If the air-bone gap at any frequency is greater than 15 dB, where the bone conduction thresholds are within the normal range but the air conduction thresholds are reduced, this is considered a CHL. This occurs when there is abnormal transmission of sound through the
outer or middle ear without damage to the inner ear.
CHL may occur due to wax impaction, otitis media (link to article on Otitis Media), eardrum perforation, ossicular disruption (link to article on the Ossicles), and a foreign body in the ear, among other causes.
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