Fig 1.

Mechanisms of air- and bone-conducted sound in healthy and third window anatomy. A, Normal air conduction. Vibrations of the tympanic membrane are transmitted inward through the auditory ossicles and oval window. Energy is then conducted through the incompressible perilymph, producing equal and outward motion of the round window. The difference in vibration between the oval and round windows generates a pressure gradient across the basilar membrane, activating hair cells and creating the perception of sound. B, Decreased air conduction in third window anatomy. Due to shunting across third windows (semicircular canal dehiscence [SCCD], EVAS, cochlear dehiscence), there is decreased energy transmission from the oval window to the round window. The decrease in pressure gradient across the basilar membrane yields reduced sound perception. C, Normal bone conduction. Vibrations are transmitted throughout the otic capsule. This transmission results in differential outward motion of the oval and round windows due to unequal impedance of these 2 structures. The resulting pressure difference across the basilar membrane enables sound perception. D, Increased bone conduction in third window anatomy. Due to shunting across third windows, there is decreased motion of the oval window on the scala vestibuli side of the cochlea. However, the motion of the round window on the scala tympani side is unchanged. This phenomenon artifactually elevates the pressure difference across the basilar membrane, resulting in increased sound perception. TM indicates tympanic membrane; yellow, auditory ossicles; beige, otic capsule; red, oval window; green, round window; blue, perilymph; purple, basilar membrane. Adapted with permission from Merchant SN, Rosowski JJ. Conductive hearing loss caused by third window lesions of the inner ear. Otol Neurotol 2008;29:282–89.