Effects of deafening and cochlear implantation procedures on postimplantation psychophysical electrical detection thresholds

Comparison of Intraoperative Neural Response Telemetry Results and Postoperative Neural Response Telemetry Results in Case of Children with Congenital Bilateral Severe to Profound Sensorineural Hearing Loss Undergoing Single Sided Cochlear Implantation

Aim and Objectives

This study is designed to compare the Intra Operative Neural Response Telemetry (NRT) results with Post-Operative NRT results of the cochlear implanted children, to assess the importance of Intra Operative NRT thresholds in device activation and/or switch on of the audio processor and to evaluate the importance of Intra and post operative AutoNRT results in predicting behavioural thresholds during mapping of prelingual Cochlear Implanted children.

Materials and methods

A total of thirty (30) children (16 boys, 14 girls) with congenital Bilateral (B/L) severe to profound Sensorineural hearing loss (SNHL) were included in this study. Children between the age range of 12 to 60 months were participated in this study. All the participants were implanted with Nucleus 24 cochlear implant system. In each patient, the intra operative NRT-thresholds were measured for all 22 active electrodes. Then Intra Operative NRT thresholds were correlated with Post-Operative NRT thresholds at the time of switch on and the behavioural map after six months of switch on of the device (Audio Processor).

Results

There is a significant enhancement observed in thresholds of postoperative NRT responses which were raised or absent during intraoperative session. There is a gain in NRT thresholds marked after 6 months of postoperative follow up in comparison with first measurement at the time of Switch On of the device but the change not that significant. During postoperative mapping, there was a significant positive correlation noticed between neural response telemetry level measurements and behavioural threshold level.

Conclusion

Absent or elevated NRT responses during intraoperative testing for some electrodes especially basal electrodes does not mean that electrode is out of order or outside the cochlea, because an improvement in NRT thresholds usually occurs postoperatively. In case of children with congenital B/L severe to profound SNHL, the NRT values have a very useful role in predicting the behavioural thresholds. Combination of NRT values with behavioural thresholds and observations of Auditory Verbal Therapist can help in providing best suitable Map to the recipient.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12070-022-03284-x.

Changes in the Electrically Evoked Compound Action Potential over time After Implantation and Subsequent Deafening in Guinea Pigs

The electrically evoked compound action potential (eCAP) is a direct measure of the responsiveness of the auditory nerve to electrical stimulation from a cochlear implant (CI). CIs offer a unique opportunity to study the auditory nerve's electrophysiological behavior in individual human subjects over time. In order to understand exactly how the eCAP relates to the condition of the auditory nerve, it is crucial to compare changes in the eCAP over time in a controlled model of deafness-induced auditory nerve degeneration. In the present study, 10 normal-hearing young adult guinea pigs were implanted and deafened 4 weeks later, so that the effect of deafening could be monitored within-subject over time. Following implantation, but before deafening, most examined eCAP characteristics significantly changed, suggesting increasing excitation efficacy (e.g., higher maximum amplitude, lower threshold, shorter latency). Conversely, inter-phase gap (IPG) effects on these measures - within-subject difference measures that have been shown to correlate well with auditory nerve survival - did not vary for most eCAP characteristics. After deafening, we observed an initial increase in excitability (steeper slope of the eCAP amplitude growth function (AGF), lower threshold, shorter latency and peak width) which typically returned to normal-hearing levels within a week, after which a slower process, probably reflecting spiral ganglion cell loss, took place over the remaining 6 weeks (e.g., decrease in maximum amplitude, AGF slope, peak area, and IPG effect for AGF slope; increase in IPG effect for latency). Our results suggest that gradual changes in peak width and latency reflect the rate of neural degeneration, while peak area, maximum amplitude, and AGF slope reflect neural population size, which may be valuable for clinical diagnostics.

Effects of Combined Gentamicin and Furosemide Treatment on Cochlear Macrophages

Combining aminoglycosides and loop diuretics often serves as an effective ototoxic approach to deafen experimental animals. The treatment results in rapid hair cell loss with extended macrophage presence in the cochlea, creating a sterile inflammatory environment. Although the early recruitment of macrophages is typically neuroprotective, the delay in the resolution of macrophage activity can be a complication if the damaged cochlea is used as a model to study subsequent therapeutic strategies. Here, we applied a high dose combination of systemic gentamicin and furosemide in C57 BL/6 and CBA/CaJ mice and studied the ototoxic consequences in the cochlea, including hair cell survival, ribbon synaptic integrity, and macrophage activation up to 15-day posttreatment. The activity of macrophages in the basilar membrane was correlated to the severity of cochlear damage, particularly the hair cell damage. Comparatively, C57 BL/6 cochleae were more vulnerable to the ototoxic challenge with escalated macrophage activation. In addition, the ribbon synaptic deterioration was disproportionately limited when compared to the degree of outer hair cell loss in CBA/CaJ mice. The innate and differential otoprotection in CBA/CaJ mice appears to be associated with the rapid activation of cochlear macrophages and a certain level of synaptogenesis after the combined gentamicin and furosemide treatment.

Early Elevation and Normalization of Electrode Impedance in Patients With Enlarged Vestibular Aqueduct Undergoing Cochlear Implantation

OBJECTIVE

To characterize early changes in impedance in patients undergoing cochlear implantation with and without enlarged vestibular aqueducts (EVA).

METHODS

Case-control retrospective study of patients undergoing cochlear implantation with and without EVA. Impedance was measured across all channels intraoperatively and within 24 hours of surgery. All patients received the same electrode array.

RESULTS

Ten patients with EVA (and matched controls were identified). The average intraoperative impedance across all electrodes was significantly higher in patients with EVA (13.1 ± 1.4 kΩ) than in controls (9.6 ± 2.5 kΩ, p  < 0.001). At 24-hour activation, the average impedance across all electrodes was roughly equal in both groups (6.8 ± 2.7 kΩ versus 6.5 ± 2.1 kΩ, p  = 0.72).

CONCLUSIONS

This study is the first identify differences in intraoperative impedance between patients with and without EVA. In addition, these data demonstrate rapid normalization within 24 hours of surgery. Such findings can give a window of insight into both the intracochlear microenvironment of patients with EVA and the important early electrode-fluid-tissue interface changes that occur within hours of surgery for all patients.

Development of a chronically-implanted mouse model for studies of cochlear health and implant function

Mice with chronic cochlear implants can significantly contribute to our understanding of the relationship between cochlear health and implant function because of the availability of molecular tools for controlling conditions in the cochlea and transgenic lines modeling human disease. To date, research in implanted mice has mainly consisted of short-term studies, but since there are large changes in implant function following implant insertion trauma, and subsequent recovery in many cases, longer-term studies are needed to evaluate function and perception under stable conditions. Because frequent anesthetic administration can be especially problematic in mice, a chronic model that can be tested in the awake condition is desirable. Electrically-evoked compound action potentials (ECAPs) recorded with multichannel cochlear implants are useful functional measures because they can be obtained daily without anesthesia. In this study, we assessed changes and stability of ECAPs, electrically-evoked auditory brainstem responses (EABRs), ensemble spontaneous activity (ESA), and impedance data over time after implanting mice with multichannel implants. We then compared these data to histological findings in these implanted cochleae, and compared results from this chronic mouse model to data previously obtained in a well-established chronically-implanted guinea pig model. We determined that mice can be chronically implanted with cochlear implants, and ECAP recordings can be obtained frequently in an awake state for up to at least 42 days after implantation. These recordings can effectively monitor changes or stability in cochlear function over time. ECAP and EABR amplitude-growth functions (AGFs), AGF slopes, ESA levels and impedances in mice with multichannel implants appear similar to those found in guinea pigs with long-term multichannel implants. Animals with better survival of spiral ganglion neurons (SGNs) and inner hair cells (IHCs) have steeper AGF slopes, and larger ESA responses. The time course of post-surgical ear recovery may be quicker in mice and can show different patterns of recovery which seem to be dependent on the degree of insertion trauma and subsequent histological conditions. Histology showed varying degrees of cochlear damage with fibrosis present in all implanted mouse ears and small amounts of new bone in a few ears. Impedance changes over time varied within and across animals and may represent changes over time in multiple variables in the cochlear environment post-implantation. Due to the small size of the mouse, susceptibility to stress, and the higher potential for implant failure, chronic implantation in mice can be challenging, but overall is feasible and useful for cochlear implant research.

Use of the guinea pig in studies on the development and prevention of acquired sensorineural hearing loss, with an emphasis on noise

Guinea pigs have been used in diverse studies to better understand acquired hearing loss induced by noise and ototoxic drugs. The guinea pig has its best hearing at slightly higher frequencies relative to humans, but its hearing is more similar to humans than the rat or mouse. Like other rodents, it is more vulnerable to noise injury than the human or nonhuman primate models. There is a wealth of information on auditory function and vulnerability of the inner ear to diverse insults in the guinea pig. With respect to the assessment of potential otoprotective agents, guinea pigs are also docile animals that are relatively easy to dose via systemic injections or gavage. Of interest, the cochlea and the round window are easily accessible, notably for direct cochlear therapy, as in the chinchilla, making the guinea pig a most relevant and suitable model for hearing. This article reviews the use of the guinea pig in basic auditory research, provides detailed discussion of its use in studies on noise injury and other injuries leading to acquired sensorineural hearing loss, and lists some therapeutics assessed in these laboratory animal models to prevent acquired sensorineural hearing loss.

Changes over time in the electrically evoked compound action potential (ECAP) interphase gap (IPG) effect following cochlear implantation in Guinea pigs

The electrically-evoked compound action potential (ECAP) is correlated with spiral ganglion neuron (SGN) density in cochlear implanted animals. In a previous study, we showed that ECAP amplitude growth function (AGF) linear slopes for stimuli with a constant interphase gap (IPG) changed significantly over time following implantation. Related studies have also shown that 1) IPG sensitivity for ECAP measures ("IPG Effect") is related to SGN density in animals and 2) the ECAP IPG Effect is related to speech recognition performance in humans with cochlear implants. The current study examined how the ECAP IPG Effect changed following cochlear implantation in four non-deafened guinea pigs with residual inner hair cells (IHCs) and 5 deafened, neurotrophin-treated guinea pigs. Simple impedances were measured on the same days as the ECAP measures. Generally, non-deafened implanted animals with higher SGN survival demonstrated higher ECAP AGF linear slope and peak amplitude values than the deafened, implanted guinea pigs. The ECAP IPG Effect for the AGF slopes and peak amplitudes was also larger in the hearing animals. The N1 latencies for a constant IPG were not different between groups, but the N1 latency IPG Effect was smaller in the non-deafened, implanted animals. Similar to previously reported results, ECAP measures using a fixed or changing IPG required as many as three months after implantation before a stable point could be calculated, but this was dependent on the animal and condition. For all ECAP measures most animals showed greater variance in the first 30 days post-implantation. Post-implantation changes in ECAPs and impedances were not correlated with one another. Results from this study are helpful for estimating the mechanisms underlying ECAP characteristics and have implications for clinical application of the ECAP measures in long-term human cochlear implant recipients. Specifically, these measures could help to monitor neural health over a period of time, or during a time of stability these measures could be used to help select electrode sites for activation in clinical programming.

Differences in the impedance of cochlear implant devices within 24 hours of their implantation

Cochlear implantation is a surgical procedure, which is performed on severely hearing-impaired patients. Impedance field telemetry is commonly used to determine the integrity of the cochlear implant device during and after surgery. At the Department of Otolaryngology, Cheng Hsin General Hospital (Taipei, Taiwan), the cochlear implant devices are switched on within 24 hours of their implantation. In the present study, the impedance changes of Advanced Bionics^™ cochlear implant devices were compared with previous studies and other devices. The aim was to confirm previous hypotheses and to explore other potential associated factors that could influence impedance following cochlear implantation. The current study included 12 patients who underwent cochlear implantation at Cheng Hsin General Hospital with Advanced Bionics cochlear implant devices. The cochlear devices were all switched on within 24 hours of their implantation. The impedance was measured and compared across all contact channels of the electrode, both intra-operatively and post-operatively. The intra-operative impedance was compared with the switch-on impedance (within 24 hours of the cochlear implantation); the impedance was notably increased for all contact channels at switch-on. Of the 16 channels examined, 4 channels had a significant increase in impedance between the intra-operative measurement and the switch-on measurement. To the best of our knowledge, the impedance of a cochlear implant device can be affected by the diameter of the electrode, the position of the electrode arrays in the scala tympani, sheath formation and fibrosis surrounding the electrode after implantation and electrical stimulation during or after surgery. When the results of the current study were compared with previous studies, it was found that the impedance changes were opposite to that of Cochlear^™ implant devices. This may be explained by the position of the electrode arrays, sheath formation, the blow-out effect and differences in electrical stimulation.

ECAP growth function to increasing pulse amplitude or pulse duration demonstrates large inter-animal variability that is reflected in auditory cortex of the guinea pig

Despite remarkable advances made to ameliorate how cochlear implants process the acoustic environment, many improvements can still be made. One of most fundamental questions concerns a strategy to simulate an increase in sound intensity. Psychoacoustic studies indicated that acting on either the current, or the duration of the stimulating pulses leads to perception of changes in how loud the sound is. The present study compared the growth function of electrically evoked Compound Action Potentials (eCAP) of the 8^th nerve using these two strategies to increase electrical charges (and potentially to increase the sound intensity). Both with chronically (experiment 1) or acutely (experiment 2) implanted guinea pigs, only a few differences were observed between the mean eCAP amplitude growth functions obtained with the two strategies. However, both in chronic and acute experiments, many animals showed larger increases of eCAP amplitude with current increase, whereas some animals showed larger of eCAP amplitude with duration increase, and other animals show no difference between either approaches. This indicates that the parameters allowing the largest increase in eCAP amplitude considerably differ between subjects. In addition, there was a significant correlation between the strength of neuronal firing rate in auditory cortex and the effect of these two strategies on the eCAP amplitude. This suggests that pre-selecting only one strategy for recruiting auditory nerve fibers in a given subject might not be appropriate for all human subjects.

Temporal changes in impedance of implanted adults for various cochlear segments

Electrode impedance (EI) is the first objective assessment carried out during the surgical procedure and follow-up of cochlear implanted patients. This measure provides information on the integrity of electrodes and on the surrounding environment. It is one of the main factors responsible for energy consumption of the cochlear implant (CI). The aim of our study is to investigate changes over time in EI in adult recipients implanted with the perimodiolar array by comparing differences in various cochlear segments. In addition, we explore the relationship between these objective measures and subjective measures such as T-level and C-level. We studied 28 adult patients. Impedance values (IVs) were calculated in "common-ground" (CG) and in monopolar (M1+2) mode for electrode groups in basal middle and apical segments. We found significant decreases in IVs between activation and 1 month. We obtained higher values for basal impedance, whereas lower IVs were found for apical electrodes at all observation times. Statistical pairing over time between impedance and T/C values showed significant correlation for both global impedance (GI) and T-C levels at CG and M1+2 mode up to 6 months. Segregated statistical analysis also showed a significant and prolonged correlation of basal IVs and fitting parameters. The higher basal impedance over time can be explained by the higher proportion of newly formed tissue in this region. The linear correlation of impedances with the fitting parameters become not significant after 3/6 months for the apical and middle segments and remained significant only for the basal region over time. This behaviour underlines the importance of persistence in intra-cochlear factors in influencing fitting parameters in the basal segment.

Inner ear structure of miniature pigs measured by multi-planar reconstruction techniques

To study the structures of the scala vestibuli and tympani of miniature pigs in order to evaluate the feasibility of using miniature pigs as the animal model for cochlear implant. The temporal bones of three miniature pigs with normal hearing were scanned by micro-CT. With the aid of the Mimics software, we reconstructed the 3D structure of inner ear basing on the serial images of the miniature pig, and obtained dimensions of the scala vestibuli and tympani with multi-planar reconstruction (MPR) technique. The constructed slicing images displayed the fine structures of the cochlea. The results of our study showed that the cross-sectional areas of the scala tympani were greatest at 2.67 ± 0.90 mm² when the circumferential length from the starting point of basal turn of the cochlea reached to 1.16 mm. The scala vestibuli has a largest width and height at the starting point of basal turn. The width and the height were 2.65 ± 0.45 mm and 2.43 ± 0.2 mm respectively. The largest width and height of the scala tympani were 2.17 ± 0.30 mm and 1.83 ± 0.42 mm. The result of our study suggests that the cochlea of miniature pigs is highly consistent with human's. Miniature pigs may be used as a new model for cochlear implant. MPR technique may be used as a new approach to obtain further information of patient's cochlea in surgeons which is helpful to select suitable cochlear implant devices and surgery approach.

Variations in electrode impedance during and after cochlear implantation: Round window versus extended round window insertions

OBJECTIVES

To assess differences in intra- and postoperative electrode impedances following cochlear implantation between round window insertions (RWI) and extended round window insertions (ERWI).

METHODS

Fifty patients with congenital hearing loss received unilateral hearing implants (Sonata Ti100, Med-El GmbH, Innsbruck, Austria) with standard electrode arrays. The patients were divided into two groups according to the surgical technique used. Thirty-five procedures were performed with RWI (group A) and 15 with ERWI (group B). Electrode impedance was measured and analysed during the operation, and one week and one month postoperatively.

RESULTS

There were no statistically significant differences (i.e., P > 0.05) in electrode impedance between groups A and B intraoperatively, or at one week or one month postoperatively. Electrode impedance at one month postoperatively was higher than the intraoperative and postoperative one week values in group A (P < 0.05), with similar results in group B.

CONCLUSION

There was no significant difference between RWI and ERWI in operative duration or complications of cochlear implantation. Moreover, no significant differences in postoperative electrode impedance values were found between the two surgical routes.

Neurotrophin Gene Therapy in Deafened Ears with Cochlear Implants: Long-term Effects on Nerve Survival and Functional Measures

Because cochlear implants function by stimulating the auditory nerve, it is assumed that the condition of the nerve plays an important role in the efficacy of the prosthesis. Thus, considerable research has been devoted to methods of preserving the nerve following deafness. Neurotrophins have been identified as a potential contributor to neural health, but most of the research to date has been done in young animals and for short periods (less than 3 to 6 months) after the onset of treatment. The first objective of the current experiment was to examine the effects of a neurotrophin gene therapy delivery method on spiral ganglion neuron (SGN) preservation and function in the long term (5 to 14 months) in mature guinea pigs with cochlear implants. The second objective was to examine several potential non-invasive monitors of auditory nerve health following the neurotrophin gene therapy procedure. Eighteen mature adult male guinea pigs were deafened by cochlear perfusion of neomycin and then one ear was inoculated with an adeno-associated viral vector with an Nft3-gene insert (AAV.Ntf3) and implanted with a cochlear implant electrode array. Five control animals were deafened and inoculated with an empty AAV and implanted. Data from 43 other guinea pig ears from this and previous experiments were used for comparison: 24 animals implanted in a hearing ear, nine animals deafened and implanted with no inoculation, and ten normal-hearing non-implanted ears. After 4 to 21 months of psychophysical and electrophysiological testing, the animals were prepared for histological examination of SGN densities and inner hair cell (IHC) survival. Seventy-eight percent of the ears deafened and inoculated with AAV.Ntf3 showed better SGN survival than the 14 deafened-control ears. The degree of SGN preservation following the gene therapy procedure was variable across animals and across cochlear turns. Slopes of psychophysical multipulse integration (MPI) functions were predictive of SGN density, but only in animals with preserved IHCs. MPI was not affected by the AAV.Ntf3 treatment, but there was a minor improvement in temporal integration (TI). AAV.Ntf3 treatment had significant effects on ECAP and EABR amplitude growth func-tion (AGF) slopes; the reduction in slope in deafened ears was ameliorated by the AAV.Ntf3 treatment. Slopes of the ECAP and EABR AGFs were predictive of SGN density in a broad area near and just apical to the implant. The highest ensemble spontaneous activity (ESA) values were seen in animals with surviving IHCs, but AAV.Ntf3 treatment in deafened ears resulted in slightly higher ESA values compared to deafened untreated ears. Overall, a combination of the psychophysical and electrophysiological measures can be useful for monitoring the health of the implanted cochlea in guinea pigs. These measures should be applicable for assessing cochlear health in human subjects.

An evidence-based guide for intraoperative cochlear implant backup use

OBJECTIVES

Intraoperative assessment of electrode function and position is essential for cochlear implant outcomes. Electrophysiological measures and radiographic imaging modalities have been used for this purpose. This study was conducted to determine the relative utility of X-ray, impedance measurements and ECAP for cochlear implant function.

METHODS

A retrospective review was conducted for 132 children (age range from 5 to 13 years old) who underwent cochlear implantation. 3 months after switch on, aided free field (FF) and Ling Six Sounds were compared between patients who had intraoperative ECAP and those who had not intraoperative ECAP (but with normally located electrode array as confirmed by plain X-ray).

RESULTS

Only 4 cases (3%) had abnormal impedance measurements that became normal after reinsertion of the same electrode array. 3 cases (2.3%) showed malposition of the electrode array on plain radiograph. They underwent immediate revision. 11 cases (8.3%) had absent intraoperative ECAP response for all electrodes, and 4 cases (3%) had absent intraoperative ECAP response for basal electrodes. These devices were not explanted at the time of surgery. 3 months after switch on, FF and the ling 6 sounds were tested for all patients. No statistical significant differences were found between cases with normal intraoperative ECAP and those who gave no response in ECAP.

CONCLUSION

From the study we can conclude that plain X-ray is the guideline for backup use. Results of plain X-ray impacted surgical decision making and led to the use of the backup device. In patients with correct electrode placement in normally developed cochlea as detected by X-ray, it is not recommended to use the backup even if there is no intraoperative ECAP. Also, it is recommended to follow how those children performed after 1 or 2 years to see if making the decision of using or not a backup device based in electrophysiology measures vs X-ray made a difference.

Insertion trauma and recovery of function after cochlear implantation: Evidence from objective functional measures

Partial loss and subsequent recovery of cochlear implant function in the first few weeks following cochlear implant surgery has been observed in previous studies using psychophysical detection thresholds. In the current study, we explored this putative manifestation of insertion trauma using objective functional measures: electrically-evoked compound action potential (ECAP) amplitude-growth functions (ECAP amplitude as a function of stimulus level). In guinea pigs implanted in a hearing ear with good post-implant hearing and good spiral ganglion neuron (SGN) survival, consistent patterns of ECAP functions were observed. The slopes of ECAP growth functions were moderately steep on the day of implant insertion, decreased to low levels over the first few days after implantation and then increased slowly over several weeks to reach a relatively stable level. In parallel, ECAP thresholds increased over time after implantation and then recovered, although more quickly, to a relatively stable low level as did thresholds for eliciting a facial twitch. Similar results were obtained in animals deafened but treated with an adenovirus with a neurotrophin gene insert that resulted in good SGN preservation. In contrast, in animals implanted in deaf ears that had relatively poor SGN survival, ECAP slopes reached low levels within a few days after implantation and remained low. These results are consistent with the idea that steep ECAP growth functions require a healthy population of auditory nerve fibers and that cochlear implant insertion trauma can temporarily impair the function of a healthy SGN population.

Importance of cochlear health for implant function

Amazing progress has been made in providing useful hearing to hearing-impaired individuals using cochlear implants, but challenges remain. One such challenge is understanding the effects of partial degeneration of the auditory nerve, the target of cochlear implant stimulation. Here we review studies from our human and animal laboratories aimed at characterizing the health of the implanted cochlea and the auditory nerve. We use the data on cochlear and neural health to guide rehabilitation strategies. The data also motivate the development of tissue-engineering procedures to preserve or build a healthy cochlea and improve performance obtained by cochlear implant recipients or eventually replace the need for a cochlear implant. This article is part of a Special Issue entitled .

Is electrode-modiolus distance a prognostic factor for hearing performances after cochlear implant surgery?

The aim of this study was to evaluate electrode array position in relation to cochlear anatomy and its influence on hearing performance in cochlear implantees. Twenty-two patients (25 ears) with Med-El cochlear implants were included in this retrospective study. A negative correlation was observed between electrode-modiolus distance (EMD) at the cochlear base and monosyllabic word discrimination 6 months after implantation. We found no correlation between EMD and hearing outcome at 12 months. The insertion depth/cochlear perimeter ratio appeared to negatively influence the EMD at the base. Indeed, deep insertions in small cochleae appeared to yield smaller EMD and better hearing performance. This observation supports the idea of preplanning the surgery by adapting the electrode array to the length of the available scala tympani.

Impedance and electrically evoked compound action potential (ECAP) drop within 24 hours after cochlear implantation

Previous animal study revealed that post-implantation electrical detection levels significantly declined within days. The impact of cochlear implant (CI) insertion on human auditory pathway in terms of impedance and electrically evoked compound action potential (ECAP) variation within hours after surgery remains unclear, since at this time frequency mapping can only commence weeks after implantation due to factors associated with wound conditions. The study presented our experiences with regards to initial switch-on within 24 hours, and thus the findings about the milieus inside cochlea within the first few hours after cochlear implantation in terms of impedance/ECAP fluctuations. The charts of fifty-four subjects with profound hearing impairment were studied. A minimal invasive approach was used for cochlear implantation, characterized by a small skin incision (≈ 2.5 cm) and soft techniques for cochleostomy. Impedance/ECAP was measured intro-operatively and within 24 hours post-operatively. Initial mapping within 24 hours post-operatively was performed in all patients without major complications. Impedance/ECAP became significantly lower measured within 24 hours post-operatively as compared with intra-operatively (p<0.001). There were no differences between pre-operative and post-operative threshold for air-conduction hearing. A significant drop of impedance/ECAP in one day after cochlear implantation was revealed for the first time in human beings. Mechanisms could be related to the restoration of neuronal sensitivity to the electrical stimulation, and/or the interaction between the matrix enveloping the electrodes and the electrical stimulation of the initial switch-on. Less wound pain/swelling and soft techniques both contributed to the success of immediate initial mapping, which implied a stable micro-environment inside the cochlea despite electrodes insertion. Our research invites further studies to correlate initial impedance/ECAP changes with long-term hearing/speech performance.

The use of a dual PEDOT and RGD-functionalized alginate hydrogel coating to provide sustained drug delivery and improved cochlear implant function

Cochlear implants provide hearing by electrically stimulating the auditory nerve. Implant function can be hindered by device design variables, including electrode size and electrode-to-nerve distance, and cochlear environment variables, including the degeneration of the auditory nerve following hair cell loss. We have developed a dual-component cochlear implant coating to improve both the electrical function of the implant and the biological stability of the inner ear, thereby facilitating the long-term perception of sound through a cochlear implant. This coating is a combination of an arginine-glycine-aspartic acid (RGD)-functionalized alginate hydrogel and the conducting polymer poly(3, 4-ethylenedioxythiophene) (PEDOT). Both in vitro and in vivo assays on the effects of these electrode coatings demonstrated improvements in device performance. We found that the coating reduced electrode impedance, improved charge delivery, and locally released significant levels of a trophic factor into cochlear fluids. This coating is non-cytotoxic, clinically relevant, and has the potential to significantly improve the cochlear implant user's experience.

Detection of pulse trains in the electrically stimulated cochlea: effects of cochlear health

Perception of electrical stimuli varies widely across users of cochlear implants and across stimulation sites in individual users. It is commonly assumed that the ability of subjects to detect and discriminate electrical signals is dependent, in part, on conditions in the implanted cochlea, but evidence supporting that hypothesis is sparse. The objective of this study was to define specific relationships between the survival of tissues near the implanted electrodes and the functional responses to electrical stimulation of those electrodes. Psychophysical and neurophysiological procedures were used to assess stimulus detection as a function of pulse rate under the various degrees of cochlear pathology. Cochlear morphology, assessed post-mortem, ranged from near-normal numbers of hair cells, peripheral processes and spiral ganglion cells, to complete absence of hair cells and peripheral processes and small numbers of surviving spiral ganglion cells. The psychophysical and neurophysiological studies indicated that slopes and levels of the threshold versus pulse rate functions reflected multipulse integration throughout the 200 ms pulse train with an additional contribution of interactions between adjacent pulses at high pulse rates. The amount of multipulse integration was correlated with the health of the implanted cochlea with implications for perception of more complex prosthetic stimuli.

Cochlear infrastructure for electrical hearing

Although the cochlear implant is already the world's most successful neural prosthesis, opportunities for further improvement abound. Promising areas of current research include work on improving the biological infrastructure in the implanted cochlea to optimize reception of cochlear implant stimulation and on designing the pattern of electrical stimulation to take maximal advantage of conditions in the implanted cochlea. In this review we summarize what is currently known about conditions in the cochlea of deaf, implanted humans and then review recent work from our animal laboratory investigating the effects of preserving or reinnervating tissues on psychophysical and electrophysiological measures of implant function. Additionally we review work from our human laboratory on optimizing the pattern of electrical stimulation to better utilize strengths in the cochlear infrastructure. Histological studies of human temporal bones from implant users and from people who would have been candidates for implants show a range of pathologic conditions including spiral ganglion cell counts ranging from approximately 2% to 92% of normal and partial hair cell survival in some cases. To duplicate these conditions in a guinea pig model, we use a variety of deafening and implantation procedures as well as post-deafening therapies designed to protect neurons and/or regenerate neurites. Across populations of human patients, relationships between nerve survival and functional measures such as speech have been difficult to demonstrate, possibly due to the numerous subject variables that can affect implant function and the elapsed time between functional measures and postmortem histology. However, psychophysical studies across stimulation sites within individual human subjects suggest that biological conditions near the implanted electrodes contribute significantly to implant function, and this is supported by studies in animal models comparing histological findings to psychophysical and electrophysiological data. Results of these studies support the efforts to improve the biological infrastructure in the implanted ear and guide strategies which optimize stimulation patterns to match patient-specific conditions in the cochlea.

Electrical stimulation of the dorsal cochlear nucleus induces hearing in rats

Auditory brainstem implants (ABIs) restore hearing by electrical stimulation of the cochlear nucleus (CN). Depending on the physiological condition, duration of the pre-existing deafness, extent of damage to the CN, and the number of channels accessible to the tonotopic frequency gradients of the CN, ABIs improve speech understanding to varying degrees. Although the ventral cochlear nucleus, a mainstream auditory structure, has been considered a logic target for ABI stimulation, it is not yet clear how the dorsal cochlear nucleus (DCN) contributes to patients' hearing during ABI stimulation. To better understand the mechanisms underlying ABIs, we tested if electrical stimulation of the rat DCN induces hearing using a novel electrical prepulse inhibition (ePPI) of startle reflex behavior model. Our results showed that bipolar electrical stimulation of all channels in the DCN induced behavioral manifestation of hearing and that electrical stimulation of certain channels in the DCN induced robust neural activity in auditory cortex channels that responded to acoustic stimulation and demonstrated well-defined frequency tuning curves. This suggests that the DCN plays an important role in electrical hearing and should be further pursued in designing new ABIs. The novel ePPI behavioral paradigm may potentially be developed into an efficient method for testing hearing in animals with an implantable prosthesis.

Effects of hearing preservation on psychophysical responses to cochlear implant stimulation

Previous studies have shown that residual acoustic hearing supplements cochlear implant function to improve speech recognition in noise as well as perception of music. The current study had two primary objectives. First, we sought to determine how cochlear implantation and electrical stimulation over a time period of 14 to 21 months influence cochlear structures such as hair cells and spiral ganglion neurons. Second, we sought to investigate whether the structures that provide acoustic hearing also affect the perception of electrical stimulation. We compared psychophysical responses to cochlear implant stimulation in two groups of adult guinea pigs. Group I (11 animals) received a cochlear implant in a previously untreated ear, while group II (ten animals) received a cochlear implant in an ear that had been previously infused with neomycin to destroy hearing. Psychophysical thresholds were measured in response to pulse-train and sinusoidal stimuli. Histological analysis of all group I animals and a subset of group II animals was performed. Nine of the 11 group I animals showed survival of the organ of Corti and spiral ganglion neurons adjacent to the electrode array. All group I animals showed survival of these elements in regions apical to the electrode array. Group II animals that were examined histologically showed complete loss of the organ of Corti in regions adjacent and apical to the electrode array and severe spiral ganglion neuron loss, consistent with previous reports for neomycin-treated ears. Behaviorally, group II animals had significantly lower thresholds than group I animals in response to 100 Hz sinusoidal stimuli. However, group I animals had significantly lower thresholds than group II animals in response to pulse-train stimuli (0.02 ms/phase; 156 to 5,000 pps). Additionally, the two groups showed distinct threshold versus pulse rate functions. We hypothesize that the differences in detection thresholds between groups are caused by the electrical activation of the hair cells in group I animals and/or differences between groups in the condition of the spiral ganglion neurons.

Enhanced survival of spiral ganglion cells after cessation of treatment with brain-derived neurotrophic factor in deafened guinea pigs

Exogenous delivery of neurotrophic factors into the cochlea of deafened animals rescues spiral ganglion cells (SGCs) from degeneration. To be clinically relevant for human cochlear implant candidates, the protective effect of neurotrophins should persist after cessation of treatment and the treated SGCs should remain functional. In this study, the survival and functionality of SGCs were investigated after temporary treatment with brain-derived neurotrophic factor (BDNF). Guinea pigs in the experimental group were deafened, and 2 weeks later, the right cochleae were implanted with an electrode array and drug delivery cannula. BDNF was administered to the implanted cochleae during a 4-week period via a mini-osmotic pump. After completion of the treatment, the osmotic pumps were removed. Two weeks later, the animals were killed and the survival of SGCs was analyzed. To monitor the functionality of the auditory nerve, electrically evoked auditory brainstem responses (eABRs) were recorded in awake animals throughout the experiment. BDNF treatment resulted in enhanced survival of SGCs 2 weeks after cessation of the treatment and prevented the decreases in size and circularity that are seen in the untreated contralateral cochleae. The amplitude of the suprathreshold eABR response in BDNF-treated animals was significantly larger than in deafened control animals and comparable to that in normal-hearing control animals. The amplitude in the BDNF-treated group did not decrease significantly after cessation of treatment. The eABR latency in BDNF-treated animals was longer than normal and comparable to that in deafened control animals. These morphological and functional findings demonstrate that neurotrophic intervention had a lasting effect, which is promising for future clinical application of neurotrophic factors in implanted human cochleae.

Over-expression of BDNF by adenovirus with concurrent electrical stimulation improves cochlear implant thresholds and survival of auditory neurons

The survival of the auditory nerve in cases of sensorineural hearing loss is believed to be a major factor in effective cochlear implant function. The current study assesses two measures of cochlear implant thresholds following a post-deafening treatment intended to halt auditory nerve degeneration. We used an adenoviral construct containing a gene insert for brain-derived neurotrophic factor (BDNF), a construct that has previously been shown to promote neuronal survival in a number of biological systems. We implanted ototoxically deafened guinea pigs with a multichannel cochlear implant and delivered a single inoculation of an adenovirus suspension coding for BDNF (Ad.BDNF) into the scala tympani at the time of implantation. Thresholds to electrical stimulation were assessed both psychophysically and electrophysiologically over a period of 80 days. Spiral ganglion cell survival was analyzed at the 80 days time point. Compared to the control group, the Ad.BDNF treated group had lower psychophysical and electrophysiological thresholds as well as higher survival of spiral ganglion cells. Electrophysiological, but not psychophysical, thresholds correlated well with the density of spiral ganglion cells. These results indicate that the changes in the anatomy of the auditory nerve induced by the combination of Ad.BDNF inoculation and the electrical stimulation used for testing improved functional measures of CI performance.