Superior-semicircular-canal dehiscence: Effects of location, shape, and size on sound conduction

Abstract

The effects of a superior-semicircular-canal (SSC) dehiscence (SSCD) on hearing sensitivity via the air-conduction (AC) and bone-conduction (BC) pathways were investigated using a three-dimensional finite-element (FE) model of a human middle ear coupled to the inner ear. Dehiscences were modeled by removing a section of the outer bony wall of the SSC and applying a zero-pressure condition to the fluid surface thus exposed. At each frequency, the basilar-membrane velocity, vBM, was separately calculated for AC and BC stimulation, under both pre- and post-dehiscence conditions. Hearing loss was calculated as the difference in the maximum magnitudes of vBM between the pre- and post-dehiscence conditions representing a change in hearing threshold. In this study, BC excitations were simulated by applying rigid-body vibrations to the model along the directions of the (arbitrarily defined) x, y, and z axes of the model. Simulation results are consistent with previous clinical measurements on patients with an SSCD and with results from earlier lumped-element electrical-circuit modeling studies, with the dehiscence decreasing the hearing threshold (i.e., increasing vBM) by about 35 dB for BC excitation at low frequencies, while for AC excitation the dehiscence increases the hearing threshold (i.e., decreases vBM) by about 15 dB. A new finding from this study is that the initial width (defined as the width of the edge of the dehiscence where the flow of the fluid-motion wave from the oval window meets it for the first time) on the vestibular side of the dehiscence has more of an effect on vBM than the area of the dehiscence. Analyses of dehiscence effects using the FE model further predict that changing the direction of the BC excitation should have an effect on vBM, with vBM being about 20 dB lower due to BC excitation parallel to the longitudinal direction of the BM in the hook region (the x direction) as compared to excitations in other directions (y and z). BC excitation in the x direction and with a ‘center’ dehiscence located midway along the length of the SSC causes a reduction in the anti-symmetric component of the fluid pressure across the BM, as compared to the other directions of BC excitation, which results in a decrease in vBM at high frequencies.

Identifying Mechanisms Behind the Tullio Phenomenon: a Computational Study Based on First Principles

“…This boundary condition was suggested in Obrist (2011) and has also been used in a computational study by Kim et al (2013). At this discontinuity, we split the perilymph domain into two parts (cf.…”

“…(21) of the soundinduced steady streaming velocity in the endolymph can be used to approximate the corresponding eye velocities α t . In order to reflect physically reasonable anatomical conditions, we use the frequency-dependent stapes motility measurements U s /p s from Kringlebotn and Gundersen (1985) in combination with numerical predictions on the fluid fraction ϕ by Kim et al (2013), using Eq. (1).…”

“…This allows us to determine the primary flow velocity amplitude U p in the perilymph for a given sound frequency f at sound pressure level SPL. Figure 16 shows predictions for five such virtual patients that differ from each other in SCD size (based on the model by Kim et al 2013) and within a presumable variance of the patient-specific frequency where the stapes motility is greatest. Some eye velocity predictions are of the same order of magnitude as the ones from real patients which we reconstructed from available search coil recordings in the literature (Minor 2000;Cremer et al 2000;Minor et al 2001;Minor 2005;Kaski et al 2012).…”

“…Measurements of the stapes motility U s /p s indicate that the stapes oscillates at the largest velocity amplitudes for sound stimuli between approximately 800 Hz and 2 kHz (Kringlebotn and Gundersen 1985;Huber et al 2001;Chien et al 2009). According to predictions for the fluid fraction ϕ by Kim et al (2013), a larger percentage of the stapes-induced flow enters the vestibular ducts at lower frequencies, whereas higher sound frequencies favor a stimulation of the cochlear ducts. Since the steady streaming was found to scale with the square of the perilymph velocity amplitude U p (see Eq.…”

“…This (primary) flow was computationally assessed and quantified by Rosowski et al (2004), Songer and Rosowski (2007), and Kim et al (2013). It remains unclear how this strongly oscillatory motion of the perilymph is able to provoke a "tonic response" (slow change) of the cupula afferents' firing rate as it has been observed by Carey et al (2004).…”