological explanation?
- A 1 kHz; due to resonance frequency of the middle ear ossicular chain being disrupted
- B 2 kHz; due to the natural resonance of the ossicular chain at 2 kHz — stapes fixation dampens this resonance, artificially reducing bone conduction at 2 kHz ✓
- C 4 kHz; due to noise-induced cochlear damage superimposed on otosclerosis
- D 500 Hz; due to loss of Bekesy traveling wave compliance
Correct answer: B. 2 kHz; due to the natural resonance of the ossicular chain at 2 kHz — stapes fixation dampens this resonance, artificially reducing bone conduction at 2 kHz
Explanation
The Carhart notch is a mechanical artefact of stapes fixation in otosclerosis, not true cochlear SNHL. It is most pronounced at 2 kHz because the ossicular chain has its natural resonance frequency near 2 kHz; stapes fixation dampens ossicular inertia and the resonance component of bone conduction testing at this frequency. After successful stapedectomy, the notch disappears — confirming its mechanical (not cochlear) origin. This differentiation is crucial for surgical counseling.
Reference: Dhingra Diseases of Ear, Nose and Throat, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.