Reflections on the role of a traveling wave along the basilar membrane in view of clinical and experimental findings

Surgical Considerations in Inner Ear Gene Therapy from Human Temporal Bone Anatomy

OBJECTIVE(S)

Recently directed methods of inner ear drug delivery underscore the necessity for understanding critical anatomical dimensions. This study examines anatomical measurements of the human middle and inner ear relevant for inner ear drug delivery studied with three different imaging modalities.

METHODS

Post-mortem human temporal bones were analyzed using human temporal bone histopathology (N = 24), micro computerized tomography (μCT; N = 4), and synchrotron radiation phase-contrast imaging (SR-PCI; N = 7). Nine measurements involving the oval and round windows were performed when relevant anatomical structures were visualized for subsequent age-controlled analysis, and comparisons were made between imaging methods.

RESULTS

Combined human temporal bone histopathology showed the mean distance to the saccule from the center of the stapes footplate (FP) was 2.07 ± 0.357 mm and the minimum distance was 1.23 mm. The mean distance from the round window membrane (RWM) to the osseous spiral lamina (OSL) was 1.75 ± 0.199 mm and the minimum distance was 1.43 mm. Instruments inserted up to 1 mm past the center of the FP are unlikely to cause saccular damage, provided there are no endolymphatic hydrops. Similarly, instruments inserted up to 1 mm through the RWM in the trajectory toward the OSL are unlikely to cause OSL damage.

CONCLUSION

The combined analyses of inner-ear dimensions of age-controlled groups and imaging modalities demonstrate critical dimensions of importance to consider when inserting delivery vehicles into the human cochlea.

LEVEL OF EVIDENCE

N/A Laryngoscope, 134:2879-2888, 2024.

The Effect of Soft Tissue Stimulation on Skull Vibrations and Hearing Thresholds in Humans

HYPOTHESIS

Hearing via soft tissue stimulation involves an osseous pathway.

BACKGROUND

A recent study that measured both hearing thresholds and skull vibrations found that vibratory stimulation of soft tissue led to hearing sensation that correlated with skull vibrations, supporting the hypothesis of an osseous pathway. It is possible, however, that a lower application force of the vibrator on the stimulated soft tissue would not be sufficient to elicit skull vibration suggesting hearing via a nonosseous pathway. The purpose of the present study was to confirm the osseous pathway by measuring skull vibrations and behavioral thresholds using a low application force on a layer of ultrasound gel. Gel was used to mimic soft tissue because of its similar acoustic impedance and to control for variability between participants.

METHODS

Hearing thresholds and the skull vibrations of five patients who were implanted with bone-anchored implants were assessed in two conditions when the bone vibrator was applied on the forehead: 1) direct application with 5N force; 2) through a layer of ultrasound gel with minimal application force. Skull vibrations were measured in both conditions by a laser Doppler vibrometer focused on the bone-anchored implant.

RESULTS

Skull vibrations were present even when minimal application force was applied on soft tissue. The difference in skull vibrations when the vibrator was directly on the forehead compared with the gel condition was consistent with the variability in hearing thresholds between the two conditions.

CONCLUSION

These results reinforce the hypothesis that skull vibrations are involved in hearing when sound is transmitted via either soft tissue or bone.

Effect of Oval Window Blockage on Bone Conduction in Cadaver Heads

OBJECTIVE

This study aimed to explore the feasibility of medical adhesive in the molding of oval window (OW) blockage in cadaver heads and to study the effect on bone conduction (BC).

METHODS

Four cadaver heads were selected to establish OW blockage model. The daub type of medical adhesive was used to immobilize OW. The vibration properties of the round window membrane (VRWM) in response to the acoustic stimulation, and the vibration properties of the round window membrane and cochlear promontory (VCP) in response to the BC transducer B-71 stimulation were assessed by laser Doppler vibrometer in both pre-OW blockage and post-OW blockage.

RESULTS

After blocking the oval window, the mean values of the sound-induced velocities amplitude responses of the round window membrane by air conduction were decreased significantly beyond 30 dB in all measured frequencies (p < 0.05). The round window membrane relative velocity (VRWM/VCP) shows a decrease of about 1 dB at 1 and 3 kHz frequencies and a slight increase of around 0.5 dB from 4 to 8 kHz frequencies in post-OW blockage. However, it should also be noted that the VRWM/VCP is a significant decrease of 1.2 dB at 3 kHz in post-OW blockage compared with pre-OW blockage (p < 0.05).

CONCLUSION

Medical adhesive was available for the immobilization of oval window. In cadaver heads, the effect of OW blockage on the BC was the notching at 3 kHz.

Does hearing in response to soft-tissue stimulation involve skull vibrations? A within-subject comparison between skull vibration magnitudes and hearing thresholds

Hearing can be elicited in response to bone as well as soft-tissue stimulation. However, the underlying mechanism of soft-tissue stimulation is under debate. It has been hypothesized that if skull vibrations were the underlying mechanism of hearing in response to soft-tissue stimulation, then skull vibrations would be associated with hearing thresholds. However, if skull vibrations were not associated with hearing thresholds, an alternative mechanism is involved. In the present study, both skull vibrations and hearing thresholds were assessed in the same participants in response to bone (mastoid) and soft-tissue (neck) stimulation. The experimental group included five hearing-impaired adults in whom a bone-anchored hearing aid was implanted due to conductive or mixed hearing loss. Because the implant is exposed above the skin and has become an integral part of the temporal bone, vibration of the implant represented skull vibrations. To ensure that middle-ear pathologies of the experimental group did not affect overall results, hearing thresholds were also obtained in 10 participants with normal hearing in response to stimulation at the same sites. We found that the magnitude of the bone vibrations initiated by the stimulation at the two sites (neck and mastoid) detected by the laser Doppler vibrometer on the bone-anchored implant were linearly related to stimulus intensity. It was therefore possible to extrapolate the vibration magnitudes at low-intensity stimulation, where poor signal-to-noise ratio limited actual recordings. It was found that the vibration magnitude differences (between soft-tissue and bone stimulation) were not different than the hearing threshold differences at the tested frequencies. Results of the present study suggest that bone vibration magnitude differences can adequately explain hearing threshold differences and are likely to be responsible for the hearing sensation. Thus, the present results support the idea that bone and soft-tissue conduction could share the same underlying mechanism, namely the induction of bone vibrations. Studies with the present methodology should be continued in future work in order to obtain further insight into the underlying mechanism of activation of the hearing system.

Soft tissue conduction as a possible contributor to the limited attenuation provided by hearing protection devices

CONTEXT

Damage to the auditory system by loud sounds can be avoided by hearing protection devices (HPDs) such as earmuffs, earplugs, or both for maximum attenuation. However, the attenuation can be limited by air conduction (AC) leakage around the earplugs and earmuffs by the occlusion effect (OE) and by skull vibrations initiating bone conduction (BC).

AIMS

To assess maximum attenuation by HPDs and possible flanking pathways to the inner ear.

SUBJECTS AND METHODS

AC attenuation and resulting thresholds were assessed using the real ear attenuation at threshold (REAT) procedure on 15 normal-hearing participants in four free-field conditions: (a) unprotected ears, (b) ears covered with earmuffs, (c) ears blocked with deeply inserted customized earplugs, and (d) ears blocked with both earplugs and earmuffs. BC thresholds were assessed with and without earplugs to assess the OE.

RESULTS

Addition of earmuffs to earplugs did not cause significantly greater attenuation than earplugs alone, confirming minimal AC leakage through the external meatus and the absence of the OE. Maximum REATs ranged between 40 and 46 dB, leading to thresholds of 46-54 dB HL. Furthermore, calculation of the acoustic impedance mismatch between air and bone predicted at least 60 dB attenuation of BC.

CONCLUSION

Results do not support the notion that skull vibrations (BC) contributed to the limited attenuation provided by traditional HPDs. An alternative explanation, supported by experimental evidence, suggests transmission of sound to inner ear via non-osseous pathways such as skin, soft tissues, and fluid. Because the acoustic impedance mismatch between air and soft tissues is smaller than that between air and bone, air-borne sounds would be transmitted to soft tissues more effectively than to bone, and therefore less attenuation is expected through soft tissue sound conduction. This can contribute to the limited attenuation provided by traditional HPDs. The present study has practical implications for hearing conservation protocols.