Alteration of the relative vibration of the round window membrane after implantation of a direct acoustic cochlear implant.

Objective: Subjects implanted with a Direct Acoustic Cochlear Implant (DACI) show improvements in their bone conduction (BC) thresholds after surgery. We hypothesised that a new pathway for BC sound is created via the DACI. The aim of this study was to investigate the contribution of this pathway to the cochlear response via measurements of the promontory and round window membrane (RWM) velocities while stimulating with a conventional bone conductor.Design: This study was a cadaver head study with a repeated measures study design.Study Sample: Eight ears of five fresh-frozen cadaveric whole heads were investigated in this trial.Results: After DACI implantation the promontory and RWM velocities did not change significantly in the frequency range 0.5-2 kHz when the DACI was switched off.Conclusions: No significant changes in the relative vibration magnitude of the RWM after DACI implantation were observed. The improvements in BC thresholds seen in patients implanted with a DACI very likely have their origin in the changed impedance at the oval window after DACI surgery leading to a more efficient contribution from the inner ear components to BC sound.

Mechanical Effects of Cochlear Implant on Acoustic Hearing.

Residual hearing loss in cochlear implant users is investigated using the mechanical-human-cochlear model. Hearing loss due to stiffening of the round window increases significantly as input frequencies decrease from 3 kHz to 1 kHz but remains constant at lower frequencies, whereas loss due to the presence of an electrode insert becomes significantly higher at lower frequencies ([Formula: see text] kHz). The latter also shifts the characteristic frequency map toward the basal end of the cochlea. In the region away from the end of the electrode insert, cochlear function recovers, but the user still suffers from hearing loss caused by round window stiffening.

Cannula-based drug delivery to the guinea pig round window causes a lasting hearing loss that may be temporarily mitigated by BDNF.

Sustained local delivery of drugs to the inner ear may be required for future regenerative and protective strategies. The round window is surgically accessible and a promising delivery route. To be viable, a delivery system should not cause hearing loss. This study determined the effect on hearing of placing a drug-delivery microcatheter on to the round window, and delivering either artificial perilymph (AP) or brain-derived neurotrophic factor (BDNF) via this catheter with a mini-osmotic pump. Auditory brainstem responses (ABRs) were monitored for 4 months after surgery, while the AP or BDNF was administered for the first month. The presence of the microcatheter - whether dry or when delivering AP or BDNF for 4 weeks - was associated with an increase in ABR thresholds of up to 15 dB, 16 weeks after implantation. This threshold shift was, in part, delayed by the delivery of BDNF. We conclude that the chronic presence of a microcatheter in the round window niche causes hearing loss, and that this is exacerbated by delivery of AP, and ameliorated temporarily by delivery of BDNF.

New chamber stapes prosthesis - A preliminary assessment of the functioning of the prototype.

Piston-stapedotomy is the most common method for hearing restoration in patients with otosclerosis. In this study, we have experimentally examined a prototype of a new chamber stapes prosthesis. The prototype was implanted in a human cadaver temporal bone. The round window vibrations before and after implantation were measured for the acoustic signal (90 dB SPL, 0.8-8 kHz) in the external auditory canal. In comparison with a 0.4-mm piston prosthesis, the chamber prosthesis induced significantly higher vibration of the round window, especially for frequencies above 1.5 kHz. Based on the results, it can be surmised that stapedotomy with a chamber stapes prosthesis could provide better hearing results in comparison with the piston-stapedotomy.

The potential use of low-frequency tones to locate regions of outer hair cell loss.

Current methods used to diagnose cochlear hearing loss are limited in their ability to determine the location and extent of anatomical damage to various cochlear structures. In previous experiments, we have used the electrical potential recorded at the round window -the cochlear response (CR) -to predict the location of damage to outer hair cells in the gerbil. In a follow-up experiment, we applied 10 mM ouabain to the round window niche to reduce neural activity in order to quantify the neural contribution to the CR. We concluded that a significant proportion of the CR to a 762 Hz tone originated from phase-locking activity of basal auditory nerve fibers, which could have contaminated our conclusions regarding outer hair cell health. However, at such high concentrations, ouabain may have also affected the responses from outer hair cells, exaggerating the effect we attributed to the auditory nerve. In this study, we lowered the concentration of ouabain to 1 mM and determined the physiologic effects on outer hair cells using distortion-product otoacoustic emissions. As well as quantifying the effects of 1 mM ouabain on the auditory nerve and outer hair cells, we attempted to reduce the neural contribution to the CR by using near-infrasonic stimulus frequencies of 45 and 85 Hz, and hypothesized that these low-frequency stimuli would generate a cumulative amplitude function (CAF) that could reflect damage to hair cells in the apex more accurately than the 762 stimuli. One hour after application of 1 mM ouabain, CR amplitudes significantly increased, but remained unchanged in the presence of high-pass filtered noise conditions, suggesting that basal auditory nerve fibers have a limited contribution to the CR at such low frequencies.

Effect of Round Window Reinforcement on Hearing: A Temporal Bone Study With Clinical Implications for Surgical Reinforcement of the Round Window.

HYPOTHESIS: Round window reinforcement leads to conductive hearing loss. BACKGROUND: The round window is stiffened surgically as therapy for various conditions, including perilymphatic fistula and superior semicircular canal dehiscence. Round window reinforcement reduces symptoms in these patients. However, it also reduces fluid displacement in the cochlea and might therefore increase conductive hearing loss. METHODS: Perichondrium was applied to the round window membrane in nine fresh-frozen, nonpathologic temporal bones. In four temporal bones cartilage was applied subsequently. Acoustic stimuli in the form of frequency sweeps from 250 to 8000 Hz were generated at 110 dB sound pressure level. A total of 16 frequencies in a 1/3-octave series were used. Stapes velocities in response to the acoustic stimuli were measured at equally spaced multiple points covering the stapes footplate using a scanning laser Doppler interferometry system. Measurements were made at baseline, after applying perichondrium, and after applying cartilage. RESULTS: At frequencies up to 1000 Hz perichondrium reinforcement decreased stapes velocities by 1.5 to 2.9 dB compared with no reinforcement (p value = 0.003). Reinforcement with cartilage led to a further deterioration of stapes velocities by 2.6 to 4.2 dB at frequencies up to 1000 Hz (p value = 0.050). The higher frequencies were not affected by perichondrium reinforcement (p value = 0.774) or cartilage reinforcement (p value = 0.644). CONCLUSION: Our results seem to suggest a modest, clinically negligible effect of reinforcement with perichondrium. Placing cartilage on the round window resulted in a graded effect on stapes velocities in keeping with the increased stiffness of cartilage compared with perichondrium. Even so, the effect was relatively small.

Effects of pulsatile electrical stimulation of the round window on central hyperactivity after cochlear trauma in guinea pig.

Partial hearing loss induced by acoustic trauma has been shown in animal models to result in an increased spontaneous firing rate in central auditory structures. This so-called hyperactivity has been suggested to be involved in the generation of tinnitus, a phantom auditory sensation. Although there is no universal cure for tinnitus, electrical stimulation of the cochlea, as achieved by a cochlear implant, can result in significant reduction of the tinnitus percept. However, the mechanism by which this tinnitus suppression occurs is as yet unknown and furthermore cochlear implantation may not be an optimal treatment option for tinnitus sufferers who are not profoundly deaf. A better understanding of the mechanism of tinnitus suppression by electrical stimulation of the cochlea, may lead to the development of more specialised devices for those for whom a cochlear implant is not appropriate. This study aimed to investigate the effects of electrical stimulation in the form of brief biphasic shocks delivered to the round window of the cochlea on the spontaneous firing rates of hyperactive inferior colliculus neurons following acoustic trauma in guinea pigs. Effects during the stimulation itself included both inhibition and excitation but spontaneous firing was suppressed for up to hundreds of ms after the cessation of the shock train in all sampled hyperactive neurons. Pharmacological block of olivocochlear efferent action on outer hair cells did not eliminate the prolonged suppression observed in inferior colliculus neurons, and it is therefore likely that activation of the afferent pathways is responsible for the central effects observed.

Retrospective audiological analysis of bone conduction versus round window vibratory stimulation in patients with mixed hearing loss.

OBJECTIVE: To compare audiological outcomes in mild-to-moderate mixed hearing loss patients treated with a bone-anchored hearing aid or an active middle-ear implant. Analysis aimed to refine criteria used in preoperative selection of implant type. DESIGN: Retrospective comparative analysis of audiological data. Follow-up time ranged between 0.55 and 8.8 years. STUDY SAMPLE: For detailed comparative analysis, 12 patients (six in each group) with comparable bone conduction thresholds and similar clinical characteristics were selected. A larger cohort of 48 patient files were used to evaluate overall audiological indication criteria (24 per group). RESULTS: In free-field tone audiometry, Baha patients showed mean aided thresholds between 40-48 dB, whereas hearing thresholds for VSB patients were 25-43 dB. Baha and VSB users had mean WRS of 56% and 82%, respectively, at 65 dB. Better speech understanding in noise was seen with the VSB. CONCLUSION: Analysis of the main cohort (n = 48) showed that treatment with round window vibroplasty leads to better hearing performance than treatment with a bone-anchored hearing device, if the bone conduction pure-tone average (0.5 to 4 kHz) is poorer than 35 dB HL. Audiological analysis in the smaller comparative analysis showed similar findings.

Electrophysiologic consequences of flexible electrode insertions in gerbils with noise-induced hearing loss.

HYPOTHESIS: Flexible electrode interaction with intracochlear structures in a noise-damaged region of the cochlea can lead to measureable electrophysiologic changes. BACKGROUND: An emerging goal in cochlear implantation is preservation of residual hearing subsequently allowing for combined electric and acoustic stimulation (EAS). However, residual hearing is at least partially lost in most patients as a result of electrode insertion. A gerbil model was used to examine changes to acoustically evoked cochlear potentials during simulated cochlear implantation. METHODS: Gerbils were partially deafened by noise exposure to mimic residual hearing in human cochlear implant candidates. After 1 month, round window and intracochlear recordings during flexible electrode insertion were made in response to 1 kHz tone burst stimuli at 80 dB SPL. After the insertion, the cochleas were histologically examined for hair cell loss because of the noise exposure and trauma because of the electrode insertion. RESULTS: Anatomic damage from the flexible electrode was not observable in most cases. However, insertions caused response declines that were, on average, greater than the controls, although some losses were similar to the controls. The CM was more sensitive than the CAP for detecting cochlear disturbance. CONCLUSION: Because response reductions occurred in the absence of anatomic damage, disturbances in the fluid at the base appear to affect responses from the apex. The losses were less than in previous experiments where the basilar membrane was penetrated.

Round window electrocochleography just before cochlear implantation: relationship to word recognition outcomes in adults.

HYPOTHESES: Electrocochleography (ECoG) to acoustic stimuli can differentiate relative degrees of cochlear responsiveness across the population of cochlear implant recipients. The magnitude of the ongoing portion of the ECoG, which includes both hair cell and neural contributions, will correlate with speech outcomes as measured by results on CNC word score tests. BACKGROUND: Postoperative speech outcomes with cochlear implants vary from almost no benefit to near normal comprehension. A factor expected to have a high predictive value is the degree of neural survival. However, speech performance with the implant does not correlate with the number and distribution of surviving ganglion cells when measured postmortem. We will investigate whether ECoG can provide an estimate of cochlear function that helps predict postoperative speech outcomes. METHODS: An electrode was placed at the round window of the ear about to be implanted during implant surgery. Tone bursts were delivered through an insert earphone. Subjects included children (n = 52, 1-18 yr) and postlingually hearing impaired adults (n = 32). Word scores at 6 months were available from 21 adult subjects. RESULTS: Significant responses to sound were recorded from almost all subjects (80/84 or 95%). The ECoG magnitudes spanned more than 50 dB in both children and adults. The distributions of ECoG magnitudes and frequencies were similar between children and adults. The correlation between the ECoG magnitude and word score accounted for 47% of the variance. CONCLUSION: ECoGs with high signal-to-noise ratios can be recorded from almost all implant candidates, including both adult and pediatric populations. In postlingual adults, the ECoG magnitude is more predictive of implant outcomes than other nonsurgical variables such as duration of deafness or degree of residual hearing.

[Effect of fenestration of the cochlea on the vibration of the round window membrane]. / Effekt der Promontorialfensterung auf die Schwingung der Rundfenstermembran - experimentelle Studie.

Drilling a promontory window and coupling an FMT into the scala tympani may be a surgical alternative to stapes surgery in obliterative tympanosclerosis. Aim of this experimental study on human temporal bones was to measure changes of the acoustic transfer function from the tympanic membrane to the round window membrane after drilling a promontory window and insertion of a floating mass transducer.Laser vibrometry and acoustic measurements were performed on 11 temporal bone preparations equipped with a microphone attached to the round window. Calibrations were carried out to allow determination of SPLs affecting the cochlea after drilling a promontory window leaving the membranous inner ear intact and after insertion of an FMT into the cavity (with or without slight pressure).Drilling a promontory window does influence the transfer function. Insertion of the FMT with additional slight pressure further changes the transfer function.The presence of a promontory window changes the acoustic transfer function to the round window. Further investigations are needed to correlate the qualitative results with the audiological results after "third window vibroplasty" (inserted floating mass transducer without stimulation).

Prosthetic reconstruction from the tympanic membrane to the stapes head or to the stapes footplate? A laser Doppler study.

BACKGROUND: In the absence of the incus, many surgeons believe that reconstruction from the tympanic membrane to the stapes head is more effective than reconstruction to the stapes footplate. This has rarely been tested empirically. Published better clinical results with reconstruction to the stapes head might simply reflect less underlying disease in ears with an intact stapes superstructure. OBJECTIVE: To compare vibration transmission of these two forms of prosthetic reconstruction. METHODS: A fresh human cadaveric temporal bone model was used. Round window vibrations in response to sound in the ear canal were measured with a laser Doppler vibrometer. After incus removal, the discontinuity was repaired using a titanium prosthesis. Reconstruction from the tympanic membrane to the stapes head was compared to reconstruction to the stapes footplate. RESULTS: Reconstruction of both types decreased round window vibrations by 10 to 15 dB between 500 and 3000 Hz compared to the intact middle ear. Reconstruction to the stapes head performed 5 to 10 dB better at lower frequencies (500-2000 Hz), but this was only statistically significant at 1 and 2 kHz. CONCLUSIONS: There is only a 5 to 10 dB mechanical advantage gained by reconstruction from the tympanic membrane to the stapes head compared to reconstruction to the footplate for frequencies between 1 and 2 kHz.

The floating mass transducer at the round window: direct transmission or bone conduction?

The round window placement of a floating mass transducer (FMT) is a new approach for coupling an implantable hearing system to the cochlea. We evaluated the vibration transfer to the cochlear fluids of an FMT placed at the round window (rwFMT) with special attention to the role of bone conduction. A posterior tympanotomy was performed on eleven ears of seven human whole head specimens. Several rwFMT setups were examined using laser Doppler vibrometry measurements at the stapes and the promontory. In three ears, the vibrations of a bone anchored hearing aid (BAHA) and an FMT fixed to the promontory (pFMT) were compared to explore the role of bone conduction. Vibration transmission to the measuring point at the stapes was best when the rwFMT was perpendicularly placed in the round window and underlayed with connective tissue. Fixation of the rwFMT to the round window exhibited significantly lower vibration transmission. Although measurable, bone conduction from the pFMT was much lower than that of the BAHA. Our results suggest that the rwFMT does not act as a small bone anchored hearing aid, but instead, acts as a direct vibratory stimulator of the round window membrane.

Performance considerations of prosthetic actuators for round-window stimulation.

Round-window (RW) stimulation has improved speech perception in patients with mixed hearing loss. In cadaveric temporal bones, we recently showed that RW stimulation with an active prosthesis produced differential pressure across the cochlear partition (a measure related to cochlear transduction) similar to normal forward sound stimulation above 1 kHz, when contact area between the prosthesis and RW is secured. However, there is large variability in the hearing improvement in patients implanted with existing modified prosthesis. This is likely because the middle-ear prosthesis used for RW stimulation was designed for a very different application. In this paper, we utilize recently developed experimental techniques that allow for the calculation of performance specifications for a RW actuator. In cadaveric human temporal bones (N=3), we simultaneously measure scala vestibuli and scala tympani intracochlear pressures, as well as stapes velocity and ear-canal pressure, during normal forward sound stimulation as well as reverse RW stimulation. We then calculate specifications such as the impedance the actuator will need to oppose at the RW, the force with which it must push against the RW, and the velocity and distance by which it must move the RW to obtain cochlear stimulation equivalent to that of specific levels of ear-canal pressure under normal sound stimulation. This information is essential for adapting existing prostheses and for designing new actuators specifically for RW stimulation.

A pilot study of the safety and performance of the Otologics fully implantable hearing device: transducing sounds via the round window membrane to the inner ear.

OBJECTIVES: The safety and performance of the Otologics fully implantable hearing device were assessed in adult patients with mixed conductive and sensorineural hearing loss. METHODS: The subcutaneous microphone of this fully implantable device picks up ambient sounds, converts them into an electrical signal, amplifies the signal according to the user's needs, and sends it to an electromechanical transducer. The transducer tip is customized with a prosthesis in order to be in contact with the round window membrane and is protected by fascia; this translates the electrical signal into a mechanical motion that directly stimulates the round window membrane and enables the user to perceive sound. The implanted battery is recharged daily via an external charger and the user can turn the implant on and off as well as adjust the volume with a hand-held remote control. In this pilot study, 6 patients with mixed conductive and sensorineural hearing loss were implanted with the Otologics fully implantable hearing device. Pre- and postoperative air conduction, bone conduction, as well as aided and unaided thresholds and speech scores were measured. RESULTS: No significant differences between preoperative and postoperative pure-tone averages were noted. Average improvement ranged from 19.16 to 35.8 dB of functional gain across audiometric frequencies with a mean of 26.17 +/- 5.15 dB. Long-term average functional gain at 12 months was 20.83 +/- 6.22 dB. Word recognition scores demonstrated significant differences between unaided and implant-aided conditions. CONCLUSIONS: Preliminary results of this trial of the Otologics fully implantable hearing device provide evidence that this fully implantable device is capable of efficiently transferring the sound to the inner ear via the round window membrane in patients with mixed hearing loss.

Frequency-specific electrocochleography indicates that presynaptic and postsynaptic mechanisms of auditory neuropathy exist.

OBJECTIVES: The physiological mechanisms underlying auditory neuropathy (AN) remain unclear and it is likely that the multiple disruptions are classified under the broadly defined term. Cochlear implantation is being more widely used in this population to bypass the suspected site-of-lesion although a number of cases have been identified within the Sydney Cochlear Implant Centre where this management strategy has been unsuccessful. It is likely that this relates to the different physiological mechanisms underlying AN. DESIGN: To investigate the site-of-lesion in AN, frequency-specific round window electrocochleography (ECochG) was used to assess local hair-cell, dendritic, and axonal currents generated within the cochlea in 14 subjects with AN and compared with responses from two normally hearing subjects. ECochG results were then compared with electrically evoked auditory brain stem response (EABR) measured after cochlear implantation. RESULTS: The results of this study demonstrate that two dominant patterns of ECochG waveforms (produced by a high-frequency alternating tone burst) can be identified in this population of AN subjects: (a) gross waveform showing a prolonged summating potential (SP) latency that, in most cases, is followed by a small compound action potential; and (b) gross waveform showing a normal latency SP waveform followed by a broad negative potential [assumed to reflect the dendritic potential (DP) identified in anaesthetized guinea-pigs]. This study demonstrates that in most subjects (n = 7) with a prolonged latency SP but no DP, normal morphology EABR waveforms were elicited for all electrode channels. On the other hand, all subjects (n = 7) who showed a normal latency SP followed by a broad negative DP, showed EABR waveforms that were absent or having poor wave V morphology. The authors' interpretation of this is that ECochG results may provide a classification of AN into pre- and postsynaptic lesions. CONCLUSIONS: We suggest that a presynaptic and postsynaptic type of AN exist that may have implications for the fitting of cochlear implants.

Non-ossicular signal transmission in human middle ears: Experimental assessment of the "acoustic route" with perforated tympanic membranes.

Direct acoustic stimulation of the cochlea by the sound-pressure difference between the oval and round windows (called the "acoustic route") has been thought to contribute to hearing in some pathological conditions, along with the normally dominant "ossicular route." To determine the efficacy of this acoustic route and its constituent mechanisms in human ears, sound pressures were measured at three locations in cadaveric temporal bones [with intact and perforated tympanic membranes (TMs)]: (1) in the external ear canal lateral to the TM, P(TM); (2) in the tympanic cavity lateral to the oval window, P(OW); and (3) near the round window, P(RW). Sound transmission via the acoustic route is described by two concatenated processes: (1) coupling of sound pressure from ear canal to middle-ear cavity, H(P(CAV) ) identical withP(CAV)P(TM), where P(CAV) represents the middle-ear cavity pressure, and (2) sound-pressure difference between the windows, H(WPD) identical with(P(OW)-P(RW))P(CAV). Results show that: H(P(CAV) ) depends on perforation size but not perforation location; H(WPD) depends on neither perforation size nor location. The results (1) provide a description of the window pressures based on measurements, (2) refute the common otological view that TM perforation location affects the "relative phase of the pressures at the oval and round windows," and (3) show with an intact ossicular chain that acoustic-route transmission is substantially below ossicular-route transmission except for low frequencies with large perforations. Thus, hearing loss from TM perforations results primarily from reduction in sound coupling via the ossicular route. Some features of the frequency dependence of H(P(CAV) ) and H(WPD) can be interpreted in terms of a structure-based lumped-element acoustic model of the perforation and middle-ear cavities.

Simultaneous measurement of electrocochleography and cochlear blood flow during cochlear hypoxia in rabbits.

In this study, a new monitoring system is developed to measure cochlear blood flow (CBF) and electrocochleography (ECochG) during transient ischemic episodes of the cochlea. A newly designed otic probe was used for the simultaneous recordings of laser-Doppler CBF and ECochG directly from the round window (RW). The probe enabled the recording of high amplitude compound action potentials (CAP) and cochlear microphonics (CM) with few averages. Experiments were conducted on rabbits to generate episodes of cochlear ischemia by using timed compressions of the internal auditory artery (IAA). The computer monitoring system extracted and measured CAP and CM components from ECochG in real-time. Results indicate that CM and CAP generally followed CBF during compressions and releases of IAA. Both CBF values and CAP amplitudes showed an overshoot following the reperfusion. CAP amplitude measures were found to be very sensitive to ischemia showing very rapid amplitude, latency and morphological changes. CM amplitude decreased more slowly than the CAP and CBF. Simultaneous recordings of CBF and ECochG using the otic probe provide a valuable neuromonitoring tool to investigate the dynamic behavior of the cochlea during ischemia.

[Effect of variations in middle ear pressure on ruptures of the round window membrane of the inner ear of the guinea pig (Cavia porcellus)]. / Untersuchungen zum Einfluss von Mittelohrdruckschwankungen bei Rupturen der runden Fenstermembran auf das Innenohr des Meerschweinchens (Cavia porcellus).

The electrical activity of the inner ear before and after rupture of the round window membrane was monitored in guinea pigs under different pressure conditions with the aid of electrocochleography. Following studies were conducted weekly over a period of 4 weeks. Findings showed that overpressure below the pressure needed to open the Eustachian tube caused strong temporary functional disturbance of the cochlea, especially at high frequencies. Irreversible changes that were pressure-dependent were observed and occurred mainly at high frequencies. Application of low pressure to the round window membrane caused a functional loss comparable to changes after overpressure. Animals with a predamaged cochlea reacted to overpressure that was below opening pressure of the tube and to corresponding low pressure with a longer lasting functional loss than did animals with an undamaged cochlea.

Modulation of cochlear responses in the guinea pig by low-frequency, phase-shifted maskers following noise trauma.

Low-frequency acoustic biasing using an intensive phase-shifted, low-frequency masker was studied according to its ability to determine disorders of cochlear micromechanics following noise trauma in the guinea pig as animal model. Statistical analyses proved that this technique allowed electrophysiological differentiation of controls versus groups with different degrees of experimentally induced threshold shifts. To substantiate group differences an intensity of at least 70 dB SPL was required for the 52 Hz masker and the difference in relation to the test-tone intensity had to be +/- 10 or +/- 20 dB SPL. The noise-traumatized cochlea could be identified by means of a threshold shift for the 5 microV pseudothreshold, a low modulation span of the compound action potential amplitude (< 25-50 microV frequency dependent), and reduced positive summating potential amplitude with negative non-modulating values within the different measurement phases for 1 and 2 kHz stimulation.