Prospective electrophysiologic findings of round window stimulation in a model of experimentally induced stapes fixation. Otol Neurotol. 2009;30:1215-1224. [PMID: 19779388 DOI: 10.1097/mao.0b013e3181bc3c06]

Review of blast noise and the auditory system

The auditory system is particularly vulnerable to blast injury due to the ear's role as a highly sensitive pressure transducer. Over the past several decades, studies have used a variety of animal models and experimental procedures to recreate blast-induced acoustic trauma. Given the developing nature of this field and our incomplete understanding of molecular mechanisms underlying blast-related auditory disturbances, an updated discussion about these studies is warranted. Here, we comprehensively review well-established blast-related auditory pathology including tympanic membrane perforation and hair cell loss. In addition, we discuss important mechanistic studies that aim to bridge gaps in our current understanding of the molecular and microstructural events underlying blast-induced cochlear, auditory nerve, brainstem, and central auditory system damage. Key findings from the recent literature include the association between endolymphatic hydrops and cochlear synaptic loss, blast-induced neuroinflammatory markers in the peripheral and central auditory system, and therapeutic approaches targeting biochemical markers of blast injury. We conclude that blast is an extreme form of noise exposure. Blast waves produce cochlear damage that appears similar to, but more extreme than, the standard noise exposure protocols used in auditory research. However, experimental variations in studies of blast-induced acoustic trauma make it challenging to compare and interpret data across studies.

Round Window Stimulation of the Cochlea

Mixed hearing loss associated with a sensorineural component and an impaired conductive mechanism for sound from the external ear canal to the cochlea represents a challenge for rehabilitation using either surgery or traditional hearing amplification. Direct stimulations of the ossicular chain and the round window (RW) membrane have allowed an improved hearing in this population. The authors review the developments in basic and clinical research that have allowed the exploration of new routes for inner ear stimulation. Similar changes occur in the electrophysiological measures in response to auditory stimulation through the traditional route and direct mechanical stimulation of the RW. The latter has proven to be very effective as a means of hearing rehabilitation in a group of patients with significant difficulties with hearing and communication.

Effect of ossicular chain deformity on reverse stimulation considering the overflow characteristics of third windows

Stimulating the round window membrane via an active actuator of the middle ear implant, named the reverse stimulation, has become an option to help patients with ossicular chain deformity (OCD) to restore hearing. However, there is still no concise description of how OCD affects reverse stimulation considering the overflow characteristics of third windows. In the present study, an impedance model considering the vestibular and cochlear aqueducts was used to investigate the dynamic response of the cochlea to reverse stimulation under OCD. First, a finite-element (FE) model of the middle ear and the ear canal was used to estimate the changes in reverse middle-ear impedance caused by ossicular chain fixation and ossicular chain interruption. Then, the impedance model was used to predict the reverse transfer function, which characterizes the effect of OCD on the dynamic response of the cochlea. The results show that ossicular chain fixation reduces the reverse stimulation's performance. Moreover, the existence of the third windows complicates the effect of ossicular chain fixation on the reverse stimulation and boosts obviously the reverse stimulation's performance at low frequencies. In contrast, regardless of the existence of third windows, ossicular chain interruption enhances the effect of reverse stimulation.

Successive ipsilateral surgery of Vibrant Soundbridge and Bonebridge devices for congenital bilateral conductive hearing loss: a case report

The Bonebridge and Vibrant Soundbridge systems are semi-implanted hearing devices, which have been widely applied in patients with congenital conductive hearing loss. However, comparison between these two hearing devices is rare, especially in the same patient. We report a 23-year-old man who underwent successive implantation of Vibrant Soundbridge and Bonebridge devices in the same ear because of dysfunction of the Vibrant Soundbridge. We provide insight on the patient’s experience and compare the audiological and subjective outcomes of satisfaction.

The chinchilla animal model for hearing science and noise-induced hearing loss

The chinchilla animal model for noise-induced hearing loss has an extensive history spanning more than 50 years. Many behavioral, anatomical, and physiological characteristics of the chinchilla make it a valuable animal model for hearing science. These include similarities with human hearing frequency and intensity sensitivity, the ability to be trained behaviorally with acoustic stimuli relevant to human hearing, a docile nature that allows many physiological measures to be made in an awake state, physiological robustness that allows for data to be collected from all levels of the auditory system, and the ability to model various types of conductive and sensorineural hearing losses that mimic pathologies observed in humans. Given these attributes, chinchillas have been used repeatedly to study anatomical, physiological, and behavioral effects of continuous and impulse noise exposures that produce either temporary or permanent threshold shifts. Based on the mechanistic insights from noise-exposure studies, chinchillas have also been used in pre-clinical drug studies for the prevention and rescue of noise-induced hearing loss. This review paper highlights the role of the chinchilla model in hearing science, its important contributions, and its advantages and limitations.

Middle ear structure and transcanal approach appropriate for middle ear surgery in rabbits

The current study aimed to investigate the middle ear structure and surgical approach appropriate for middle ear surgery in rabbits. A total of eight healthy New Zealand rabbits (16 ears) were dissected under a surgical microscope. The dimensions of the auditory canal and the middle ear were measured. In the present study, the transcanal surgical approach to the middle ear in rabbits was performed without complications, the anatomical landmarks in the auricle and the external auditory canal were apparent, no large vessels were present in the surgical zone and the bleeding was minor. Furthermore, the surgical procedure did not require removal of large bone sections of the external auditory canal. Additionally, the constitution of the ossicular chain, the leverage ratio of the ossicular chain and the constitution of ligaments and muscles in rabbits were similar to humans. Otherwise, the facial nerve canal in rabbits was more prominent compared with humans and the mobility of pars flaccida in rabbits was more noticeable compared with humans. The results of the current study indicate that the transcanal surgical approach was suitable to study the middle ear in rabbits. Furthermore, the rabbit middle ear may be used as a model for ossicular surgery and facial nerve research.

Round window application of an active middle ear implant (AMEI) system in congenital oval window atresia

CONCLUSION

Application of the Vibrant Soundbridge to the round window (RW) membrane can be utilized as an efficient therapy for congenital oval window (OW) atresia.

OBJECTIVE

To report the surgical technique and auditory outcome of an active middle ear implant (AMEI) system used in patients with congenital OW atresia.

METHODS

Nine subjects with congenital OW atresia (six males and three females, ranging in age from 5.5 to 25 years, average 12.5 years) were implanted with an AMEI (Vibrant Soundbridge) at the round window (RW-Vibroplasty). Five cases were diagnosed as having isolated congenital OW atresia while four patients presented with combined external/middle ear malformation.

RESULTS

An improvement of 30 dB in average pure-tone air conduction thresholds (0.5-4 kHz) was achieved, with the high frequencies showing greater results. The subjects achieved postoperative speech recognition scores of 80-100% on the Computerized Mandarin Speech Test System (CMSTS) sentence test. Bone conduction thresholds were confirmed as stable in all subjects postoperatively. Decline in auditory benefit was noticed in two subjects, who then underwent revision surgery. One of these revision surgery patients then experienced stable hearing recovery, while the other patient's hearing declined.

The mechanism of direct stimulation of the cochlea by vibrating the round window

BACKGROUND

Active middle ear implants such as the vibrant sound bridge (VSB) have been placed on the round window (RW) in patients with conductive or mixed hearing loss, with satisfactory hearing results. Several observations show that the mechanism of RW stimulation is not completely understood. The purpose of the present study was to compare different coupling procedures between the transducer and the RW in order to contribute to an understanding of the mechanism of RW stimulation.

METHODS

Five fat sand rats underwent ablation of the left ear and opening of the right bulla, followed by baseline measurements of thresholds of auditory nerve brainstem evoked responses (ABR) to air and bone conduction click stimuli. Subsequently the malleus and incus were removed from the right middle ear, modeling a conductive hearing loss in which the VSB on the RW is indicated. In the next stage of the experiment, a rod attached to the bone vibrator was placed gently on the RW membrane and then on saline fluid applied to the RW niche. ABR thresholds were recorded following both placements.

RESULTS

Mean baseline ABR threshold in response to air conduction stimuli was 48 ± 4 dB; mean ABR threshold when the rod was placed on the dry RW membrane was 99 ± 12 dB; mean ABR threshold when the rod was in the saline on RW niche was 79 ± 7 dB.

CONCLUSIONS

ABR thresholds were better (lower) with stimulation of fluid on the RW membrane compared to direct stimulation of the RW, providing further evidence of a direct fluid pathway.

Vibromechanical assessment of active middle ear implant stimulation in simulated middle ear effusion: a temporal bone study

HYPOTHESIS

Active middle ear implant (AMEI) generated vibromechanical stimulation of the ossicular chain (ossicular chain vibroplasty [OCV]) or the round window (round window vibroplasty [RWV]) is not significantly affected by simulated middle ear effusion in a human temporal bone model.

BACKGROUND

OCV and RWV may be employed for sensorineural, mixed, and conductive hearing losses. Although middle ear effusions may be encountered across patient populations, little is known about how effusions may affect AMEI vibromechanical efficiency.

METHODS

Laser Doppler vibrometry of stapes velocities (SVs) were performed in a human temporal bone model of simulated effusion (N = 5). Baseline measurements to acoustic sinusoidal stimuli, OCV, and RWV (0.25-8 kHz) were made without effusion. The measurements were repeated with simulated middle ear effusion and compared with baseline measurements. Data were analyzed across 3 frequency bands: low (0.25-1 kHz), medium (1-3 kHz), and high (3-8 kHz).

RESULTS

Acoustic stimulation with simulated middle ear effusion resulted in a significant (p < 0.001) frequency-dependent attenuation of SVs of 4, 10, and 7 dB (low, medium, and high ranges, respectively). OCV in simulated effusion resulted in attenuated SVs of 1, 5, and 14 dB (low, medium, and high) compared to without effusion; however, this attenuation was not significant (p = 0.07). Interestingly, in the setting of RWV, simulated effusion resulted in significantly (p = 0.001) increased SVs of 16, 11, and 8 dB (low, medium, and high). A 3-dB variance in AMEI efficiency was observed in repeated measurements in a single temporal bone.

CONCLUSION

The efficiency of OCV was not significantly affected by the presence of a middle ear effusion. Improved efficiency, however, was observed with RWV.

Otologics active middle ear implants

This article describes outcomes for the Otologics active middle ear implant for the semi-implantable and fully implantable (Carina, Otologics LLC, Boulder, CO) devices. Inclusion and exclusion criteria are reported in detail for surgical and audiologic management. Results from the clinical trial demonstrated no change for unaided air and bone conduction thresholds and no significant change in monosyllabic word scores or sentences in noise. Experiments are reported for conductive and mixed types of hearing losses in animal and human cadaveric models. These devices are in their infancy, and further study is needed to better identify candidates and develop appropriate expectations.

Comparison of auditory responses determined by acoustic stimulation and by mechanical round window stimulation at equivalent stapes velocities

Active middle ear implants (AMEIs) have been studied to overcome the limitations of conventional hearing aids such as howling, occlusion, and social discrimination. AMEIs usually drive the oval window (OW) by means of transmitting vibrational force through the ossicles and the vibrational force corresponding to sound is generated from a mechanical actuator. Recently, round window (RW) stimulation using an AMEI such as a floating mass transducer (FMT) to deliver sound to the cochlea has been introduced and hearing improvement in clinical use has been reported. Although previous studies demonstrated that the auditory response to RW stimulation was comparable to a sound-evoked auditory response, few studies have investigated the quantification of the physiologic performance of an AMEI through RW stimulation on the inner ear in vivo. There is no established relationship between the cochlear responses and mechanical stimulation to RW. The aim of this study is to assess the physiologic response in RW stimulation by an AMEI. The transferred energy through the RW to the inner ear could estimate the response corresponding to acoustic stimulation in order to quantify the AMEI output in the ossicular chain or OW stimulation. The parameters of the auditory brainstem responses (ABRs) were measured and compared based on stapes velocities similar enough to be regarded as the same for acoustic stimulation to the external auditory canal (EAC) and mechanical stimulation to the RW in an in vivo system. In conclusion, this study showed that the amplitudes and latencies of the ABRs of acoustic and RW stimulation showed significant differences at comparable stapes velocities in an in vivo system. These differences in the ABR amplitudes and latencies reflect different output functions of the cochlea in response to different stimulation pathways. Therefore, it is necessary to develop a new method for quantifying the output of the cochlea in the case of RW stimulation.

A novel mechanism of cochlear excitation during simultaneous stimulation and pressure relief through the round window

The round window (RW) membrane provides pressure relief when the cochlea is excited by sound. Here, we report measurements of cochlear function from guinea pigs when the cochlea was stimulated at acoustic frequencies by movements of a miniature magnet which partially occluded the RW. Maximum cochlear sensitivity, corresponding to subnanometre magnet displacements at neural thresholds, was observed for frequencies around 20 kHz, which is similar to that for acoustic stimulation. Neural response latencies to acoustic and RW stimulation were similar and taken to indicate that both means of stimulation resulted in the generation of conventional travelling waves along the cochlear partition. It was concluded that the relatively high impedance of the ossicles, as seen from the cochlea, enabled the region of the RW not occluded by the magnet, to act as a pressure shunt during RW stimulation. We propose that travelling waves, similar to those owing to acoustic far-field pressure changes, are driven by a jet-like, near-field component of a complex pressure field, which is generated by the magnetically vibrated RW. Outcomes of research described here are theoretical and practical design principles for the development of new types of hearing aids, which use near-field, RW excitation of the cochlea.

Systematic review of animal models of middle ear surgery

Animal models of middle ear surgery help us to explore disease processes and intervention outcomes in a manner not possible in patients. This review begins with an overview of animal models of middle ear surgery which outlines the advantages and limitations of such models. Procedures of interest include myringoplasty/tympanoplasty, mastoidectomy, ossiculoplasty, stapedectomy, and active middle ear implants. The most important issue is how well the model reflects the human response to surgery. Primates are most similar to humans with respect to anatomy; however, such studies are uncommon now due to expense and ethical issues. Conversely, small animals are easily obtained and housed, but experimental findings may not accurately represent what happens in humans. We then present a systematic review of animal models of middle ear surgery. Particular attention is paid to any distinctive anatomical features of the middle ear, the method of accessing the middle ear and the chosen outcomes. These outcomes are classified as either physiological in live animals, (e.g., behavioural or electrophysiological responses), or anatomical in cadaveric animals, (e.g., light or electron microscopy). Evoked physiological measures are limited by the disruption of the evoking air-conducted sound across the manipulated middle ear. The eleven identified species suitable as animal models are mouse, rat, gerbil, chinchilla, guinea pig, rabbit, cat, dog, sheep, pig and primate. Advantages and disadvantages of each species as a middle ear surgical model are outlined, and a suggested framework to aid in choosing a particular model is presented.

Application of the Vibrant Soundbridge middle-ear implant for aural atresia in patients with Treacher Collins syndrome

OBJECTIVE

To present results for the auditory rehabilitation of patients with Treacher Collins syndrome with bilateral osseous atresia, using middle-ear implantation with a Vibrant Soundbridge.

METHODS

Three patients underwent vibroplasty for aural atresia with moderate to severe conductive hearing loss. The pre-operative Jahrsdoerfer radiological score was 4 for all patients. Patients underwent active middle-ear implantation of a Vibrant Soundbridge implant (coupling the floating mass transducer to the rudimentary stapes or footplate distally, and positioning it adjacent to the round window membrane proximally), with audiological analysis as follow up.

RESULTS

After implant activation, the mean air conduction threshold ± standard deviation decreased to 22.8 ± 5.5 dB HL, representing a mean functional gain of 44.5 dB. The mean word recognition score (for bisyllabic words at 65 dB SPL) increased from 0 to 97 per cent.

CONCLUSION

Vibrant Soundbridge implantation is an effective hearing rehabilitation procedure in patients with Treacher Collins syndrome with bilateral osseous atresia. This is a versatile implant which can achieve coupling even in cases of severe middle-ear malformation.

Third-window vibroplasty with an active middle ear implant: assessment of physiologic responses in a model of stapes fixation in Chinchilla lanigera

HYPOTHESIS

Mechanical stimulation through a cochlear third window into the scala tympani in a chinchilla model with normal and fixed stapes can generate cochlear responses equivalent to acoustic stimuli.

BACKGROUND

Cochlear stimulation via the round window (RW) using active middle ear implants (AMEIs) can produce physiologic responses similar to acoustic stimulation including in a model of stapes fixation. However, pathologic conditions, such as advanced otosclerosis, can preclude delivery of sound energy to the cochlea through the oval window and/or the RW.

METHODS

Cochlear microphonic (CM) and laser Doppler vibrometer measurements of stapes and RW velocities were performed in 6 ears of 4 chinchillas. Baseline measurements to acoustic sinusoidal stimuli (0.25-8 kHz) were made. Measurements were repeated with an AMEI driving the RW or a third window to the scala tympani before and after stapes fixation.

RESULTS

AMEI stimulation of the third window produced CM waveforms with morphologies similar to acoustic stimuli. CM thresholds with RW and third-window stimulation were frequency dependent but ranged from 0.25 to 10 and 0.5 to 40 mV, respectively. Stapes fixation, confirmed by laser Doppler vibrometer measurements, resulted in a significant frequency dependent impairment in CM thresholds up to 13 dB (at <3 kHz) for RW stimulation and a nonsignificant frequency-dependent improvement of up to 10 dB (at >3 kHz) via third-window stimulation.

CONCLUSION

AMEI mechanical stimulation through a third window into the scala tympani produces physiologic responses nearly identical to acoustic stimulation including in a model of stapes fixation with decreased efficiency.

Controlled round-window stimulation in human temporal bones yielding reproducible and functionally relevant stapedial responses

Stimulation of the round window (RW) has gained increasing clinical importance. Clinical, as well as human temporal bone and in-vivo animal studies show considerable variability. The influence of RW stimulation on the cochlea remains unclear. We designed a human temporal-bone study with controlled direct mechanical stimulation of the RW membrane to identify conditions for successful RW stimulation. Eight human temporal bones were stimulated on the RW by piezoelectric stack actuators with cylindrical aluminium rods of diameter 0.5 mm and with either flat or 30° inclined top surface. Using a dedicated two-stage positioning protocol for the actuator, we achieved highly reproducible measurements of the stimulus vibration at the RW and of the resultant vibration of the stapes footplate. The reverse transmission, characterized by the displacement ratio of the stapes-footplate relative to the actuator tip on the RW membrane, yielded an average displacement ratio of 0.089 up to 1.2 kHz when the actuator was coupled without angular misalignment to the RW membrane. The results suggest that 90-μm pretension of the RW membrane is essential for optimum and reproducible RW stimulation. The displacements are shown to be roughly consistent with the equal-volume displacement hypothesis under specific assumptions about the displacement mode of the RW membrane. It is further suggested that the large inter-patient variability in the effectiveness of RW stimulation might be due primarily to the success of coupling, rather than to the variability of functionally relevant anatomical parameters.

Physiological assessment of active middle ear implant coupling to the round window in Chinchilla lanigera

OBJECTIVE

To study the effects of various active middle ear implant loading parameters on round window stimulation in an animal model.

STUDY DESIGN

Physiological measurements of the cochlear microphonic and stapes velocity were made from active middle ear implant-generated sinusoidal stimuli with controlled changes in loading parameters.

SETTING

Prospective study at an academic research institution.

SUBJECTS AND METHODS

Cochlear microphonic and stapes velocities (H(EV)) were measured in 6 study subjects (Chinchilla lanigera) in response to active middle ear implant (Otologics MET, Boulder, Colorado) round window stimulation with assessment of effects of varying parameters of loading pressure, interposed connective tissue, and angle of stimulation with respect to the round window membrane.

RESULTS

The measured performance variabilities in repeated applications of the active middle ear implant to the round window were 2.5 dB and 5.0 dB for H(EV) and cochlear microphonic thresholds, respectively. Loading pressure applied to the round window (51-574 dynes) and angle of approach (±30° with respect to coronal plane) did not have a significant effect on cochlear microphonic thresholds or H(EV). Significant improvements in cochlear microphonic thresholds and H(EV) were observed for interposed connective tissue regardless of tissue type.

CONCLUSION

Variability in performance due to repeated couplings of the active middle ear implant to the round window is small and reproducible. Interposition of connective tissue significantly improves vibration energy transfer to the cochlea. Neither changes in loading pressure nor angle of stimulation of the round window affected active middle ear implant performance.

Active middle ear implant application in case of stapes fixation: a temporal bone study

HYPOTHESIS

Driving the oval window directly with an active middle ear implant (AMEI) can produce high levels of input to the inner ear.

BACKGROUND

Treatment of otosclerosis bypasses the stapes with a piston that penetrates the vestibule. Although this treats the conductive component of hearing loss, it does not treat the sensorineural part, which can be improved using an additional conventional hearing aid. Active middle ear implants have been proposed to be an alternative in treating otosclerosis in cases of mixed hearing losses.

METHODS

Seven temporal bones were prepared to expose the stapes and round window (RW). Stapes and RW velocities were measured while driving with an AMEI the stapes head with a bell-shaped tip. The stapes footplate was then fixed with acrylic cement; fixation was confirmed through attenuated RW velocities. A cylinder tip (0.5 mm) was then used to drive the inner ear through a stapedotomy with and without interposition of fascia.

RESULTS

Driving the stapes with an AMEI produced mean maximum equivalent ear canal sound pressure levels (SPL) of 138 dB (0.25-8 kHz at 1 V [RMS]). Stapes fixation caused a approximately 25-dB attenuation. Driving with a cylinder tip through the stapedotomy produced 114 dB SPL (24 dB less than normal) and 110 dB SPL (28 dB less than normal) performance with and without fascia, respectively. Performance with fascia was greater than without.

CONCLUSION

Driving the oval window with an AMEI in a scenario of stapes fixation was demonstrated to be feasible, with performance comparable to traditional AMEI coupling to the incus or stapes. These possibilities offer new perspectives to treat mixed hearing loss in case of fixed footplate.