Abstract
A lumped element impedance model of the inner ear with sources based on wave propagation in the skull bone was used to investigate the mechanisms of hearing sensitivity changes with semi-circular canal dehiscence (SSCD) and alterations of the size of the vestibular aqueduct. The model was able to replicate clinical and experimental findings reported in the literature. For air conduction, the reduction in cochlear impedance due to a SSCD reduces the intra-cochlear pressure at low frequencies resulting in a reduced hearing sensation. For bone conduction, the reduced impedance in the vestibular side due to the SSCD facilitates volume velocity caused by inner ear fluid inertia, and this effect dominates BC hearing with a third window opening on the vestibular side. The SSCD effect is generally greater for BC than for AC. Moreover, the effect increases with increased area of the dehiscence, but areas more than the cross section area of the semi-circular canal itself leads to small alterations. The model-predicted air-bone gap for a SSCD of 1 mm2 is 30 dB at 100 Hz that decreases with frequency and become non-existent at frequencies above 1 kHz. According to the model, this air-bone gap is similar to the air-bone gap of an early stage otosclerosis. The normal variation of the size of the vestibular aqueduct do not affect air conduction hearing, but can vary bone conduction sensitivity by up to 15 dB at low frequencies. Reinforcement of the OW to mitigate hyperacusis with SSCD is inefficient while a RW reinforcement can reset the bone conduction sensitivity to near normal.
Highlights
In normal function of hearing, the ear canal sound pressure is transmitted to the inner ear via the tympanic membrane (TM) and middle ear ossicles
The sound pressures at both positions are in general agreement with the experimentally obtained scala vestibuli sound pressures shown in Figure 3A, where the difference between the model-predicted sound pressures and experimentally obtained sound pressures are similar to the difference between the two experimentally obtained sound pressures
The CSF pressure transmission decreases by 12 dB with halving the duct diameter and improves by 10 dB with doubling the duct diameter, almost independent of frequency. These results indicate that the vestibular aqueduct is important for low frequency bone conduction (BC) hearing
Summary
In normal function of hearing, the ear canal sound pressure is transmitted to the inner ear via the tympanic membrane (TM) and middle ear ossicles. This results in a motion of the stapes in the oval window (OW) that is mimicked in terms of fluid displaced by the motion of the round window (RW), but with opposite phase [1]. Blood vessels and neural tissue entering the inner ear may transmit pressure in and out of the inner ear All these small channels were collectively referred to as the third window by Ranke et al [2]. For air-conduction (AC) hearing in normal ears, the impedance of these narrow channels are much greater than the impedances of the inner ear fluids, basilar membrane (BM), OW, and RW [3, 4], and they do not affect the volume velocity exciting the BM
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