Objective: The assessment of bone conduction thresholds in patients with conductive hearing loss is not a full measure of the function of the inner ear due to the weakening of the influence of middle ear components on bone conduction. This relationship has been called the 'Carhart effect'.
Methods: The retrospective analysis covered 977 patients diagnosed and treated for middle ear diseases from 2010 to 2020. The Carhart effect was considered to be an increase in the bone conduction threshold by a minimum of 10 dB relative to adjacent frequencies. The study was performed with the aim of assessing the presence of the Carhart effect in the course of middle ear diseases in pretreatment analysis.
Results: The Carhart effect was observed in 532 cases, most often in patients with chronic otitis media and otosclerosis. It was least often observed in patients with otitis media with effusion. In otitis media with effusion, the Carhart effect was more often noted for the frequency of 4000 Hz, in otosclerosis for the frequency of 2000 Hz. In patients with chronic otitis media, this effect for the frequency of 4000 Hz was correlated with the location of inflammatory changes in the attic area. The presence of inflammatory lesions in the oval window area was associated with the presence of the Carhart effect for the frequency of 2000 Hz.
Conclusions: 1. The frequency of the Carhart effect observed in diseases of the middle ear does not depend on the disease entity but on the type and location of abnormalities in the middle ear. 2. The Carhart effect observed for the frequency of 4000 Hz coexists with the localization of lesions in the range of the malleus and incus, and for the frequency of 2000 Hz, it is partially associated with abnormalities in the range of stapes and oval window.
Keywords: Carhart effect; bone conduction; chronic otitis media; otitis media with effusion; otosclerosis.