A simple electrical lumped-element model simulates intra-cochlear sound pressures and cochlear impedance below 2 kHz

A mechanical lumped-element model of the human middle ear for bone conduction hearing

Bone conduction (BC) is an important modality of hearing. It enables us to differentiate conductive and sensorineural hearing loss, perceive sounds despite a disabled middle ear, and listen to conversation and music privately without blocking the ear canal. Yet the mechanism underlying BC is not fully understood mainly because the bone-conducted vibrations in the skull simultaneously stimulate the outer ear, the middle ear, and the cochlea. The nature of the parallel stimulation on those interconnected parts makes it difficult to contemplate the dynamics in each compartment and the influences they impose on each other. In the present study, a computational lumped-element human ear model for BC is developed. The model comprises lumped mechanical components - masses, springs and dampers - to represent structures such as eardrum, ossicles, ligaments, joints, and cochlear fluid. The parameters of those components are determined by fitting the simulated ossicular vibrations to the measured counterparts reported by Stenfelt et al., the most extensive BC middle-ear dataset. The results show that the model-predicted vibrations of the umbo and stapes generally match the experimental results not just in the normal ear condition but also after various perturbations such as adding mass on the eardrum and separating the incudostapedial joint. It is believed this is the first lumped-element model that can correctly simulate the vibrations of the human middle ear in BC. The model can serve as the bedrock not only for better understanding the dynamics of the entire ear in BC but also for developing new diagnostics for middle-ear conditions and assisting design of novel hearing prostheses.

Lumped element models of sound conduction in the human ear: A systematic review

Lumped element models facilitate investigating the fundamental mechanisms of human ear sound conduction. This systematic review aims to guide researchers to the optimal model for the investigated parameters. For this purpose, the literature was reviewed up to 12 July 2023, according to the PRISMA guidelines. Seven models are included via database searching, and another 19 via cross-referencing. The quality of the models is assessed by comparing the predicted middle ear transfer function, the tympanic membrane impedance, the energy reflectance, and the intracochlear pressures (ICPs) (scala vestibuli, scala tympani, and differential) with experimental data. Regarding air conduction (AC), the models characterize the pathway from the outer to the inner ear and accurately predict all six aforementioned parameters. This contrasts with the few existing bone conduction (BC) models that simulate only a part of the ear. In addition, these models excel at predicting one observable parameter, namely, ICP. Thus, a model that simulates BC from the coupling site to the inner ear is still lacking and would increase insights into the human ear sound conduction. Last, this review provides insights and recommendations to determine the appropriate model for AC and BC implants, which is highly relevant for future clinical applications.

The Effect of Endolymphatic Hydrops and Mannitol Dehydration Treatment on Guinea Pigs

Background

Endolymphatic hydrops (EH) is considered as the pathological correlate of Menière’s disease (MD) and cause of hearing loss. The mechanism of EH, remaining unrevealed, poses challenges for formalized clinical trials.

Objective

This study aims to investigate the development of hearing loss, as well as the effect of dehydration treatment on EH animal models.

Methods

In this study, different severity EH animal models were created. The laser Doppler vibrometer (LDV) and auditory brainstem responses (ABR) were used to study the effects of EH and the dehydration effects of mannitol. The LDV was used to measure the vibration of the round window membrane (RWM) reflecting the changes in inner ear impedance. ABR was used to evaluate the hearing changes. Furthermore, tissue section and scanning electron microscopy (SEM) observations were used to analyze the anatomical change to the cochlea and outer hair cells.

Results

The RWM vibrations decreased with the severity of EH, indicating an increase in the cochlear impedance. The dehydration therapy lowered the impedance to restore acoustic transduction in EH 10- and 20-day animal models. Simultaneously, the ABR thresholds increased in EH models and were restored after dehydration. Moreover, a difference in the hearing was found between ABR and LDV results in severe EH animal models, and the dehydration therapy was less effective, indicating a sensorineural hearing loss (SNHL).

Conclusion

Endolymphatic hydrops causes hearing loss by increasing the cochlear impedance in all tested groups, and mannitol dehydration is an effective therapy to restore hearing. However, SNHL occurs for the EH 30-day animal models, limiting the effectiveness of dehydration. Our results suggest the use of dehydrating agents in the early stage of EH.

Conductive hearing loss in large vestibular aqueduct syndrome -clinical observations and proof-of-concept predictive modeling by a biomechanical approach

OBJECTIVE

The purpose of this study was to investigate the effect of a dilated vestibular aqueduct on conductive hearing loss (CHL). A biomechanical method was proposed for modeling the patterns of CHL in patients with large vestibular aqueduct syndrome (LVAS).

STUDY DESIGN

High resolution computed tomography (CT) scans and pure tone audiometry (PTA) were retrospectively collected from 16 patients who were diagnosed with LVAS. Seventeen ears with measurable air-bone gaps (ABGs) on PTA were applied for model development. The sizes of midpoint, operculum and distal segment were measured from CT to calculate the biomechanical parameters of each vestibular aqueduct. The mechanical effect of the dilated vestibular aqueduct on sound conduction was simulated using a lumped-parameter model. The CHL levels predicted by the model were compared with the observed ABGs at 250, 500 and 1000 Hz respectively.

RESULTS

The model was able to predict the trend that greater ABGs occurred at lower frequencies, which were consistent with clinical observations. However, deviations between the predicted and the observed ABGs became larger as the frequency increased. None of the correlation coefficients between the radiologic measures and the observed ABG levels were significant.

CONCLUSION

These findings lend support to the feasibility of this approach in modeling CHL in LVAS. The presence of a dilated vestibular aqueduct leads to altered impedance and sound pressure, suggesting the impact of a pathological third window. High individual variability of the observed ABGs implies additional factors may also be involved, especially at 500 Hz and 1000 Hz.

Investigation of Mechanisms in Bone Conduction Hyperacusis With Third Window Pathologies Based on Model Predictions

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.

The detailed shapes of equal-loudness-level contours at low frequencies

High-resolution equal-loudness-level contours (ELCs) were measured over the frequency range 10-250 Hz using 19 normal-hearing subjects. Three levels of the 50-Hz reference sound were used, corresponding to the levels at 50 Hz of the 30-, 50-, and 70-phon standardized ELCs given in ISO-226:2003. The dynamic range of the contours generally decreased with increasing reference level, and the slope was shallow between 10 and 20 Hz, consistent with previous studies. For the lowest level, the ELCs were sometimes but not always smooth and on average followed the standardized 30-phon contour for frequencies above 40 Hz. For the two higher levels, the individual ELCs showed a distinct non-monotonic feature in a "transition region" between about 40 and 100 Hz, where the slope could reach near-zero or even positive values. The pattern of the non-monotonic feature was similar across levels for the subjects for whom it was observed, but the pattern varied across subjects. Below 40 Hz, the slopes of the ELCs increased markedly for all loudness levels, and the levels exceeded those of the standardized ELCs. Systematic deviations from the standardized ELCs were largest for frequencies below 40 Hz for all levels and within the transition region for the two higher levels.

Limits on normal cochlear 'third' windows provided by previous investigations of additional sound paths into and out of the cat inner ear

While most models of cochlear function assume the presence of only two windows into the mammalian cochlea (the oval and round windows), a position that is generally supported by several lines of data, there is evidence for additional sound paths into and out of the inner ear in normal mammals. In this report we review the existing evidence for and against the 'two-window' hypothesis. We then determine how existing data and inner-ear anatomy restrict transmission of sound through these additional sound pathways in cat by utilizing a well-tested model of the cat inner ear, together with anatomical descriptions of the cat cochlear and vestibular aqueducts (potential additional windows to the cochlea). We conclude: (1) The existing data place limits on the size of the cochlear and vestibular aqueducts in cat and are consistent with small volume-velocities through these ducts during ossicular stimulation of the cochlea, (2) the predicted volume velocities produced by aqueducts with diameters half the size of the bony diameters match the functional data within ±10 dB, and (3) these additional volume velocity paths contribute to the inner ear's response to non-acoustic stimulation and conductive pathology.

The effect of the helicotrema on low-frequency loudness perception

Below approximately 40 Hz, the cochlear travelling wave reaches the apex, and differential pressure is shunted through the helicotrema, reducing hearing sensitivity. Just above this corner frequency, a resonance feature is often observed in objectively measured middle-ear-transfer functions (METFs). This study inquires whether overall and fine structure characteristics of the METF are also perceptually evident. Equal-loudness-level contours (ELCs) were measured between 20 and 160 Hz for 14 subjects in a purpose-built test chamber. In addition, the inverse shapes of their METFs were obtained by adjusting the intensity of a low-frequency suppressor tone to maintain an equal suppression depth of otoacoustic emissions for various suppressor tone frequencies (20-250 Hz). For 11 subjects, the METFs showed a resonance. Six of them had coinciding features in both ears, and also in their ELC. For two subjects only the right-ear METF was obtainable, and in one case it was consistent with the ELC. One other subject showed a consistent lack of the feature in their ELC and in both METFs. Although three subjects displayed clear inconsistencies between both measures, the similarity between inverse METF and ELC for most subjects shows that the helicotrema has a marked impact on low-frequency sound perception.

Modelling the effect of round window stiffness on residual hearing after cochlear implantation

Preservation of residual hearing after cochlear implantation is now considered an important goal of surgery. However, studies indicate an average post-operative hearing loss of around 20 dB at low frequencies. One factor which may contribute to post-operative hearing loss, but which has received little attention in the literature to date, is the increased stiffness of the round window, due to the physical presence of the cochlear implant, and to its subsequent thickening or to bone growth around it. A finite element model was used to estimate that there is approximately a 100-fold increase in the round window stiffness due to a cochlear implant passing through it. A lumped element model was then developed to study the effects of this change in stiffness on the acoustic response of the cochlea. As the round window stiffness increases, the effects of the cochlear and vestibular aqueducts become more important. An increase of round window stiffness by a factor of 10 is predicted to have little effect on residual hearing, but increasing this stiffness by a factor of 100 reduces the acoustic sensitivity of the cochlea by about 20 dB, below 1 kHz, in reasonable agreement with the observed loss in residual hearing after implantation. It is also shown that the effect of this stiffening could be reduced by incorporating a small gas bubble within the cochlear implant.