Cochlear pathology following chronic electrical stimulation of the auditory nerve: II. Deafened kittens

Deciphering platinum dissolution in neural stimulation electrodes: Electrochemistry or biology?

Platinum (Pt) is the metal of choice for electrodes in implantable neural prostheses like the cochlear implants, deep brain stimulating devices, and brain-computer interfacing technologies. However, it is well known since the 1970s that Pt dissolution occurs with electrical stimulation. More recent clinical and in vivo studies have shown signs of corrosion in explanted electrode arrays and the presence of Pt-containing particulates in tissue samples. The process of degradation and release of metallic ions and particles can significantly impact on device performance. Moreover, the effects of Pt dissolution products on tissue health and function are still largely unknown. This is due to the highly complex chemistry underlying the dissolution process and the difficulty in decoupling electrical and chemical effects on biological responses. Understanding the mechanisms and effects of Pt dissolution proves challenging as the dissolution process can be influenced by electrical, chemical, physical, and biological factors, all of them highly variable between experimental settings. By evaluating comprehensive findings on Pt dissolution mechanisms reported in the fuel cell field, this review presents a critical analysis of the possible mechanisms that drive Pt dissolution in neural stimulation in vitro and in vivo. Stimulation parameters, such as aggregate charge, charge density, and electrochemical potential can all impact the levels of dissolved Pt. However, chemical factors such as electrolyte types, dissolved gases, and pH can all influence dissolution, confounding the findings of in vitro studies with multiple variables. Biological factors, such as proteins, have been documented to exhibit a mitigating effect on the dissolution process. Other biological factors like cells and fibro-proliferative responses, such as fibrosis and gliosis, impact on electrode properties and are suspected to impact on Pt dissolution. However, the relationship between electrical properties of stimulating electrodes and Pt dissolution remains contentious. Host responses to Pt degradation products are also controversial due to the unknown chemistry of Pt compounds formed and the lack of understanding of Pt distribution in clinical scenarios. The cytotoxicity of Pt produced via electrical stimulation appears similar to Pt-based compounds, including hexachloroplatinates and chemotherapeutic agents like cisplatin. While the levels of Pt produced under clinical and acute stimulation regimes were typically an order of magnitude lower than toxic concentrations observed in vitro, further research is needed to accurately assess the mass balance and type of Pt produced during long-term stimulation and its impact on tissue response. Finally, approaches to mitigating the dissolution process are reviewed. A wide variety of approaches, including stimulation strategies, coating electrode materials, and surface modification techniques to avoid excess charge during stimulation and minimise tissue response, may ultimately support long-term and safe operation of neural stimulating devices.

Bioelectronic Neural Interfaces: Improving Neuromodulation Through Organic Conductive Coatings

Integration of bioelectronic devices in clinical practice is expanding rapidly, focusing on conditions ranging from sensory to neurological and mental health disorders. While platinum (Pt) electrodes in neuromodulation devices such as cochlear implants and deep brain stimulators have shown promising results, challenges still affect their long-term performance. Key among these are electrode and device longevity in vivo, and formation of encapsulating fibrous tissue. To overcome these challenges, organic conductors with unique chemical and physical properties are being explored. They hold great promise as coatings for neural interfaces, offering more rapid regulatory pathways and clinical implementation than standalone bioelectronics. This study provides a comprehensive review of the potential benefits of organic coatings in neuromodulation electrodes and the challenges that limit their effective integration into existing devices. It discusses issues related to metallic electrode use and introduces physical, electrical, and biological properties of organic coatings applied in neuromodulation. Furthermore, previously reported challenges related to organic coating stability, durability, manufacturing, and biocompatibility are thoroughly reviewed and proposed coating adhesion mechanisms are summarized. Understanding organic coating properties, modifications, and current challenges of organic coatings in clinical and industrial settings is expected to provide valuable insights for their future development and integration into organic bioelectronics.

Effect of Hydrogel-based Model Fibrosis on Electrical Properties of Bioelectrodes

Fibrous tissue encapsulation can impact the performance of bioelectrodes following implantation. For example, significant increases in electrode impedance can occur within four weeks post-implantation. A key limitation hindering the understanding of host response-mediated impedance change is the reliance on animal models or complex in vitro cell cultures for electrode testing. This study aimed to develop an in vitro acellular model that can reproduce the changes in electrical properties of bioelectrodes that occur due to host responses following implantation. Specifically, the effect of synthetic, biological, and bio-synthetic co-polymer hydrogel coatings on electrode impedance was measured. Poly(vinyl alcohol) (PVA), gelatin, and PVA-gelatin co-polymers (10 and 20 wt%) were coated onto platinum (Pt) electrodes. Polarisation and access voltage, key components of the voltage response that relate to cell adhesion and protein adsorption respectively, were measured pre and post hydrogel coating and the impedance change was calculated. Results showed that increasing the polymer concentration affects the access resistance regardless of the hydrogel chemistry but only high content gelatin hydrogels increased the polarisation resistance. The increase in total impedance was ~ 2-fold of bare Pt, similar to clinical observations. This study demonstrated that an acellular fibrosis model using hydrogels could reproduce the impedance changes observed in vivo. Such a model system will support research to better understand in vivo changes in electrical properties and the longer term function of neuroprosthetic electrodes.Clinical Relevance-This study proposes an acellular fibrosis model for preclinical research. This will support the design of improved clinical stimulation strategies and better understanding of the mechanisms of impedance change at the device-tissue interface.

Electrode impedances in children with cochlear implants: Comparison between intra-operative Switch ON and post-operative Switch ON

INTRODUCTION

Intra-operative Switch ON (IOSO) is a novel clinical approach of activating the cochlear implant during the surgery adopted at our cochlear implantation center.We compared the electrode impedances in two conditions of Switch ON of cochlear implants; IOSO and post-operative Switch ON (POSO, 21st day of surgery).

METHODS

Electrode impedances of 185 cochlear implants, 93 of whom received IOSO and 92 POSO, recorded over 10 years were analyzed retrospectively.

RESULTS

Electrode impedances of IOSO group were significantly lower than POSO group at Switch ON and 3rd, 6th, 9th, and 12th months post cochlear implantation. In IOSO group, 3rd month's electrode impedances were high when compared to electrode impedances at Switch ON. Beyond the 3rd months, electrode impedance remained unchanged. In POSO group, there were no significant differences in electrode impedances between any measurement schedule.

CONCLUSIONS

To our knowledge, this is the first study to investigate in detail the electrode impedances of the two above-said conditions of Switch ON in the process of cochlear implantation. This study concludes that timing of CI Switch ON has a significant effect on the electrode impedances. These results may affect the choice of cochlear implant Switch ON timing.

Closing the Gap between the Auditory Nerve and Cochlear Implant Electrodes: Which Neurotrophin Cocktail Performs Best for Axonal Outgrowth and Is Electrical Stimulation Beneficial?

Neurotrophins promote neurite outgrowth of auditory neurons and may help closing the gap to cochlear implant (CI) electrodes to enhance electrical hearing. The best concentrations and mix of neurotrophins for this nerve regrowth are unknown. Whether electrical stimulation (ES) during outgrowth is beneficial or may direct axons is another open question. Auditory neuron explant cultures of distinct cochlear turns of 6-7 days old mice were cultured for four days. We tested different concentrations and combinations of BDNF and NT-3 and quantified the numbers and lengths of neurites with an advanced automated analysis. A custom-made 24-well electrical stimulator based on two bulk CIs served to test different ES strategies. Quantification of receptors trkB, trkC, p75^NTR, and histological analysis helped to analyze effects. We found 25 ng/mL BDNF to perform best, especially in basal neurons, a negative influence of NT-3 in combined BDNF/NT-3 scenarios, and tonotopic changes in trk and p75^NTR receptor stainings. ES largely impeded neurite outgrowth and glia ensheathment in an amplitude-dependent way. Apical neurons showed slight benefits in neurite numbers and length with ES at 10 and 500 µA. We recommend BDNF as a potent drug to enhance the man-machine interface, but CIs should be better activated after nerve regrowth.

Cochlear implants: Causes, effects and mitigation strategies for the foreign body response and inflammation

Cochlear implants provide effective auditory rehabilitation for patients with severe to profound sensorineural hearing loss. Recent advances in cochlear implant technology and surgical approaches have enabled a greater number of patients to benefit from this technology, including those with significant residual low frequency acoustic hearing. Nearly all cochleae implanted with a cochlear implant electrode array develop an inflammatory and fibrotic response. This tissue reaction can have deleterious consequences for implant function, residual acoustic hearing, and the development of the next generation of cochlear prosthetics. This article reviews the current understanding of the inflammatory/foreign body response (FBR) after cochlear implant surgery, its impact on clinical outcome, and therapeutic strategies to mitigate this response. Findings from both in human subjects and animal models across a variety of species are highlighted. Electrode array design, surgical techniques, implant materials, and the degree and type of electrical stimulation are some critical factors that affect the FBR and inflammation. Modification of these factors and various anti-inflammatory pharmacological interventions have been shown to mitigate the inflammatory/FBR response. Ongoing and future approaches that seek to limit surgical trauma and curb the FBR to the implanted biomaterials of the electrode array are discussed. A better understanding of the anatomical, cellular and molecular basis of the inflammatory/FBR response after cochlear implantation has the potential to improve the outcome of current cochlear implants and also facilitate the development of the next generation of neural prostheses.

Analysis of electrode impedance and its subcomponents for lateral wall, mid-scala, and perimodiolar electrodes in cochlear implants

OBJECTIVE

Electrode impedances play an important role in cochlear implant patient management. During clinical visits, electrode impedances are calculated from a single point voltage waveform. In the present study, multipoint electrode impedance analysis was performed to study electrode impedance and its subcomponents in patients with three different types of cochlear implant electrode arrays.

DESIGN

Voltage waveforms were measured at six different time points during the cathodic phase of a biphasic pulse in forty-seven cochlear implant patients with perimodiolar, mid-scala, or lateral wall electrode arrays. Multipoint electrode impedances were used to determine access resistance and polarization impedance.

RESULTS

Access resistance of approximately 5 kΩ was calculated across the three different electrode arrays. Mid-scala electrodes showed a smaller increase in impedances as a function of pulse duration compared to the other electrodes. Patients with lower impedances showed higher capacitance and lower resistance, suggesting that differences in electrochemical reaction at the electrodes' surface can influence impedances in cochlear implants.

CONCLUSIONS

Analysis of cochlear implant electrode impedances and their subcomponents provides valuable information about resistance to the flow of current between stimulating and return electrodes, and build an understanding of the contribution of electrochemical processes used to deliver electrical stimulation to the auditory nerve.

Predictors of Fibrotic and Bone Tissue Formation With 3-D Reconstructions of Post-implantation Human Temporal Bones

HYPOTHESIS

Years of implantation, surgical insertion approach, and electrode length will impact the volume of new tissue formation secondary to cochlear implantation.

BACKGROUND

New tissue formation, fibrosis, and osteoneogenesis after cochlear implantation have been implicated in increasing impedance and affecting performance of the cochlear implant.

METHODS

3-D reconstructions of 15 archival human temporal bones from patients with a history of cochlear implantation (CI) were generated from H&E histopathologic slides to study factors which affect volume of tissue formation.

RESULTS

Years of implantation was a predictor of osteoneogenesis (r = 0.638, p-value = 0.011) and total new tissue formation (r = 0.588, p-value = 0.021), however not of fibrosis (r = 0.235, p-value = 0.399). Median total tissue formation differed between cochleostomy and round window insertions, 25.98 and 10.34%, respectively (Mann-Whitney U = 7, p = 0.018). No correlations were found between electrode length or angular insertion depth and total new tissue (p = 0.192, p = 0.35), osteoneogenesis (p = 0.193, p = 0.27), and fibrosis (p = 0.498, p = 0.83), respectively. However, the type II error for electrode length and angular insertion depth ranged from 0.73 to 0.90, largely due to small numbers of the shorter electrodes.

CONCLUSIONS

With numbers of cochlear implant recipients increasing worldwide, an understanding of how to minimize intracochlear changes from implantation is important. The present study demonstrates that increasing years of implantation and inserting electrodes via a cochleostomy compared with a round window approach are associated with significantly greater degree of new tissue volume formation. While previous studies have demonstrated increased intracochlear damage in the setting of translocation with longer electrodes, length, and angular insertion depth of CI electrodes were not associated with increased tissue formation.

Does early activation within hours after cochlear implant surgery influence electrode impedances?

OBJECTIVE

This study aims to determine if early device activation can influence cochlear implant electrode impedances by providing electrical stimulation within hours after cochlear implant surgery.

DESIGN

Electrode impedances were measured intraoperatively, at device activation, and one-month after device activation in three groups: users whose devices were activated (1) on the same day (Same Day), (2) the next day (Next Day), and (3) 10-14 days (Standard), after cochlear implant surgery.

STUDY SAMPLE

Electrode impedances are reported in fifty-one patients implanted with a Cochlear™ Nucleus^® Cochlear Implant.

RESULTS

Compared to intraoperative levels, impedances dropped within hours for the Same Day activation group (p < 0.001) and continued dropping on the next day after surgery (p < 0.001). Similarly, electrode impedances were significantly (p < 0.001) lower at device activation for the Next Day group as compared to their intraoperative measurements. For Standard activation, impedances increased significantly from intraoperative levels, prior to device activation (p < 0.001). One-month after initial activation, impedances were not statistically different between the Same Day, Next Day, and Standard activation groups.

CONCLUSIONS

Early device activation does not influence long-term impedances in a clinically meaningful manner.

Current perspectives on the safe electrical stimulation of peripheral nerves with platinum electrodes

This review details some peripheral nervous system (PNS) targets and electrode designs used for electrical stimulation. It investigates limitations in current knowledge of safe electrical stimulation and possible future electrode developments. Current PNS targets are large, leading to poor resolution and off-target side-effects. Most clinical devices are platinum or platinum/iridium embedded in an insulation material. Their safety is usually guided by the Shannon plot, which is not valid for the PNS. New electrode designs are needed to target smaller nerve fibers, enabling higher resolution electrical therapies with fewer off-target side-effects. Damage can occur through biological and electrochemical mechanisms. Greater mechanistic understanding is required to ensure safe and efficacious, long-term electrical stimulation with new electrode materials, geometries and stimulation waveforms.

Consecutive Treatment with Brain-Derived Neurotrophic Factor and Electrical Stimulation Has a Protective Effect on Primary Auditory Neurons

Degeneration of neurons, such as the inner ear spiral ganglion neurons (SGN), may be decelerated or even stopped by neurotrophic factor treatment, such as brain-derived neurotrophic factor (BDNF), as well as electrical stimulation (ES). In a clinical setting, drug treatment of the SGN could start directly during implantation of a cochlear implant, whereas electrical stimulation begins days to weeks later. The present study was conducted to determine the effects of consecutive BDNF and ES treatments on SGN density and electrical responsiveness. An electrode drug delivery device was implanted in guinea pigs 3 weeks after deafening and five experimental groups were established: two groups received intracochlear infusion of artificial perilymph (AP) or BDNF; two groups were treated with AP respectively BDNF in addition to ES (AP + ES, BDNF + ES); and one group received BDNF from the day of implantation until day 34 followed by ES (BDNF ⇨ ES). Electrically evoked auditory brainstem responses were recorded. After one month of treatment, the tissue was harvested and the SGN density was assessed. The results show that consecutive treatment with BDNF and ES was as successful as the simultaneous combined treatment in terms of enhanced SGN density compared to the untreated contralateral side but not in regard to the numbers of protected cells.

Increase in cochlear implant electrode impedances with the use of electrical stimulation

OBJECTIVE

Electrode impedances play a critical role in cochlear implant programming. It has been previously shown that impedances rise during periods of non-use, such as the post-operative recovery period. Then when the device is activated and use is initiated, impedances fall and are typically stable. In this study, we report a new pattern where electrode impedances increase with device use and decrease with device rest.

DESIGN

Electrode impedances were measured three to four times every day over a span of 1-3 months for two cochlear implant patients.

STUDY SAMPLE

Two patients with a Nucleus cochlear implant participated in this study.

RESULTS

Both subjects in this study show wide fluctuations in electrode impedances. By taking serial electrode impedance measurements throughout a day of use, we observe that electrode impedances consistently increase with device use and decrease with device rest.

CONCLUSION

In this study, we report two cases of electrode impedances increasing as a function of device use. Numerous management strategies were employed to reduce this effect but none prevailed; a clear pathophysiologic mechanism remains elusive. Further study into the cause of this electrode impedance pattern is warranted to establish a management strategy for these cochlear implant users.

In Vivo Real-time Remote Cochlear Implant Capacitive Impedance Measurements: A Glimpse Into the Implanted Inner Ear

OBJECTIVES

To propose a remote, real-time, safe, and easy systematic method to determine electrode electric impedance components: access resistance, polarization capacitance, and polarization resistance.

PATIENTS

Patients who received a cochlear implant and had normal cochlear anatomy and complete array insertion were recruited. A total of four adult patients were included and separated in two groups according to implantation time.

INTERVENTION

Cochlear implant electrical impedance and its components were measured in all patients by using a novel diagnostic tool: a custom made software running in the patient's computer. Data is transmitted in real time to the investigator. Various stimulation and measuring strategies were used to obtain specific information in each cochlear region.

MAIN OUTCOME MEASURES

Access resistance, polarization capacitance, and resistance of each patient were measured. Measurement success rate and required time for the patient were recorded.

RESULTS

Access resistance, polarization capacitance, and resistance were obtained in different modes, thus in every specific region of the cochlea. All measurements were successful. Each measurement took approximately 7 minutes and was transmitted in real time to the investigators.

CONCLUSION

Routine use of this tool may allow constant assessment of cochlear health and could be eventually used to monitor the effect of drugs in the inner ear. This methodology provides an in vivo "electrical view" of the inside of the implanted cochlea.

Intracochlear fibrosis and the foreign body response to cochlear implant biomaterials

OBJECTIVE

To report current knowledge on the topic of intracochlear fibrosis and the foreign body response following cochlear implantation (CI).

METHODS

A literature search was performed in PubMed to identify peer-reviewed articles. Search components included "cochlear implant," "Foreign body response (FBR)," and "fibrosis." Original studies and review articles relevant to the topic were included.

RESULTS

Ninety peer-reviewed articles describing the foreign body response or intracochlear fibrosis following CI were included.

CONCLUSIONS

Intracochlear fibrosis following CI represents a significant limiting factor for the success of CI users. Several strategies have been employed to mitigate the foreign body response within the cochlea including drug delivery systems and modifications in surgical technique and electrode design. A better understanding of the FBR has the potential to improve CI outcomes and the next generation of cochlear prostheses.

Electrical Stimulation Degenerated Cochlear Synapses Through Oxidative Stress in Neonatal Cochlear Explants

Neurostimulation devices use electrical stimulation (ES) to substitute, supplement or modulate neural function. However, the impact of ES on their modulating structures is largely unknown. For example, recipients of cochlear implants using electroacoustic stimulation experienced delayed loss of residual hearing over time after ES, even though ES had no impact on the morphology of hair cells. In this study, using a novel model of cochlear explant culture with charge-balanced biphasic ES, we found that ES did not change the quantity and morphology of hair cells but decreased the number of inner hair cell (IHC) synapses and the density of spiral ganglion neuron (SGN) peripheral fibers. Inhibiting calcium influx with voltage-dependent calcium channel (VDCC) blockers attenuated the loss of SGN peripheral fibers and IHC synapses induced by ES. ES increased ROS/RNS in cochlear explants, but the inhibition of calcium influx abolished this effect. Glutathione peroxidase 1 (GPx1) and GPx2 in cochlear explants decreased under ES and ebselen abolished this effect and attenuated the loss of SGN peripheral fibers. This finding demonstrated that ES induced the degeneration of SGN peripheral fibers and IHC synapses in a current intensity- and duration-dependent manner in vitro. Calcium influx resulting in oxidative stress played an important role in this process. Additionally, ebselen might be a potential protector of ES-induced cochlear synaptic degeneration.

A mouse model of cochlear implantation with chronic electric stimulation

Objectives

Cochlear implants provide an effective treatment option for those with severe hearing loss, including those with preserved low frequency hearing. However, certain issues can reduce implant efficacy including intracochlear tissue response and delayed loss of residual acoustic hearing. We describe a mouse model of cochlear implantation with chronic electric stimulation that can be used to study cochlear implant biology and related pathologies.

Methods

Twelve normal hearing adult CBA/J mice underwent unilateral cochlear implantation and were evenly divided into one group receiving electric stimulation and one not. Serial impedance and neural response telemetry (NRT) measurements were made to assess implant functionality. Functionality was defined as having at least one electrode with an impedance ≤ 35 kOhms. Mouse cochleae were harvested for histology and 3D x-ray microscopy 21 days post-operatively, or, in case the implant was still functional, at a later time point when the implant failed. A separate experiment measured the hearing preservation rate in 7 adult CBA/J mice undergoing unilateral cochlear implantation with serial auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAE).

Results

Implants maintained functionality for a mean of 35 days in the non-stimulated group and 19.8 days in the stimulated group. Reliable NRT and behavioral responses to electric stimulation were recorded. A robust intracochlear peri-implant tissue response with neo-ossification was seen in all cochleae. Six of seven mice maintained intact low frequency hearing up to 6 weeks following cochlear implantation.

Conclusions

We demonstrate the feasibility of cochlear implantation and behaviorally significant electric stimulation in the mouse, with the potential for hearing preservation. This model may be combined with established mouse models of hearing loss and the large genetic and molecular research toolkit unique to the mouse for mechanistic and therapeutic investigations of cochlear implant biology.

Semaphorin 3A induces acute changes in membrane excitability in spiral ganglion neurons in vitro

The development and survival of spiral ganglion neurons (SGNs) are dependent on multiple trophic factors as well as membrane electrical activity. Semaphorins (Sema) constitute a family of membrane-associated and secreted proteins that have garnered significant attention as a potential SGN "navigator" during cochlea development. Previous studies using mutant mice demonstrated that Sema3A plays a role in the SGN pathfinding. The mechanisms, however, by which Sema3A shapes SGNs firing behavior are not known. In these studies, we found that Sema3A plays a novel role in regulating SGN resting membrane potential and excitability. Using dissociated SGN from pre-hearing (P3-P5) and post-hearing mice (P12-P15), we recorded membrane potentials using whole-cell patch clamp recording techniques in apical and basal SGN populations. Recombinant Sema3A was applied to examine the effects on intrinsic membrane properties and action potentials evoked by current injections. Apical and basal SGNs from newborn mice treated with recombinant Sema3A (100 ng/ml) displayed a higher resting membrane potential, higher threshold, decreased amplitude, and prolonged latency and duration of spikes. Although a similar phenomenon was observed in SGNs from post-hearing mice, the resting membrane potential was essentially indistinguishable before and after Sema3A exposure. Sema3A-mediated changes in membrane excitability were associated with a significant decrease in K⁺ and Ca²⁺ currents. Sema3A acts through linopirdine-sensitive K⁺ channels in apical, but not in the basal SGNs. Therefore, Sema3A induces differential effects in SGN membrane excitability that are dependent on age and location, and constitutes an additional early and novel effect of Sema3A SGNs in vitro.

Exploiting Routine Clinical Measures to Inform Strategies for Better Hearing Performance in Cochlear Implant Users

Neuroprostheses designed to interface with the nervous system to replace injured or missing senses can significantly improve a patient's quality of life. The challenge remains to provide implants that operate optimally over several decades. Changes in the implant-tissue interface may precede performance problems. Tools to identify and characterize such changes using existing clinical measures would be highly valuable. Modern cochlear implant (CI) systems allow easy and regular measurements of electrode impedance (EI). This measure is routinely performed as a hardware integrity test, but it also allows a level of insight into the immune-mediated response to the implant, which is associated with performance outcomes. This study is a 5-year retrospective investigation of MED-EL CI users at the University of Southampton Auditory Implant Service including 176 adult ears (18-91) and 74 pediatric ears (1-17). The trend in EI in adults showed a decrease at apical electrodes. An increase was seen at the basal electrodes which are closest to the surgery site. The trend in the pediatric cohort was increasing EI over time for nearly all electrode positions, although this group showed greater variability and had a smaller sample size. We applied an outlier-labeling rule to statistically identify individuals that exhibit raised impedance. This highlighted 14 adult ears (8%) and 3 pediatric ears (5%) with impedance levels that deviated from the group distribution. The slow development of EI suggests intra-cochlear fibrosis and/or osteogenesis as the underlying mechanism. The usual clinical intervention for extreme impedance readings is to deactivate the relevant electrode. Our findings highlight some interesting clinical contradictions: some cases with raised (but not extreme) impedance had not prompted an electrode deactivation; and many cases of electrode deactivation had been informed by subjective patient reports. This emphasizes the need for improved objective evidence to inform electrode deactivations in borderline cases, for which our outlier-labeling approach is a promising candidate. A data extraction and analysis protocol that allows ongoing and automated statistical analysis of routinely collected data could benefit both the CI and wider neuroprosthetics communities. Our approach provides new tools to inform practice and to improve the function and longevity of neuroprosthetic devices.

Evaluation of focused multipolar stimulation for cochlear implants: a preclinical safety study

OBJECTIVE

Cochlear implants (CIs) have a limited number of independent stimulation channels due to the highly conductive nature of the fluid-filled cochlea. Attempts to develop highly focused stimulation to improve speech perception in CI users includes the use of simultaneous stimulation via multiple current sources. Focused multipolar (FMP) stimulation is an example of this approach and has been shown to reduce interaction between stimulating channels. However, compared with conventional biphasic current pulses generated from a single current source, FMP is a complex stimulus that includes extended periods of stimulation before charge recovery is achieved, raising questions on whether chronic stimulation with this strategy is safe. The present study evaluated the long-term safety of intracochlear stimulation using FMP in a preclinical animal model of profound deafness.

APPROACH

Six cats were bilaterally implanted with scala tympani electrode arrays two months after deafening, and received continuous unilateral FMP stimulation at levels that evoked a behavioural response for periods of up to 182 d. Electrode impedance, electrically-evoked compound action potentials (ECAPs) and auditory brainstem responses (EABRs) were monitored periodically over the course of the stimulation program from both the stimulated and contralateral control cochleae. On completion of the stimulation program cochleae were examined histologically and the electrode arrays were evaluated for evidence of platinum (Pt) corrosion.

MAIN RESULTS

There was no significant difference in electrode impedance between control and chronically stimulated electrodes following long-term FMP stimulation. Moreover, there was no significant difference between ECAP and EABR thresholds evoked from control or stimulated cochleae at either the onset of stimulation or at completion of the stimulation program. Chronic FMP stimulation had no effect on spiral ganglion neuron (SGN) survival when compared with unstimulated control cochleae. Long-term implantation typically evoked a mild foreign body reaction proximal to the electrode array; however stimulated cochleae exhibited a small but statistically significant increase in the tissue response. Finally, there was no evidence of Pt corrosion following long-term FMP stimulation; stimulated electrodes exhibited the same surface features as the unstimulated control electrodes.

SIGNIFICANCE

Chronic intracochlear FMP stimulation at levels used in the present study did not adversely affect electrically-evoked neural thresholds or SGN survival but evoked a small, benign increase in inflammatory response compared to control ears. Moreover chronic FMP stimulation does not affect the surface of Pt electrodes at suprathreshold stimulus levels. These findings support the safe clinical application of an FMP stimulation strategy.

Challenging aspects of contemporary cochlear implant electrode array design

Objective

A design comparison of current perimodiolar and lateral wall electrode arrays of the cochlear implant (CI) is provided. The focus is on functional features such as acoustic frequency coverage and tonotopic mapping, battery consumption and dynamic range. A traumacity of their insertion is also evaluated.

Methods

Review of up-to-date literature.

Results

Perimodiolar electrode arrays are positioned in the basal turn of the cochlea near the modiolus. They are designed to initiate the action potential in the proximity to the neural soma located in spiral ganglion. On the other hand, lateral wall electrode arrays can be inserted deeper inside the cochlea, as they are located along the lateral wall and such insertion trajectory is less traumatic. This class of arrays targets primarily surviving neural peripheral processes. Due to their larger insertion depth, lateral wall arrays can deliver lower acoustic frequencies in manner better corresponding to cochlear tonotopicity. In fact, spiral ganglion sections containing auditory nerve fibres tuned to low acoustic frequencies are located deeper than 1 and half turn inside the cochlea. For this reason, a significant frequency mismatch might be occurring for apical electrodes in perimodiolar arrays, detrimental to speech perception. Tonal languages such as Mandarin might be therefore better treated with lateral wall arrays. On the other hand, closer proximity to target tissue results in lower psychophysical threshold levels for perimodiolar arrays. However, the maximal comfort level is also lower, paradoxically resulting in narrower dynamic range than that of lateral wall arrays. Battery consumption is comparable for both types of arrays.

Conclusions

Lateral wall arrays are less likely to cause trauma to cochlear structures. As the current trend in cochlear implantation is the maximal protection of residual acoustic hearing, the lateral wall arrays seem more suitable for hearing preservation CI surgeries. Future development could focus on combining the advantages of both types: perimodiolar location in the basal turn extended to lateral wall location for higher turn locations.

Secondary Degeneration of Auditory Neurons after Topical Aminoglycoside Administration in a Gerbil Model

Hair cells in the cochlea can be damaged by various causes. Damaged hair cells can lead to additional destruction of parts of the auditory afferent pathway sequentially, which is called secondary degeneration. Recently, researches regarding cochlear implants have been actively carried out for clinical purposes; secondary degeneration in animals is a much more practical model for identifying the prognosis of cochlear implants. However, an appropriate model for this research is not established yet. Thus, we developed a secondary degeneration model using an ototoxic drug. 35 gerbils were separated into four different groups and kanamycin was applied via various approaches. ABR was measured several times after drug administration. SGCs were also counted to identify any secondary degeneration. The results showed that outer and inner HCs were damaged in all kanamycin-treated groups. Twelve weeks after kanamycin treatment, the round window membrane injection group showed severe subject differences in hair cells and SGC damage, whereas the gelfoam group showed consistent and severe damage in hair cells and SGCs. In this study, we successfully induced secondary degeneration in hair cells in a gerbil model. This model can be used for various purposes in the hearing research area either for treatment or for preservation.

The pattern and degree of capsular fibrous sheaths surrounding cochlear electrode arrays

An inflammatory tissue reaction around the electrode array of a cochlear implant (CI) is common, in particular at the electrode insertion region (cochleostomy) where mechanical trauma often occurs. However, the factors determining the amount and causes of fibrous reaction surrounding the stimulating electrode, especially medially near the perimodiolar location, are unclear. Temporal bone (TB) specimens from patients who had undergone cochlear implantation during life with either Advanced Bionics (AB) Clarion ™ or HiRes90K™ (Sylmar, CA, USA) devices that have a half-band and a pre-curved electrode, or Cochlear ™ Nucleus (Sydney, Australia) device that have a full-band and a straight electrode were evaluated. The thickness of the fibrous tissue surrounding the electrode array of both types of CI devices at both the lower (LB) and upper (UB) basal turns of the cochlea was quantified at three locations: the medial, inferior, and superior aspects of the sheath. Fracture of the osseous spiral lamina and/or marked displacement of the basilar membrane were interpreted as evidence of intracochlear trauma. In addition, post-operative word recognition scores, duration of implantation, and post-operative programming data were evaluated. Seven TBs from six patients implanted with AB devices and five TBs from five patients implanted with Nucleus devices were included. A fibrous capsule around the stimulating electrode array was present in all twelve specimens. TBs implanted with AB device had a significantly thicker fibrous capsule at the medial aspect than at the inferior or superior aspects at both locations (LB and UB) of the cochlea (Wilcoxon signed-ranks test, p < 0.01). TBs implanted with a Nucleus device had no difference in the thickness of the fibrous capsule surrounding the track of the electrode array (Wilcoxon signed-ranks test, p > 0.05). Nine of fourteen (64%) basal turns of the cochlea (LB and UB of seven TBs) implanted with AB devices demonstrated intracochlear trauma compared to two of ten (20%) basal turns of the cochlea (LB and UB of five TBs) with Nucleus devices, (Fisher exact test, p < 0.05). There was no significant correlation between the thickness of the fibrous tissue and the duration of implantation or the word recognition scores (Spearman rho, p = 0.06, p = 0.4 respectively). Our outcomes demonstrated the development of a robust fibrous tissue sheath medially closest to the site of electric stimulation in cases implanted with the AB device electrode, but not in cases implanted with the Nucleus device. The cause of the asymmetric fibrous sheath may be multifactorial including insertional trauma, a foreign body response, and/or asymmetric current flow.

Effects of intracochlear factors on spiral ganglion cells and auditory brain stem response after long-term electrical stimulation in deafened kittens

Using an animal model, we have studied the response of the auditory brain stem to cochlear implantation and the effect of intracochlear factors on this response. Neonatally, pharmacologically deafened cats (100 to more than 180 days old) were implanted with a 4-electrode array in both cochleas. Then, the left cochlea of each cat was electrically stimulated for total periods of up to 1000 hours. After a terminal 14C-2-deoxyglucose (2DG) experiment, the fraction of the right inferior colliculus with a significant accumulation of 2DG label was calculated. Using 3-dimensional computer-aided reconstruction, we examined the cochleas of these animals for spiral ganglion cell (SGC) survival and intracochlear factors such as electrode positions, degeneration of the organ of Corti, and the degree of fibrosis of the scala tympani. The distribution of each parameter was calculated along the organ of Corti from the basal end. There was a positive correlation between SGC survival and the level of fibrosis in the scala tympani, and a negative correlation between SGC survival and the degree of organ of Corti degeneration. Finally, there was a negative correlation between the 2DG-labeled inferior colliculus volume fraction and the degree of fibrosis, particularly in the 1-mm region nearest the pair of electrodes, and presumably in the basal turn.

Impedance Changes and Fibrous Tissue Growth after Cochlear Implantation Are Correlated and Can Be Reduced Using a Dexamethasone Eluting Electrode

BACKGROUND

The efficiency of cochlear implants (CIs) is affected by postoperative connective tissue growth around the electrode array. This tissue formation is thought to be the cause behind post-operative increases in impedance. Dexamethasone (DEX) eluting CIs may reduce fibrous tissue growth around the electrode array subsequently moderating elevations in impedance of the electrode contacts.

METHODS

For this study, DEX was incorporated into the silicone of the CI electrode arrays at 1% and 10% (w/w) concentration. Electrodes prepared by the same process but without dexamethasone served as controls. All electrodes were implanted into guinea pig cochleae though the round window membrane approach. Potential additive or synergistic effects of electrical stimulation (60 minutes) were investigated by measuring impedances before and after stimulation (days 0, 7, 28, 56 and 91). Acoustically evoked auditory brainstem responses were recorded before and after CI insertion as well as on experimental days 7, 28, 56, and 91. Additionally, histology performed on epoxy embedded samples enabled measurement of the area of scala tympani occupied with fibrous tissue.

RESULTS

In all experimental groups, the highest levels of fibrous tissue were detected in the basal region of the cochlea in vicinity to the round window niche. Both DEX concentrations, 10% and 1% (w/w), significantly reduced fibrosis around the electrode array of the CI. Following 3 months of implantation impedance levels in both DEX-eluting groups were significantly lower compared to the control group, the 10% group producing a greater effect. The same effects were observed before and after electrical stimulation.

CONCLUSION

To our knowledge, this is the first study to demonstrate a correlation between the extent of new tissue growth around the electrode and impedance changes after cochlear implantation. We conclude that DEX-eluting CIs are a means to reduce this tissue reaction and improve the functional benefits of the implant by attenuating electrode impedance.

Intracochlear inflammatory response to cochlear implant electrodes in humans

HYPOTHESIS

This study evaluates the types and degrees of tissue response adjacent to the electrode of multichannel cochlear implants.

BACKGROUND

Cochlear implant electrodes have been classified as biocompatible prostheses. Nevertheless, in some reports, electrode extrusion, chronic inflammation, and even soft failure of the implant system have been attributed to a tissue response to the electrode.

METHODS

All celloidin-embedded temporal bones with multichannel cochlear implants from the temporal bone collection of the Massachusetts Eye and Ear Infirmary were included in the study. A total of 28 temporal bones from 21 subjects were identified and processed for histology. The severity of cellular response including eosinophil and lymphocytic infiltration, giant cell reaction, new bone formation, and fibrosis were scored on a scale from 0 to 3 at three 1-mm segments along the electrode: first 1 mm at the cochleostomy, last 1 mm from the tip of the electrode, and midway between these proximal and distal segments. The values were compared using the Wilcoxon test.

RESULTS

A granulomatous reaction to the electrode was observed in 27 (96.4%) temporal bones. Eosinophil infiltration was observed in 7 (25%) temporal bones, suggesting an allergic reaction. The Inflammatory response to the electrode was significantly greater at the basal turn of cochlea close to the cochleostomy (p < 0.05) than distal to it.

CONCLUSION

An inflammatory response is common after cochlear implantation, and it is more robust at the cochleostomy than distal to it, suggesting the role of trauma of insertion as a contributing factor.

Neurotrophins and cochlear implants: a solution to sensorineural deafness?

OBJECTIVES

To review current trends for treating sensorineural deafness by enhancing spiral ganglion neuron (SGN) survival using neurotrophins combined with cochlear implants and identify areas for future research and development.

METHODS

A literature search was undertaken on PubMed and Google scholar using terms: neurotrophins, cochlear implants (CIs), and sensorineural to identify the most recent and significant publications. The abstracts were read to identify relevant papers; these were accessed in full and analysed for this review.

RESULTS

Neurotrophins have a known role in cochlear development and the maintenance of SGNs. So far experiments using osmotic pumps to deliver neurotrophins have been successful for short-term enhanced survival of SGN's following aminoglycoside ototoxicity in animal models. They have demonstrated the re-sprouting of radial nerve fibres from SGN's towards the source of delivery. In addition electrical stimulation, gene and cell-based therapy have increased SGN survival to varying degrees.

DISCUSSION

Osmotic pumps carry a high risk of infection therefore CIs coated in a drug containing polymer or hydrogel are a realistic alternative for sustained delivery of neurotrophins. Increased SGN survival combined with neuronal re-growth raises the possibility for CIs to stimulate discrete SGN populations. Unfortunately, the duration of treatment needed for long-term survival still remains unclear and further work is needed. Nevertheless the combination of regenerative medicine to CI technology presents a novel approach to developing CI technology.

Targeted delivery of brain-derived neurotrophic factor for the treatment of blindness and deafness

Neurodegenerative causes of blindness and deafness possess a major challenge in their clinical management as proper treatment guidelines have not yet been found. Brain-derived neurotrophic factor (BDNF) has been established as a promising therapy against neurodegenerative disorders including hearing and visual loss. Unfortunately, the blood–retinal barrier and blood–cochlear barrier, which have a comparable structure to the blood–brain barrier prevent molecules of larger sizes (such as BDNF) from exiting the circulation and reaching the targeted cells. Anatomical features of the eye and ear allow use of local administration, bypassing histo-hematic barriers. This paper focuses on highlighting a variety of strategies proposed for the local administration of the BDNF, like direct delivery, viral gene therapy, and cell-based therapy, which have been shown to successfully improve development, survival, and function of spiral and retinal ganglion cells. The similarities and controversies for BDNF treatment of posterior eye diseases and inner ear diseases have been analyzed and compared. In this review, we also focus on the possibility of translation of this knowledge into clinical practice. And finally, we suggest that using nanoparticulate drug-delivery systems may substantially contribute to the development of clinically viable techniques for BDNF delivery into the cochlea or posterior eye segment, which, ultimately, can lead to a long-term or permanent rescue of auditory and optic neurons from degeneration.

Relationship among the physiologic channel interactions, spectral-ripple discrimination, and vowel identification in cochlear implant users

The hypothesis of this study was that broader patterns of physiological channel interactions in the local region of the cochlea are associated with poorer spectral resolution in the same region. Electrically evoked compound action potentials (ECAPs) were measured for three to six probe electrodes per subject to examine the channel interactions in different regions across the electrode array. To evaluate spectral resolution at a confined location within the cochlea, spectral-ripple discrimination (SRD) was measured using narrowband ripple stimuli with the bandwidth spanning five electrodes: Two electrodes apical and basal to the ECAP probe electrode. The relationship between the physiological channel interactions, spectral resolution in the local cochlear region, and vowel identification was evaluated. Results showed that (1) there was within- and across-subject variability in the widths of ECAP channel interaction functions and in narrowband SRD performance, (2) significant correlations were found between the widths of the ECAP functions and narrowband SRD thresholds, and between mean bandwidths of ECAP functions averaged across multiple probe electrodes and broadband SRD performance across subjects, and (3) the global spectral resolution reflecting the entire electrode array, not the local region, predicts vowel identification.

Postsurgical pathologies associated with intradural electrical stimulation in the central nervous system: design implications for a new clinical device

Spinal cord stimulation has been utilized for decades in the treatment of numerous conditions such as failed back surgery and phantom limb syndromes, arachnoiditis, cancer pain, and others. The placement of the stimulating electrode array was originally subdural but, to minimize surgical complexity and reduce the risk of certain postsurgical complications, it became exclusively epidural eventually. Here we review the relevant clinical and experimental pathologic findings, including spinal cord compression, infection, hematoma formation, cerebrospinal fluid leakage, chronic fibrosis, and stimulation-induced neurotoxicity, associated with the early approaches to subdural electrical stimulation of the central nervous system, and the spinal cord in particular. These findings may help optimize the safety and efficacy of a new approach to subdural spinal cord stimulation now under development.

Impedance changes in chronically implanted and stimulated cochlear implant electrodes

OBJECTIVES

Electrode impedance increases following implantation and undergoes transitory reduction with onset of electrical stimulation. The studies in this paper measured the changes in access resistance and polarization impedance in vivo before and following electrical stimulation, and recorded the time course of these changes.

DESIGN

Impedance measures recorded in (a) four cats following 6 months of cochlear implant use, and (b) three cochlear implant recipients with 1.5-5 years cochlear implant experience.

RESULTS

Both the experimental and clinical data exhibited a reduction in electrode impedance, 20 and 5% respectively, within 15-30 minutes of stimulation onset. The majority of these changes occurred through reduction in polarization impedance. Cessation of stimulation was followed by an equivalent rise in impedance measures within 6-12 hours.

CONCLUSIONS

Stimulus-induced reductions in impedance exhibit a rapid onset and are evident in both chronic in vivo models tested, even several years after implantation. Given the impedance changes were dominated by the polarization component, these findings suggest that the electrical stimulation altered the electrode surface rather than the bulk tissue and fluid in the cochlea.

Cochlear implantation for chronic electrical stimulation in the mouse

The mouse is becoming an increasingly attractive model for auditory research due to the number of genetic deafness models available. These genetic models offer the researcher an array of congenital causes of hearing impairment, and are therefore of high clinical relevance. To date, the use of mice in cochlear implant research has not been possible due to the lack of an intracochlear electrode array and stimulator small enough for murine use, coupled with the difficulty of the surgery in this species. Here, we present a fully-implantable intracochlear electrode stimulator assembly designed for chronic implantation in the mouse. We describe the surgical approach for implantation, as well as presenting the first functional data obtained from intracochlear electrical stimulation in the mouse.

Effect of monopolar and bipolar electric stimulation on survival and size of human spiral ganglion cells as studied by postmortem histopathology

The spiral ganglion cell (SGC) is the target of electrical stimulation in cochlear implants. This study is designed to test the hypothesis that chronic electrical stimulation tends to preserve SGCs in implanted hearing-impaired ears. A total of 26 pairs of temporal bones were studied from 26 individuals who in life suffered bilateral profound hearing impairment that was symmetric (in degree of impairment and etiology) across ears and then underwent unilateral cochlear implantation. The subjects were divided in two groups by stimulus configuration: bipolar (n = 16) or monopolar (n = 10). The temporal bones were prepared for histological review by standard methods and two measures of SGC status were made by cochlear segment: count and maximal cross-sectional area. Within-subject comparison of the measures between the implanted-stimulated and the unimplanted ears showed: (1) for both stimulus configurations, the mean (across subjects and segments) of the count difference (implanted ear - unimplanted ear) was significantly less than zero; (2) the mean (across subject) count difference for cochlear segments I, II and III (segments with electrode contacts in the implanted ear) was significantly less negative than the mean difference for cochlear segment IV (no electrode in implanted ear) for bipolar but not for monopolar stimulation; (3) neither implantation-stimulation nor stimulus configuration significantly influenced the measures of maximum cross-sectional cell area. The SGC count results are consistent with the hypothesis that implantation results in a propensity across the whole cochlea for SGCs to degenerate and with chronic bipolar stimulation ameliorating this propensity in those cochlear segments with electrodes present.

Effects of brain-derived neurotrophic factor (BDNF) and electrical stimulation on survival and function of cochlear spiral ganglion neurons in deafened, developing cats

Both neurotrophic support and neural activity are required for normal postnatal development and survival of cochlear spiral ganglion (SG) neurons. Previous studies in neonatally deafened cats demonstrated that electrical stimulation (ES) from a cochlear implant can promote improved SG survival but does not completely prevent progressive neural degeneration. Neurotrophic agents combined with an implant may further improve neural survival. Short-term studies in rodents have shown that brain-derived neurotrophic factor (BDNF) promotes SG survival after deafness and may be additive to trophic effects of stimulation. Our recent study in neonatally deafened cats provided the first evidence of BDNF neurotrophic effects in the developing auditory system over a prolonged duration Leake et al. (J Comp Neurol 519:1526-1545, 2011). Ten weeks of intracochlear BDNF infusion starting at 4 weeks of age elicited significant improvement in SG survival and larger soma size compared to contralateral. In the present study, the same deafening and BDNF infusion procedures were combined with several months of ES from an implant. After combined BDNF + ES, a highly significant increase in SG numerical density (>50 % improvement re: contralateral) was observed, which was significantly greater than the neurotrophic effect seen with ES-only over comparable durations. Combined BDNF + ES also resulted in a higher density of myelinated radial nerve fibers within the osseous spiral lamina. However, substantial ectopic and disorganized sprouting of these fibers into the scala tympani also occurred, which may be deleterious to implant function. EABR thresholds improved (re: initial thresholds at time of implantation) on the chronically stimulated channels of the implant. Terminal electrophysiological studies recording in the inferior colliculus (IC) revealed that the basic cochleotopic organization was intact in the midbrain in all studied groups. In deafened controls or after ES-only, lower IC thresholds were correlated with more selective activation widths as expected, but no such correlation was seen after BDNF + ES due to much greater variability in both measures.

Feline deafness

Cats have among the best hearing of all mammals in that they are extremely sensitive to a broad range of frequencies. The ear is a highly complex structure that is delicately balanced in terms of its biochemistry, types of receptors, ion channels, mechanical properties, and cellular organization. Sensorineural deafness is caused by "flawed" genes that are inherited from one or both parents. Hearing loss can also be acquired as a result of noise trauma from industrialized environment, viral infection, or blunt trauma. To date, it is not practical to intervene and attempt to correct these forms of deafness in cats.

Human cochlea: anatomical characteristics and their relevance for cochlear implantation

This is a review of the anatomical characteristics of human cochlea and the importance of variations in this anatomy to the process of cochlear implantation (CI). Studies of the human cochlea are essential to better comprehend the physiology and pathology of man's hearing. The human cochlea is difficult to explore due to its vulnerability and bordering capsule. Inner ear tissue undergoes quick autolytic changes making investigations of autopsy material difficult, even though excellent results have been presented over time. Important issues today are novel inner ear therapies including CI and new approaches for inner ear pharmacological treatments. Inner ear surgery is now a reality, and technical advancements in the design of electrode arrays and surgical approaches allow preservation of remaining structure/function in most cases. Surgeons should aim to conserve cochlear structures for future potential stem cell and gene therapies. Renewal interest of round window approaches necessitates further acquaintance of this complex anatomy and its variations. Rough cochleostomy drilling at the intricate "hook" region can generate intracochlear bone-dust-inducing fibrosis and new bone formation, which could negatively influence auditory nerve responses at a later time point. Here, we present macro- and microanatomic investigations of the human cochlea viewing the extensive anatomic variations that influence electrode insertion. In addition, electron microscopic (TEM and SEM) and immunohistochemical results, based on specimens removed at surgeries for life-threatening petroclival meningioma and some well-preserved postmortal tissues, are displayed. These give us new information about structure as well as protein and molecular expression in man. Our aim was not to formulate a complete description of the complex human anatomy but to focus on aspects clinically relevant for electric stimulation, predominantly, the sensory targets, and how surgical atraumaticity best could be reached.

Influence of cAMP and protein kinase A on neurite length from spiral ganglion neurons

Regrowth of peripheral spiral ganglion neuron (SGN) fibers is a primary objective in efforts to improve cochlear implant outcomes and to potentially reinnervate regenerated hair cells. Cyclic adenosine monophosphate (cAMP) regulates neurite growth and guidance via activation of protein kinase A (PKA) and Exchange Protein directly Activated by Cylic AMP (Epac). Here we explored the effects of cAMP signaling on SGN neurite length in vitro. We find that the cAMP analog, cpt-cAMP, exerts a biphasic effect on neurite length; increasing length at lower concentrations and reducing length at higher concentrations. This biphasic response occurs in cultures plated on laminin, fibronectin, or tenascin C suggesting that it is not substrate dependent. cpt-cAMP also reduces SGN neurite branching. The Epac-specific agonist, 8-pCPT-2'-O-Me-cAMP, does not alter SGN neurite length. Constitutively active PKA isoforms strongly inhibit SGN neurite length similar to higher levels of cAMP. Chronic membrane depolarization activates PKA in SGNs and also inhibits SGN neurite length. However, inhibition of PKA fails to rescue neurite length in depolarized cultures implying that activation of PKA is not necessary for the inhibition of SGN neurite length by chronic depolarization. Expression of constitutively active phosphatidylinositol 3-kinase, but not c-Jun N-terminal kinase, isoforms partially rescues SGN neurite length in the presence of activated PKA. Taken together, these results suggest that activation of cAMP/PKA represents a potential strategy to enhance SGN fiber elongation following deafness; however such therapies will likely require careful titration so as to promote rather than inhibit nerve fiber regeneration.

Nerve maintenance and regeneration in the damaged cochlea

Following the onset of sensorineural hearing loss, degeneration of mechanosensitive hair cells and spiral ganglion cells (SGCs) in humans and animals occurs to variable degrees, with a trend for greater neural degeneration with greater duration of deafness. Emergence of the cochlear implant prosthesis has provided much needed aid to many hearing impaired patients and has become a well-recognized therapy worldwide. However, ongoing peripheral nerve fiber regression and subsequent degeneration of SGC bodies can reduce the neural targets of cochlear implant stimulation and diminish its function. There is increasing interest in bio-engineering approaches that aim to enhance cochlear implant efficacy by preventing SGC body degeneration and/or regenerating peripheral nerve fibers into the deaf sensory epithelium. We review the advancements in maintaining and regenerating nerves in damaged animal cochleae, with an emphasis on the therapeutic capacity of neurotrophic factors delivered to the inner ear after an insult. Additionally, we summarize the histological process of neuronal degeneration in the inner ear and describe different animal models that have been employed to study this mechanism. Research on enhancing the biological infrastructure of the deafened cochlea in order to improve cochlear implant efficacy is of immediate clinical importance.

Combining cell-based therapies and neural prostheses to promote neural survival

Cochlear implants provide partial restoration of hearing for profoundly deaf patients by electrically stimulating spiral ganglion neurons (SGNs); however, these neurons gradually degenerate following the onset of deafness. Although the exogenous application of neurotrophins (NTs) can prevent SGN loss, current techniques to administer NTs for long periods of time have limited clinical applicability. We have used encapsulated choroid plexus cells (NTCells; Living Cell Technologies, Auckland, New Zealand) to provide NTs in a clinically viable manner that can be combined with a cochlear implant. Neonatal cats were deafened and unilaterally implanted with NTCells and a cochlear implant. Animals received chronic electrical stimulation (ES) alone, NTs alone, or combined NTs and ES (ES + NT) for a period of as much as 8 months. The opposite ear served as a deafened unimplanted control. Chronic ES alone did not result in increased survival of SGNs or their peripheral processes. NT treatment alone resulted in greater SGN survival restricted to the upper basal cochlear region and an increased density of SGN peripheral processes. Importantly, chronic ES in combination with NTs provided significant SGN survival throughout a wider extent of the cochlea, in addition to an increased peripheral process density. Re-sprouting peripheral processes were observed in the scala media and scala tympani, raising the possibility of direct contact between peripheral processes and a cochlear implant electrode array. We conclude that cell-based therapy is clinically viable and effective in promoting SGN survival for extended durations of cochlear implant use. These findings have important implications for the safe delivery of therapeutic drugs to the cochlea.

Spiral ganglion neuron survival and function in the deafened cochlea following chronic neurotrophic treatment

Cochlear implants electrically stimulate residual spiral ganglion neurons (SGNs) to provide auditory cues for the severe-profoundly deaf. However, SGNs gradually degenerate following cochlear hair cell loss, leaving fewer neurons available for stimulation. Providing an exogenous supply of neurotrophins (NTs) has been shown to prevent SGN degeneration, and when combined with chronic intracochlear electrical stimulation (ES) following a short period of deafness (5 days), may also promote the formation of new neurons. The present study assessed the histopathological response of guinea pig cochleae treated with NTs (brain-derived neurotrophic factor and neurotrophin-3) with and without ES over a four week period, initiated two weeks after deafening. Results were compared to both NT alone and artificial perilymph (AP) treated animals. AP/ES treated animals exhibited no evidence of SGN rescue compared with untreated deafened controls. In contrast, NT administration showed a significant SGN rescue effect in the lower and middle cochlear turns (two-way ANOVA, p < 0.05) compared with AP-treated control animals. ES in combination with NT did not enhance SGN survival compared with NT alone. SGN function was assessed by measuring electrically-evoked auditory brainstem response (EABR) thresholds. EABR thresholds following NT treatment were significantly lower than animals treated with AP (two-way ANOVA, p = 0.033). Finally, the potential for induced neurogenesis following the combined treatment was investigated using a marker of DNA synthesis. However, no evidence of neurogenesis was observed in the SGN population. The results indicate that chronic NT delivery to the cochlea may be beneficial to cochlear implant patients by increasing the number of viable SGNs and decreasing activation thresholds compared to chronic ES alone.