Bone-conduction hearing and the occlusion effect in otosclerosis and normal controls

The goal of this study was to better understand bone-conduction hearing in subjects with normal hearing and in those with otosclerosis through the occlusion effect. With this study, the authors hope to lend credence to commonly accepted theories of bone-conduction hearing and the effect of lateralization during the Weber tuning fork test. There are three accepted theories defining bone-conduction hearing: compressional bone conduction describes an auditory percept produced by the compression and expansion of the cochlea leading to basilar membrane vibration; inertial bone conduction describes the inertia of the ossicular chain as a result of skull vibration during bone conduction testing; whereas skull vibration may also be transmitted to the external auditory canal, surrounding soft tissues, and para-auditory structures to illicit tympanic membrane vibration known as osseotympanic bone conduction. Twenty normal volunteers and 17 unilateral otosclerosis patients underwent external canal sound pressure level measurement during bone-conduction testing using a standardized bone oscillator placement and stimulation paradigm. Sound was detected with a probe microphone placed in the external auditory canal in nonoccluded and occluded conditions after a 50-dB hearing level bone-conduction stimulus. There was no significant difference in sound pressure level between otosclerosis and normal subjects when the external auditory canals were nonoccluded. With occlusion, sound pressure level increased in both groups, but at a statistically significantly higher level for the otosclerosis group. Sound measured in the external canal likely represents energy lost to the environment transmitted through the middle and external ear systems, aided by the effect of both inertial and osseotympanic bone conduction. Occluding the ear leads to sound trapping and amplification. Also, the pressure exerted against the tympanic membrane reduces middle ear compliance and increases the impedance mismatch between air and the middle ear system, reflecting sound back into the external canal. This effect is further enhanced by stapes fixation to explain our data in both groups of subjects. The final common pathway in "lateralization" is probably a product of higher than normal impedance mismatch at the oval window.