Electrical stimulation of the auditory nerve: the effect of electrode position on neural excitation

The Inter-Phase Gap Offset Effect as a Measure of Neural Health in Cochlear Implant Users With Residual Acoustic Hearing

OBJECTIVES

The inter-phase gap (IPG) offset effect is defined as the dB offset between the linear parts of electrically evoked compound action potential (ECAP) amplitude growth functions for two stimuli differing only in IPG. The method was recently suggested to represent neural health in cochlear implant (CI) users while being unaffected by CI electrode impedances. Hereby, a larger IPG offset effect should reflect better neural health. The aims of the present study were to (1) examine whether the IPG offset effect negatively correlates with the ECAP threshold and the preoperative pure-tone average (PTA) in CI recipients with residual acoustic hearing and (2) investigate the dependency of the IPG offset effect on hair cell survival and intracochlear electrode impedances.

DESIGN

Seventeen adult study participants with residual acoustic hearing at 500 Hz undergoing CI surgery at the University Hospital of Zurich were prospectively enrolled. ECAP thresholds, IPG offset effects, electrocochleography (ECochG) responses to 500 Hz tone bursts, and monopolar electrical impedances were obtained at an apical, middle, and basal electrode set during and between 4 and 12 weeks after CI surgery. Pure-tone audiometry was conducted within 3 weeks before surgery and approximately 6 weeks after surgery. Linear mixed regression analyses and t tests were performed to assess relationships between (changes in) ECAP threshold, IPG offset, impedance, PTA, and ECochG amplitude.

RESULTS

The IPG offset effect positively correlated with the ECAP threshold in intraoperative recordings (p < 0.001) and did not significantly correlate with the preoperative PTA (p = 0.999). The IPG offset showed a postoperative decrease for electrode sets that showed an ECochG amplitude drop. This IPG offset decrease was significantly larger than for electrode sets that showed no ECochG amplitude decrease, t(17) = 2.76, p = 0.014. Linear mixed regression analysis showed no systematic effect of electrode impedance changes on the IPG offset effect (p = 0.263) but suggested a participant-dependent effect of electrode impedance on IPG offset.

CONCLUSIONS

The present study results did not reveal the expected relationships between the IPG offset effect and ECAP threshold values or between the IPG offset effect and preoperative acoustic hearing. Changes in electrode impedance did not exhibit a direct impact on the IPG offset effect, although this impact might be individualized among CI recipients. Overall, our findings suggest that the interpretation and application of the IPG offset effect in clinical settings should be approached with caution considering its complex relationships with other cochlear and neural health metrics.

NRV: An open framework for in silico evaluation of peripheral nerve electrical stimulation strategies

Electrical stimulation of peripheral nerves has been used in various pathological contexts for rehabilitation purposes or to alleviate the symptoms of neuropathologies, thus improving the overall quality of life of patients. However, the development of novel therapeutic strategies is still a challenging issue requiring extensive in vivo experimental campaigns and technical development. To facilitate the design of new stimulation strategies, we provide a fully open source and self-contained software framework for the in silico evaluation of peripheral nerve electrical stimulation. Our modeling approach, developed in the popular and well-established Python language, uses an object-oriented paradigm to map the physiological and electrical context. The framework is designed to facilitate multi-scale analysis, from single fiber stimulation to whole multifascicular nerves. It also allows the simulation of complex strategies such as multiple electrode combinations and waveforms ranging from conventional biphasic pulses to more complex modulated kHz stimuli. In addition, we provide automated support for stimulation strategy optimization and handle the computational backend transparently to the user. Our framework has been extensively tested and validated with several existing results in the literature.

Electrically Evoked Auditory Brainstem Response Using Extracochlear Stimulation at Different Cochlear Sites: A Comparison With Intracochlear Stimulation

OBJECTIVES

The distribution and extent of excitable spiral ganglion neurons (SGNs) have been investigated using the electrically evoked auditory brainstem response (EABR) during preoperative and perioperative periods. In this study, we investigated the EABR with extracochlear stimulation (eEABR) as a preoperative test to estimate these factors.

METHODS

Sixteen male Sprague-Dawley rats were used in this study. Experiments were conducted in nine rats with normal hearing and seven rats that were partially deafened with ouabain treatment. Each experiment involved the following steps: extracochlear stimulating electrode placement at three different sites along the axis of the cochlea and eEABR recordings; cochleostomy and four-channel intracochlear array implantation, followed by EABR recordings with various electrode pair combinations; and after electrophysiological measurements, harvest of the cochleae for histopathological evaluation. The slope characteristics of the amplitude growth function measured from eEABR and EABR, frequency-specific auditory thresholds, and the density of SGNs were compared.

RESULTS

Similar trends were observed in slope changes on different sites of stimulation with both types of stimulation in normal-hearing animals-specifically, a monotonically increasing slope with increasing distance between bipolar pairs. In addition, eEABR slopes showed significant correlations with EABR slopes when the expected cochlear regions of stimulation were similar in normal-hearing animals. In partially deaf animals, the auditory thresholds at several frequencies had a significant correlation with the eEABR slopes of each extracochlear electrode at the apical, middle, and basal cochlear positions. This indicated that increasing the regions of cochlear stimulation had a differential impact on eEABR slopes, depending on the neural conditions.

CONCLUSION

Our results indicated that eEABR slopes showed significant spatial correlations with the functionality of the auditory nerve. Therefore, eEABR tests at various cochlear positions might be used for estimating the extent of excitable SGNs in cochlear implant candidates prior to implantation.

A Virtual Inner Ear Model Selects Ramped Pulse Shapes for Vestibular Afferent Stimulation

Bilateral vestibular deficiency (BVD) results in chronic dizziness, blurry vision when moving the head, and postural instability. Vestibular prostheses (VPs) show promise as a treatment, but the VP-restored vestibulo-ocular reflex (VOR) gain in human trials falls short of expectations. We hypothesize that the slope of the rising ramp in stimulation pulses plays an important role in the recruitment of vestibular afferent units. To test this hypothesis, we utilized customized programming to generate ramped pulses with different slopes, testing their efficacy in inducing electrically evoked compound action potentials (eCAPs) and current spread via bench tests and simulations in a virtual inner model created in this study. The results confirmed that the slope of the ramping pulses influenced the recruitment of vestibular afferent units. Subsequently, an optimized stimulation pulse train was identified using model simulations, exhibiting improved modulation of vestibular afferent activity. This optimized slope not only reduced the excitation spread within the semicircular canals (SCCs) but also expanded the neural dynamic range. While the model simulations exhibited promising results, in vitro and in vivo experiments are warranted to validate the findings of this study in future investigations.

A Multicenter Comparison of 1-yr Functional Outcomes and Programming Differences Between the Advanced Bionics Mid-Scala and SlimJ Electrode Arrays

OBJECTIVE

To determine if there is a difference in hearing outcomes or stimulation levels between Advanced Bionics straight and precurved arrays.

STUDY DESIGN

Retrospective chart review across three implant centers.

SETTING

Tertiary centers for cochlear and auditory brainstem implantation.

PATIENTS

One hundred fifteen pediatric and 205 adult cochlear implants (CIs) were reviewed. All patients were implanted under the National Institute for Health and Care Excellence 2009 guidelines with a HiRes Ultra SlimJ or Mid-Scala electrode array.

MAIN OUTCOME MEASURES

Hearing preservation after implantation, as well as CI-only listening scores for Bamford-Kowal-Bench sentences were compared 1 year after implantation. Stimulation levels for threshold and comfort levels were also compared 1 year after implantation.

RESULTS

Hearing preservation was significantly better with the SlimJ compared with the Mid-Scala electrode array. Bamford-Kowal-Bench outcomes were not significantly different between the two arrays in any listening condition. Stimulation levels were not different between arrays but did vary across electrode contacts. At least one electrode was deactivated in 33% of implants but was more common for the SlimJ device.

CONCLUSION

Modern straight and precurved arrays from Advanced Bionics did not differ in hearing performance or current requirements. Although hearing preservation was possible with both devices, the SlimJ array would still be the preferred electrode in cases where hearing preservation was a priority. Unfortunately, the SlimJ device was also prone to poor sound perception on basal electrodes. Further investigation is needed to determine if deactivated electrodes are associated with electrode position/migration, and if programming changes are needed to optimize the use of these high-frequency channels.

Facilitation properties in electrically evoked compound action potentials depending on spatial location and on threshold

Spiral ganglion neurons (SGNs) facilitation properties can be recorded utilizing electrically evoked compound action potential (ECAP). While intracochlear variation of the ECAP threshold in relation to its electrode channel is reported, no study investigated its impact on facilitation. In this study, we quantified intracochlear variation of the facilitation properties in cochlear implants (CI) using ECAPs. We hypothesized that the facilitation effect is dependent on the electrode channel and its ECAP threshold. Therefore, ECAPs were recorded in 23 CI subjects. For each subject, five default (channel-derived) and up to two additional (threshold-derived) stimulation sites were defined. Facilitation was quantified by the paradigm introduced by (Hey et al., 2017) with optimized parameter settings. For each channel the maximum facilitated amplitude was determined by a series of ECAP measurements. A linear mixed-effects model was used to investigate the impact of the electrode channel and ECAP threshold on the maximum facilitated amplitude. The maximum facilitated amplitude was found to be dependent on the ECAP threshold and independent on the electrode channel. We conclude that the facilitation paradigm is a useful and feasible tool to gain local information on the SGNs temporal processing patterns.

Evaluation of CI electrode position from imaging: comparison of an automated technique with the established manual method

BACKGROUND

A manual evaluation of the CI electrode position from CT and DVT scans may be affected by diagnostic errors due to cognitive biases. The aim of this study was to compare the CI electrode localization using an automated method (image-guided cochlear implant programming, IGCIP) with the clinically established manual method.

METHODS

This prospective experimental study was conducted on a dataset comprising N=50 subjects undergoing cochlear implantation with a Nucleus® CI532 or CI632 Slim Modiolar electrode. Scalar localization, electrode-to-modiolar axis distances (EMD) and angular insertion depth (aDOI) were compared between the automated IGCIP tool and the manual method. Two raters made the manual measurements, and the interrater reliability (±1.96·SD) was determined as the reference for the method comparison. The method comparison was performed using a correlation analysis and a Bland-Altman analysis.

RESULTS

Concerning the scalar localization, all electrodes were localized both manually and automatically in the scala tympani. The interrater differences ranged between ±0.2 mm (EMD) and ±10° (aDOI). There was a bias between the automatic and manual method in measuring both localization parameters, which on the one hand was smaller than the interrater variations. On the other hand, this bias depended on the magnitude of the EMD respectively aDOI. A post-hoc analysis revealed that the deviations between the methods were likely due to a different selection of mid-modiolar axis.

CONCLUSIONS

The IGCIP is a promising tool for automated processing of CT and DVT scans and has useful functionality such as being able to segment the cochlear using post-operative scans. When measuring EMD, the IGCIP tool is superior to the manual method because the smallest possible distance to the axis is determined depending on the cochlear turn, whereas the manual method selects the helicotrema as the reference point rigidly. Functionality to deal with motion artifacts and measurements of aDOI according to the consensus approach are necessary, otherwise the IGCIP is not unrestrictedly ready for clinical use.

Dynamic Behavior and Insertional Forces of a Precurved Electrode Using the Pull-Back Technique in a Fresh Microdissected Cochlea

HYPOTHESIS

This study evaluated the utility of the pull-back technique in improving perimodiolar positioning of a precurved cochlear implant (CI) electrode array (EA) with simultaneous insertion force profile measurement and direct observation of dynamic EA behavior.

BACKGROUND

Precurved EAs with perimodiolar positioning have improved outcomes compared with straight EAs because of lowered charge requirements for stimulation and decreased spread of excitation. The safety and efficacy of the pull-back technique in further improving perimodiolar positioning and its associated force profile have not been adequately demonstrated.

METHODS

The bone overlying the scala vestibuli was removed in 15 fresh cadaveric temporal bones, leaving the scala tympani unviolated. Robotic insertions of EAs were performed with simultaneous force measurement and video recording. Force profiles were obtained during standard insertion, overinsertion, and pull-back. Postinsertion CT scans were obtained during each of the three conditions, enabling automatic segmentation and calculation of angular insertion depth, mean perimodiolar distance ( Mavg ), and cochlear duct length.

RESULTS

Overinsertion did not result in significantly higher peak forces than standard insertion (mean [SD], 0.18 [0.06] and 0.14 [0.08] N; p = 0.18). Six temporal bones (40%) demonstrated visibly improved perimodiolar positioning after the protocol, whereas none worsened. Mavg significantly improved after the pull-back technique compared with standard insertion (mean [SD], 0.34 [0.07] and 0.41 [0.10] mm; p < 0.01).

CONCLUSIONS

The pull-back technique was not associated with significantly higher insertional forces compared with standard insertion. This technique was associated with significant improvement in perimodiolar positioning, both visually and quantitatively, independent of cochlear size.

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.

Electrical Field Interactions during Adjacent Electrode Stimulations: eABR Evaluation in Cochlear Implant Users

The present study investigates how electrically evoked Auditory Brainstem Responses (eABRs) can be used to measure local channel interactions along cochlear implant (CI) electrode arrays. eABRs were recorded from 16 experienced CI patients in response to electrical pulse trains delivered using three stimulation configurations: (1) single electrode stimulations (E11 or E13); (2) simultaneous stimulation from two electrodes separated by one (E_n and E_n+2, E11 and E13); and (3) stimulations from three consecutive electrodes (E11, E12, and E13). Stimulation level was kept constant at 70% electrical dynamic range (EDR) on the two flanking electrodes (E11 and E13) and was varied from 0 to 100% EDR on the middle electrode (E12). We hypothesized that increasing the middle electrode stimulation level would cause increasing local electrical interactions, reflected in characteristics of the evoked compound eABR. Results show that group averaged eABR wave III and V latency and amplitude were reduced when stimulation level at the middle electrode was increased, in particular when stimulation level on E12 reached 40, 70, and 100% EDR. Compound eABRs can provide a detailed individual quantification of electrical interactions occurring at specific electrodes along the CI electrode array. This approach allows a fine determination of interactions at the single electrode level potentially informing audiological decisions regarding mapping of CI systems.

The 100 Most-Cited Manuscripts in Hearing Implants: A Bibliometrics Analysis

The aim of the study was to characterise the most frequently cited articles on the topic of hearing implants. A systematic search was carried out using the Thomson Reuters Web of Science Core Collection database. Eligibility criteria restricted the results to primary studies and reviews published from 1970 to 2022 in English dealing primarily with hearing implants. Data including the authors, year of publication, journal, country of origin, number of citations and average number of citations per year were extracted, as well as the impact factors and five-year impact factor of journals publishing the articles. The top 100 papers were published across 23 journals and were cited 23,139 times. The most-cited and influential article describes the first use of the continuous interleaved sampling (CIS) strategy utilised in all modern cochlear implants. More than half of the studies on the list were produced by authors from the United States, and the Ear and Hearing journal had both the greatest number of articles and the greatest number of total citations. To conclude, this research serves as a guide to the most influential articles on the topic of hearing implants, although bibliometric analyses mainly focus on citations. The most-cited article was an influential description of CIS.

Speech Recognition Performance Differences Between Precurved and Straight Electrode Arrays From a Single Manufacturer

OBJECTIVE

Precurved cochlear implant (CI) electrode arrays have demonstrated superior audiometric outcomes compared with straight electrodes in a handful of studies. However, previous comparisons have often failed to account for preoperative hearing and age. This study compares hearing outcomes for precurved and straight electrodes by a single manufacturer while controlling for these and other factors in a large cohort.

STUDY DESIGN

Retrospective cohort study.

SETTING

Tertiary academic medical center.

PATIENTS

Two hundred thirty-one adult CI recipients between 2015 and 2021 with cochlear (Sydney, Australia) 522/622 (straight) or 532/632 (precurved) electrode arrays.

INTERVENTIONS

Postactivation speech recognition and audiometric testing.

MAIN OUTCOME MEASURES

Speech recognition testing (consonant-nucleus-consonant word [CNCw] and AzBio) was collected at 6 and 12 months postactivation. Hearing preservation was characterized by a low-frequency pure-tone average shift, or the change between preoperative and postoperative low-frequency pure-tone average.

RESULTS

Two hundred thirty-one patients (253 ears) with 6-month and/or 12-month CNCw or AzBio testing were included. One hundred forty-nine (59%) and 104 (41%) ears were implanted with straight and precurved electrode arrays, respectively. Average age at implantation was 70 years (interquartile range [IQR], 58-77 y). There was no significant difference in mean age between groups. CNCw scores were significantly different ( p = 0.001) between straight (51%; IQR, 36-67%) and precurved arrays (64%; IQR, 48-72%). AzBio scores were not significantly different ( p = 0.081) between straight (72%; IQR, 51-87%) and precurved arrays (81%; IQR, 57-90%). Controlling for age, race, sex, preoperative hearing, and follow-up time, precurved electrode arrays performed significantly better on CNCw (b = 10.0; 95% confidence interval, 4.2-16.0; p < 0.001) and AzBio (b = 8.9; 95% confidence interval, 1.8-16.0;, p = 0.014) testing. Hearing preservation was not different between electrodes on adjusted models.

CONCLUSION

During the study period, patients undergoing placement of precurved electrode arrays had significantly higher CNC and AzBio scores than patients receiving straight electrodes, even after controlling for age, preoperative hearing, and follow-up time.

PROFESSIONAL PRACTICE GAP AND EDUCATIONAL NEED

Understanding the difference in audiometric outcomes between precurved and straight electrode arrays will help to guide electrode selection.

LEARNING OBJECTIVE

To understand differences in speech recognition scores postoperatively by electrode array type (precurved versus straight).

DESIRED RESULT

To demonstrate a difference in hearing performance postoperatively by electrode type.

LEVEL OF EVIDENCE

III.

INDICATE IRB OR IACUC

Approved by the Institutional IRB (090155).

Self-assessment of cochlear health by cochlear implant recipients

Recent technological advances in cochlear implant (CI) telemetry have enabled, for the first time, CI users to perform cochlear health (CH) measurements through self-assessment for prolonged periods of time. This is important to better understand the influence of CH on CI outcomes, and to assess the safety and efficacy of future novel treatments for deafness that will be administered as adjunctive therapies to cochlear implantation. We evaluated the feasibility of using a CI to assess CH and examined patterns of electrode impedances, electrically-evoked compound action potentials (eCAPs) and electrocochleography (ECochGs), over time, in a group of adult CI recipients. Fifteen subjects were trained to use the Active Insertion Monitoring tablet by Advanced Bionics, at home for 12 weeks to independently record impedances twice daily, eCAPs once weekly and ECochGs daily in the first week, and weekly thereafter. Participants also completed behavioral hearing and speech assessments. Group level measurement compliance was 98.9% for impedances, 100% for eCAPs and 99.6% for ECochGs. Electrode impedances remained stable over time, with only minimal variation observed. Morning impedances were significantly higher than evening measurements, and impedances increased toward the base of the cochlea. eCAP thresholds were also highly repeatable, with all subjects showing 100% measurement consistency at, at least one electrode. Just over half of all subjects showed consistently absent thresholds at one or more electrodes, potentially suggesting the existence of cochlear dead regions. All subjects met UK NICE guidelines for cochlear implantation, so were expected to have little residual hearing. ECochG thresholds were, unsurprisingly, highly erratic and did not correlate with audiometric thresholds, though lower ECochG thresholds showed more repeatability over time than higher thresholds. We conclude that it is feasible for CI users to independently record CH measurements using their CI, and electrode impedances and eCAPs are promising measurements for objectively assessing CH.

Bridging the electrode-neuron gap: finite element modeling of in vitro neurotrophin gradients to optimize neuroelectronic interfaces in the inner ear

Although cochlear implant (CI) technology has allowed for the partial restoration of hearing over the last few decades, persistent challenges (e.g., poor performance in noisy environments and limited ability to decode intonation and music) remain. The "electrode-neuron gap" is inherent to these challenges and poses the most significant obstacle to advancing past the current plateau in CI performance. We propose the development of a "neuro-regenerative nexus"-a biological interface that doubly preserves native spiral ganglion neurons (SGNs) while precisely directing the growth of neurites arising from transplanted human pluripotent stem cell (hPSC)-derived otic neuronal progenitors (ONPs) toward the native SGN population. We hypothesized that the Polyhedrin Delivery System (PODS®-recombinant human brain-derived neurotrophic factor [rhBDNF]) could stably provide the adequate BDNF concentration gradient to hPSC-derived late-stage ONPs to facilitate otic neuronal differentiation and directional neurite outgrowth. To test this hypothesis, a finite element model (FEM) was constructed to simulate BDNF concentration profiles generated by PODS®-rhBDNF based on initial concentration and culture device geometry. For biological validation of the FEM, cell culture experiments assessing survival, differentiation, neurite growth direction, and synaptic connections were conducted using a multi-chamber microfluidic device. We were able to successfully generate the optimal BDNF concentration gradient to enable survival, neuronal differentiation toward SGNs, directed neurite extension of hPSC-derived SGNs, and synaptogenesis between two hPSC-derived SGN populations. This proof-of-concept study provides a step toward the next generation of CI technology. STATEMENT OF SIGNIFICANCE: Our study demonstrates that the generation of in vitro neurotrophin concentration gradients facilitates survival, neuronal differentiation toward auditory neurons, and directed neurite extension of human pluripotent stem cell-derived auditory neurons. These findings are indispensable to designing a bioactive cochlear implant, in which stem cell-derived neurons are integrated into a cochlear implant electrode strip, as the strategy will confer directional neurite growth from the transplanted cells in the inner ear. This study is the first to present the concept of a "neuro-regenerative nexus" congruent with a bioactive cochlear implant to eliminate the electrode-neuron gapthe most significant barrier to next-generation cochlear implant technology.

Recent Advances in Cochlear Implant Electrode Array Design Parameters

Cochlear implants are neural implant devices that aim to restore hearing in patients with severe sensorineural hearing impairment. Here, the main goal is to successfully place the electrode array in the cochlea to stimulate the auditory nerves through bypassing damaged hair cells. Several electrode and electrode array parameters affect the success of this technique, but, undoubtedly, the most important one is related to electrodes, which are used for nerve stimulation. In this paper, we provide a comprehensive resource on the electrodes currently being used in cochlear implant devices. Electrode materials, shape, and the effect of spacing between electrodes on the stimulation, stiffness, and flexibility of electrode-carrying arrays are discussed. The use of sensors and the electrical, mechanical, and electrochemical properties of electrode arrays are examined. A large library of preferred electrodes is reviewed, and recent progress in electrode design parameters is analyzed. Finally, the limitations and challenges of the current technology are discussed along with a proposal of future directions in the field.

Concept and first Implementation of an intracochlearly navigated Electrode Array for Cochlear Implantation

Cochlear implants (CI) are an established treatment for people with deafness or severe hearing loss. To restore patients' hearing an electrode array (EA) of the CI is inserted into the cochlea to stimulate the auditory nerve. Thereby, the exact positioning and gentle insertion of the EA is crucial for optimal hearing perception outcome. Currently, only microscopic vision is available for entering the cochlea, but the critical intracochlear process during EA insertion is like a "black box" and the surgeon has to rely on haptic feedback. Methods for visualizing the insertion process during surgery are inaccurate or not suitable for routine use due to radiation exposure. To address this problem, we developed a computer-assisted and image-guided cochlear implantation system with an exact real-time visualization of the EA position during the insertion process. The system is based on an electromagnetic tracking system that measures the position and orientation of a sensor integrated into the tip of a EA prototype and visualizes it in presurgical image data. A first experiment with our system showed that a EA prototype could be inserted into a cochlea of a human temporal bone and placed with an accuracy of [Formula: see text]. A maximum insertion angle of 120° was achieved.

Medical-Grade Silicone Rubber-Hydrogel-Composites for Modiolar Hugging Cochlear Implants

The gold standard for the partial restoration of sensorineural hearing loss is cochlear implant surgery, which restores patients’ speech comprehension. The remaining limitations, e.g., music perception, are partly due to a gap between cochlear implant electrodes and the auditory nerve cells in the modiolus of the inner ear. Reducing this gap will most likely lead to improved cochlear implant performance. To achieve this, a bending or curling mechanism in the electrode array is discussed. We propose a silicone rubber–hydrogel actuator where the hydrogel forms a percolating network in the dorsal silicone rubber compartment of the electrode array to exert bending forces at low volume swelling ratios. A material study of suitable polymers (medical-grade PDMS and hydrogels), including parametrized bending curvature measurements, is presented. The curvature radii measured meet the anatomical needs for positioning electrodes very closely to the modiolus. Besides stage-one biocompatibility according to ISO 10993-5, we also developed and validated a simplified mathematical model for designing hydrogel-actuated CI with modiolar hugging functionality.

Variations in microanatomy of the human modiolus require individualized cochlear implantation

Cochlear variability is of key importance for the clinical use of cochlear implants, the most successful neuroprosthetic device that is surgically placed into the cochlear scala tympani. Despite extensive literature on human cochlear variability, few information is available on the variability of the modiolar wall. In the present study, we analyzed 108 corrosion casts, 95 clinical cone beam computer tomographies (CTs) and 15 µCTs of human cochleae and observed modiolar variability of similar and larger extent than the lateral wall variability. Lateral wall measures correlated with modiolar wall measures significantly. ~ 49% of the variability had a common cause. Based on these data we developed a model of the modiolar wall variations and related the model to the design of cochlear implants aimed for perimodiolar locations. The data demonstrate that both the insertion limits relevant for lateral wall damage (approximate range of 4–9 mm) as well as the dimensions required for optimal perimodiolar placement of the electrode (the point of release from the straightener; approximate range of 2–5mm) are highly interindividually variable. The data demonstrate that tip fold-overs of preformed implants likely result from the morphology of the modiolus (with radius changing from base to apex), and that optimal cochlear implantation of perimodiolar arrays cannot be guaranteed without an individualized surgical technique.

Electrode Spacing and Current Distribution in Electrical Stimulation of Peripheral Nerve: A Computational Modeling Study using Realistic Nerve Models

Electrical stimulation of peripheral nerves has long been used and proven effective in restoring function caused by disease or injury. Accurate placement of electrodes is often critical to properly excite the nerve and yield the desired outcome. Computational modeling is becoming an important tool that can guide the rapid development and optimization of such implantable neural stimulation devices. Here, we developed a heterogeneous very high-resolution computational model of a realistic peripheral nerve stimulated by a current source through cuff electrodes. We then calculated the current distribution inside the nerve and investigated the effect of electrodes spacing on current penetration. In the present study, we first describe model implementation and calibration; we then detail the methodology we use to calculate current distribution and apply it to characterize the effect of electrodes distance on current penetration. Our computational results indicate that when the source and return cuff electrodes are placed close to each other, the penetration depth in the nerve is shallower than the cases in which the electrode distance is larger. This study outlines the utility of the proposed computational methods and anatomically correct high-resolution models in guiding and optimizing experimental nerve stimulation protocols.

Tight modiolar proximity and feasibility of slim modiolar cochlear implant electrode array insertion in diverse etiologies of hearing loss

PURPOSE

To report on our experience with the slim modiolar electrode (SME) especially focusing on the wide range of etiologies including inner ear anomalies, tumors, ossifications, and even revision surgeries.

METHODS

All the cochlear implantation cases performed from June 2018 to September 2019 by a single surgeon was prospectively recruited. The molecular/radiological etiology of hearing loss, intraoperative outcomes, and radiographic studies of cases where the SME was implanted was reviewed to evaluate compatibility of SME for the wide range of etiologies. For cases where SME replaced the other electrode as a revision, audiologic assessment was also made.

RESULTS

Among the 99 ears implanted during the study period, the SME was successfully implanted in 86 ears. These SME cases comprised inner ear anomaly/cochear nerve deficiency (n = 21) including cochlear hypoplasia type IV with the modiolus, intracochlear schwannoma (n = 1), far advanced otosclerosis (n = 1) and 7 revision cases. The SME was successfully used in 7 revision surgeries to replace the existing electrode. Shorter spiral diameter and decreased intracochlear position index for SME was found compared with their previous electrodes. Four out of the 6 patients who received revision implantation showed better speech perception after their surgeries.

CONCLUSION

The SME can be implanted in any cases unless the integrity of the modiolus is totally compromised. Due to its slim design and tight modiolar-hugging feature, good functional outcome can also be anticipated. Additionally, it is suitable for revision surgeries possibly allowing better hearing outcomes which may be attributed to its closer proximity to the modiolus.

3D printed biomimetic cochleae and machine learning co-modelling provides clinical informatics for cochlear implant patients

Cochlear implants restore hearing in patients with severe to profound deafness by delivering electrical stimuli inside the cochlea. Understanding stimulus current spread, and how it correlates to patient-dependent factors, is hampered by the poor accessibility of the inner ear and by the lack of clinically-relevant in vitro, in vivo or in silico models. Here, we present 3D printing-neural network co-modelling for interpreting electric field imaging profiles of cochlear implant patients. With tuneable electro-anatomy, the 3D printed cochleae can replicate clinical scenarios of electric field imaging profiles at the off-stimuli positions. The co-modelling framework demonstrated autonomous and robust predictions of patient profiles or cochlear geometry, unfolded the electro-anatomical factors causing current spread, assisted on-demand printing for implant testing, and inferred patients' in vivo cochlear tissue resistivity (estimated mean = 6.6 kΩcm). We anticipate our framework will facilitate physical modelling and digital twin innovations for neuromodulation implants.

Analysis of Neural Interface When Using Modiolar Electrode Stimulation. Radiological Evaluation, Trans-Impedance Matrix Analysis and Effect on Listening Effort in Cochlear Implantation

Background: The proximity of the electrode to the modiolar wall may be of interest to investigate the effect of pitch discrimination. This research establishes the relation between these factors and whether perimodiolar positions may provide benefits regarding improved electrode discrimination. Methods: A prospective randomized study including 24 post-lingual deaf adults was performed. A psychoacoustic study was done by using a psychoacoustic research platform. Radiological study, and a cone-beam computed tomography was used to assess post cochlear implantation electrodes’ position. Trans-impedance matrix (TIM) analysis was performed after cochlear implant insertion in all cases, and pupillometry test was also performed. Results: 12 patients received a slim perimodiolar electrode array, and 12 patients received a straight electrode array. Although all the patients showed similar speech test results after 12 months follow-up, those implanted with a perimodiolar electrode obtained better scores in electrode discrimination test and pupillometry test, and showed more homogenous TIM patterns. Conclusions: The better positioning of the electrode array seams to provide a better hearing resolution and less listening effort trans-impedance matrix seems to be a useful tool to analyze positioning of the perimodiolar array.

Advanced Metallic and Polymeric Coatings for Neural Interfacing: Structures, Properties and Tissue Responses

Neural electrodes are essential for nerve signal recording, neurostimulation, neuroprosthetics and neuroregeneration, which are critical for the advancement of brain science and the establishment of the next-generation brain-electronic interface, central nerve system therapeutics and artificial intelligence. However, the existing neural electrodes suffer from drawbacks such as foreign body responses, low sensitivity and limited functionalities. In order to overcome the drawbacks, efforts have been made to create new constructions and configurations of neural electrodes from soft materials, but it is also more practical and economic to improve the functionalities of the existing neural electrodes via surface coatings. In this article, recently reported surface coatings for neural electrodes are carefully categorized and analyzed. The coatings are classified into different categories based on their chemical compositions, i.e., metals, metal oxides, carbons, conducting polymers and hydrogels. The characteristic microstructures, electrochemical properties and fabrication methods of the coatings are comprehensively presented, and their structure-property correlations are discussed. Special focus is given to the biocompatibilities of the coatings, including their foreign-body response, cell affinity, and long-term stability during implantation. This review article can provide useful and sophisticated insights into the functional design, material selection and structural configuration for the next-generation multifunctional coatings of neural electrodes.

Speech Recognition Outcomes in Adults With Slim Straight and Slim Modiolar Cochlear Implant Electrode Arrays

OBJECTIVE

To compare differences in audiologic outcomes between slim modiolar electrode (SME) CI532 and slim lateral wall electrode (SLW) CI522 cochlear implant recipients.

STUDY DESIGN

Retrospective cohort study.

SETTING

Tertiary academic hospital.

METHODS

Comparison of postoperative AzBio sentence scores in quiet (percentage correct) in adult cochlear implant recipients with SME or SLW matched for preoperative AzBio sentence scores in quiet and aided and unaided pure tone average.

RESULTS

Patients with SLW (n = 52) and patients with SME (n = 37) had a similar mean (SD) age (62.0 [18.2] vs 62.6 [14.6] years, respectively), mean preoperative aided pure tone average (55.9 [20.4] vs 58.1 [16.4] dB; P = .59), and mean AzBio score (percentage correct, 11.1% [13.3%] vs 8.0% [11.5%]; P = .25). At last follow-up (SLW vs SME, 9.0 [2.9] vs 9.9 [2.6] months), postoperative mean AzBio scores in quiet were not significantly different (percentage correct, 70.8% [21.3%] vs 65.6% [24.5%]; P = .29), and data log usage was similar (12.9 [4.0] vs 11.3 [4.1] hours; P = .07). In patients with preoperative AzBio <10% correct, the 6-month mean AzBio scores were significantly better with SLW than SME (percentage correct, 70.6% [22.9%] vs 53.9% [30.3%]; P = .02). The intraoperative tip rollover rate was 8% for SME and 0% for SLW.

CONCLUSIONS

Cochlear implantation with SLW and SME provides comparable improvement in audiologic functioning. SME does not exhibit superior speech recognition outcomes when compared with SLW.

Identifying Cochlear Implant Channels With Relatively Poor Electrode-Neuron Interfaces Using the Electrically Evoked Compound Action Potential

OBJECTIVES

The primary objective of this study was to quantify local (within ear) and global (between ear) variation in the cochlear implant (CI) electrode-neuron interface (ENI) using the electrically evoked compound action potential (ECAP). We tested the hypothesis that, within an ear, ECAP measures can be used to identify channels with presumed good and poor ENIs, which may be influenced by a combination of spiral ganglion neuron (SGN) density, electrode position, and cochlear resistivity. We also hypothesized that ECAP responses would reflect age-related differences in the global quality of the ENI between younger and older listeners who theoretically differ in SGN density.

DESIGN

Data were obtained from 18 implanted ears (13 individuals) with Advanced Bionics HiRes 90K devices. Six participants (8 ears) were adolescents or young adults (age range: 14-32 years), and 7 participants (10 ears) were older adults (age range: 54-88 years). In each ear, single-channel auditory detection thresholds were measured on channels 2 through 15 in response to a spatially focused electrode configuration (steered quadrupolar; focusing coefficient = 0.9). ECAP amplitudes, amplitude growth function (AGF) slopes, and thresholds were assessed on a subset of channels in each ear in response to three interphase gaps (0, 7, and 30 µs). ECAP peak amplitudes were assessed on all channels between 2 and 15. AGFs and ECAP thresholds were measured on the two nonadjacent channels with the lowest and highest focused behavioral thresholds in each ear. ECAP responses were compared across low- and high-threshold channels and between younger and older CI listeners.

RESULTS

Channels that were estimated to interface poorly with the auditory nerve (i.e., high-focused-threshold channels) had steeper ECAP AGF slopes, smaller dynamic ranges, and higher ECAP thresholds than channels with low focused thresholds. Younger listeners had steeper ECAP AGF slopes and larger ECAP peak amplitudes than older listeners. Moreover, younger listeners showed greater interphase gap sensitivity for ECAP amplitude than older listeners.

CONCLUSIONS

ECAP responses may be used to quantify both local (within ear) and global (between ear) variation in the quality of the ENI. Results of this study support future investigation into the use of ECAP responses in site-selection CI programming strategies. The present results also support a growing body of evidence suggesting that adolescents and young adults with CIs may have denser populations of functional SGNs relative to older adults. Potential differences in global SGN integrity between younger and older listeners warrant investigation of optimal CI programming interventions based on their divergent hearing histories.

Using the electrically-evoked compound action potential (ECAP) interphase gap effect to select electrode stimulation sites in cochlear implant users

Studies in cochlear implanted animals show that the IPG Effect for ECAP growth functions (i.e., the magnitude of the change in ECAP amplitude growth function (AGF) slope or peak amplitude when the interphase gap (IPG) is increased) can be used to estimate the densities of spiral ganglion neurons (SGNs) near the electrode stimulation and recording sites. In humans, the same ECAP IPG Effect measures correlate with speech recognition performance. The present study examined the efficacy of selecting electrode sites for stimulation based on the IPG Effect, in order to improve performance of CI users on speech recognition tasks. We measured the ECAP IPG Effect for peak amplitude in adult (>18 years old) CI users (N= 18 ears), and created experimental programs to stimulate electrodes with either the highest or lowest ECAP IPG Effect for peak amplitude. Subjects also listened to a program without any electrodes deactivated. In a subset of subject ears (11/18), we compared performance differences between the experimental programs to post-operative computerized tomography (CT) scans to examine underlying factors that might contribute to the efficacy of an electrode site-selection approach. For sentences-in-noise, average performance was better when subjects listened to the experimental program that stimulated electrodes with the highest rather than the lowest IPG Effect for ECAP peak amplitude. A similar pattern was noted for transmission and perception of consonant place cues in a consonant recognition task. However, on average, performance when listening to a program with higher IPG Effect values was equal to that when listening with all electrodes activated. Results also suggest that scalar location (scala tympani or vestibuli) should be considered when using an ECAP-based electrode site-selection procedure to optimize CI performance.

Audiologic Outcomes of Cochlear Implantation in Cochlear Malformations: A Comparative Analysis of Lateral Wall and Perimodiolar Electrode Arrays

OBJECTIVE

Cochlear implantation in children with inner ear malformations has been shown to be beneficial. The aims of this study are to evaluate open set word recognition outcomes among children with cochlear implants who have cochlear malformations, and to further assess if either the lateral wall (LW) or perimodiolar (PM) electrode arrays confer any performance outcome advantages.

STUDY DESIGN

Retrospective case series.

SETTING

Tertiary referral center.

PATIENTS

Pediatric cochlear implant recipients with cochlear malformations who were implanted at our institution within the last 10 years and had speech perception scores were eligible for inclusion in the study. Potential participants were excluded if they had less than 1 year of listening experience with the cochlear implant or suspected cochlear nerve deficiency.

INTERVENTION

None.

MAIN OUTCOME MEASURE

Most recent consonant-nucleus-consonant word score.

RESULTS

ANOVA analysis demonstrated that the type of cochlear malformation was significantly associated with speech perception outcome (p = 0.006). Those with IP2 malformations had significantly better word recognition outcomes than the remaining cochlear malformations. Array type (LW or PM) was not associated with better word recognition outcomes in long-term follow-up of patients with IP2 malformations (p = 0.13).

CONCLUSIONS

In children who have cochlear malformations, cochlear implantation results in varying word recognition outcomes based on the type of malformation. While the participants in this study demonstrated postoperative open set word recognition skills, those with IP2 malformations demonstrated the most benefit. Electrode type was not found to significantly impact outcomes in this cohort.

Relations Between Scalar Shift and Insertion Depth in Human Cochlear Implantation

OBJECTIVE

The intracochlear position of an electrode array may influence the outcome after cochlear implantation. The design of the electrode array can increase the risk of trauma causing penetration of the basilar membrane or shift of the electrode array into the scala vestibuli. The aim of the present study was to identify a scalar shift after implantation of two different electrode arrays developed by one manufacturer.

STUDY DESIGN

Retrospective analysis.

SETTING

Tertiary referral center.

PATIENTS AND INTERVENTION

Cochlear implant recipients implanted between 2010 and 2014 and receiving either a mid-scala (n = 30) or a perimodiolar (n = 30) electrode array.

MAIN OUTCOME MEASURE

Occurrence of scalar shift in association with the electrode type.

RESULTS

Scalar shift occurred in 26.7% (8 of 30) of the patients implanted with a perimodiolar electrode array and in 6.7% (2 of 30) of the patients implanted with the mid-scala electrode array. The mean insertion depth in the patients experiencing scalar shift after implantation of the mid-scala electrode was much deeper (21.59 ± 0.34 mm) when compared with the mean insertion depth of the patients with scalar shift after implantation with a perimodiolar electrode array (17.85 ± 2.19 mm). There tends to be a correlation between the cochlear length and the occurrence of a scalar shift. However, the number of patients with scalar shift in the mid-scala group is rather small.

CONCLUSION

Based on the presented data, more patients implanted with a perimodiolar electrode array have a scalar shift when compared with the midscalar electrode array.

How I do it: Proximal cochlear implant electrode fixation using Ned's Knot

OBJECTIVES

Electrode extrusion is an under-recognized complication of cochlear implants, especially in those with straight electrodes.

METHODS

This paper describes in details the steps to perform proximal fixation of an electrode around the incus buttress using Ned's knot technique. Written and video illustration is included.

CONCLUSIONS

Ned's Knot is an easy technique that can help diminish the extrusion rate of straight cochlear implants electrodes.

Radiohistologic Comparison Study of Temporal Bone Specimens After Cochlear Implant Electrode Array Insertion: Is Cone-Beam CT Superior to MDCT?

OBJECTIVE. The purpose of this article is to evaluate subjective image quality and diagnostic accuracy to determine cochlear implant (CI) electrode position in a temporal bone (TB) specimen on cone-beam CT (CBCT) versus MDCT. MATERIALS AND METHODS. In this retrospective study, two radiologists independently reviewed CBCT (96-kV and 120-kV settings) and MDCT images of 20 TB specimens after electrode implantation. Qualitative evaluation of bone structures of the otic capsule, inner and outer cochlear wall, osseous spiral lamina, electrode position relative to the osseous spiral lamina, visualization of single electrode contacts on the array, metal artifacts, and overall image quality was performed using a five-point scale. Intracochlear electrode position was subsequently correlated with histologic examination. RESULTS. Radiologic assessment of bone structures of the otic capsule, the cochlear wall (except the outer part), osseous spiral lamina, electrode position, visualization of single electrode contacts on the array, metal artifacts, and overall image quality were significantly higher in CBCT compared with MDCT (maximum p = .04). No significant differences were found between CBCT at 96 kV and 120 kV (minimum p = .21). The intracochlear electrode position with histologic correlation was correctly diagnosed in 100% and 97.5% of specimens on 120-kV and 96-kV CBCT, respectively, whereas 77.5% were correctly assessed using MDCT. CONCLUSION. The data suggest that CBCT shows a higher diagnostic accuracy in TB specimen imaging after CI compared with MDCT, in particular to determine the intracochlear localization of the implant.

Electrophysiological effects of slim straight intracochlear electrode position

OBJECTIVE

The electrical current distribution of a cochlear implant electrode within the cochlea is essential for post-operative hearing performance. The slim straight electrode is designed to enable the placement of contacts in a lateral or medial direction to the modiolus. The electrophysiological effect of this different contact direction is so far unknown. The aim of this study was to determine the influence of intracochlear laterally or medially directed electrode contacts on electrophysiological behaviour.

METHOD

A slim straight electrode was inserted into the cochleae of five patients, and the neural response threshold was measured in a laterally and medially directed contact position. The cochleae in five temporal bone specimens were de-capped allowing an insertional observation of the contact position (lateral versus medial) of the electrode.

RESULTS

There was no difference in neural response threshold between a lateral and a medial position of the contacts. Temporal bone study indicated no intracochlear torsion of the electrode.

CONCLUSION

Our study provides evidence that the intracochlear position of slim straight electrode contacts does not affect the neural response threshold.

Dendritic Degeneration of Human Auditory Nerve Fibers and Its Impact on the Spiking Pattern Under Regular Conditions and During Cochlear Implant Stimulation

Due to limitations of human in vivo studies, detailed computational models enable understanding the neural signaling in the degenerated auditory system and cochlear implants (CIs). Four human cochleae were used to quantify hearing levels depending on dendritic changes in diameter and myelination thickness from type I of the auditory nerve fibers (ANFs). Type I neurons transmit the auditory information as spiking pattern from the inner hair cells (IHCs) to the cochlear nucleus. The impact of dendrite diameter and degree of myelination on neural signal transmission was simulated for (1) synaptic excitation via IHCs and (2) stimulation from CI electrodes. An accurate three-dimensional human cochlear geometry, along with 30 auditory pathways, mimicked the CI environment. The excitation properties of electrical potential distribution induced by two CI were analyzed. Main findings: (1) The unimodal distribution of control dendrite diameters becomes multimodal for hearing loss cases; a group of thin dendrites with diameters between 0.3 and 1 μm with a peak at 0.5 μm appeared. (2) Postsynaptic currents from IHCs excite such thin dendrites easier and earlier than under control conditions. However, this advantage is lost as their conduction velocity decreases proportionally with the diameter and causes increased spike latency and jitter in soma and axon. Firing probability reduces through the soma passage due to the low intracellular current flow in thin dendrites during spiking. (3) Compared with dendrite diameter, variations in myelin thickness have a small impact on spiking performance. (4) Contrary to synaptic excitation, CIs cause several spike initiation sites in dendrite, soma region, and axon; moreover, fiber excitability reduces with fiber diameter. In a few cases, where weak stimuli elicit spikes of a target neuron (TN) in the axon, dendrite diameter reduction has no effect. However, in many cases, a spike in a TN is first initiated in the dendrite, and consequently, dendrite degeneration demands an increase in threshold currents. (5) Threshold currents of a TN and co-stimulation of degenerated ANFs in other frequency regions depend on the electrode position, including its distance to the outer wall, the cochlear turn, and the three-dimensional pathway of the TN.

Evaluating and Comparing Behavioural and Electrophysiological Estimates of Neural Health in Cochlear Implant Users

Variations in neural health along the cochlea can degrade the spectral and temporal representation of sounds conveyed by cochlear implants (CIs). We evaluated and compared one electrophysiological measure and two behavioural measures that have been proposed as estimates of neural health patterns, in order to explore the extent to which the different measures provide converging and consistent neural health estimates. All measures were obtained from the same 11 users of the Cochlear Corporation CI. The two behavioural measures were multipulse integration (MPI) and the polarity effect (PE), both measured on each of seven electrodes per subject. MPI was measured as the difference between thresholds at 80 pps and 1000 pps, and PE as the difference in thresholds between cathodic- and anodic-centred quadraphasic (QP) 80-pps pulse trains. It has been proposed that good neural health corresponds to a large MPI and to a large negative PE (lower thresholds for cathodic than anodic pulses). The electrophysiological measure was the effect of interphase gap (IPG) on the offset of the ECAP amplitude growth function (AGF), which has been correlated with spiral ganglion neuron density in guinea pigs. This 'IPG offset' was obtained on the same subset of electrodes used for the behavioural measures. Despite high test-retest reliability, there were no significant correlations between the neural health estimates for either within-subject comparisons across the electrode array, or between-subject comparisons of the means. A phenomenological model of a population of spiral ganglion neurons was then used to investigate physiological mechanisms that might underlie the different neural health estimates. The combined experimental and modelling results provide evidence that PE, MPI and IPG offset may reflect different characteristics of the electrode-neural interface.

Radiological evaluation of a new straight electrode array compared to its precursors

Objective

The aim of this study is to examine electrode array coverage, scalar position and dislocation rate in straight electrode arrays with special focus on a new electrode array with 26 mm in lengths.

Study design

Retrospective study.

Setting

Tertiary academic center.

Patients

201 ears implanted between 2013 and 2019.

Main outcome measures

We conducted a comparative analysis of patients implanted with lateral wall electrode arrays of different lengths (F24 = MED-EL Flex²⁴, F26 = MED-EL Flex²⁶, F28 = MED-EL Flex²⁸ and F31.5 = MED-EL Flex^Soft). Cone beam computed tomography was used to determine electrode array position (scala tympani (ST) versus scala vestibuli (SV), intracochlear dislocation, position of dislocation and insertion angle).

Results

Study groups show no significant differences regarding cochlear size which excludes influences by cochlear morphology. As expected, the F24 showed significant shorter insertion angles compared to the longer electrode arrays. The F26 electrode array showed no signs of dislocation or SV insertion. The electrode array with the highest rate of ST dislocations was the F31.5 (26.3%). The electrode array with the highest rates of SV insertions was the F28 (5.75%). Most of the included electrode arrays dislocate between 320° and 360° (mean: 346.4°; range from 166° to 502°).

Conclusion

The shorter F24 and the new straight electrode array F26 show less or no signs of scalar dislocation, neither for round window nor for cochleostomy insertion than the longer F28 and the F31.5 array. As expected, the cochlear coverage is increasing with length of the electrode array itself but with growing risk for scalar dislocation and with the highest rates of dislocation for the longest electrode array F31.5. Position of intracochlear dislocation is in the apical cochlear part in the included lateral wall electrode arrays.

Level coding by phase duration and asymmetric pulse shape reduce channel interactions in cochlear implants

Conventional loudness coding with CIs by pulse current amplitude has a disadvantage: Increasing the stimulation current increases the spread of excitation in the auditory nerve, resulting in stronger channel interactions at high stimulation levels. These limit the number of effective information channels that a CI user can perceive. Stimulus intensity information (loudness) can alternatively be transmitted via pulse phase duration. We hypothesized that loudness coding by phase duration avoids the increase in the spread of the electric field and thus leads to less channel interactions at high stimulation levels. To avoid polarity effects, we combined this coding with pseudomonophasic stimuli. To test whether this affects the spread of excitation, 16 acutely deafened guinea pigs were implanted with CIs and neural activity from the inferior colliculus was recorded while stimulating with either biphasic, amplitude-coded pulses, or pseudomonophasic, duration- or amplitude-coded pulses. Pseudomonophasic stimuli combined with phase duration loudness coding reduced the lowest response thresholds and the spread of excitation. We investigated the channel interactions at suprathreshold levels by computing the phase-locking to a pulse train in the presence of an interacting pulse train on a different electrode on the CI. Pseudomonophasic pulses coupled with phase duration loudness coding reduced the interference by 4-5% compared to biphasic pulses, depending on the place of stimulation. This effect of pseudomonophasic stimuli was achieved with amplitude coding only in the basal cochlea, indicating a distance- or volume dependent effect. Our results show that pseudomonophasic, phase-duration-coded stimuli slightly reduce channel interactions, suggesting a potential benefit for speech understanding in humans.

Late electrically-evoked compound action potentials as markers for acute micro-lesions of spiral ganglion neurons

Cochlear implants (CIs) are the treatment of choice for profoundly hearing impaired people. It has been proposed that speech perception in CI users is influenced by the neural health (deafferentation, demyelination and degeneration) of the cochlea, which may be heterogeneous along an individual cochlea. Several options have been put forward to account for these local differences in neural health when fitting the speech processor settings, however with mixed results. The interpretation of the results is hampered by the fact that reliable markers of locally restricted changes in spiral ganglion neuron (SGN) health are lacking. The aim of the study was (i) to establish mechanical micro-lesions in the guinea pig as a model of heterogeneous SGN deafferentation and degeneration and (ii) to assess potential electrophysiological markers that can also be used in human subjects. First, we defined the extent of micro-lesions in normal hearing animals using acoustically-evoked compound action potentials (aCAPs); second, we measured electrically-evoked CAPs (eCAPs) before and after focal lesioning in neomycin-deafened and implanted animals. Therefore, we inserted guinea pig adjusted 6-contact CIs through a cochleostomy in the scala tympani. The eCAP was recorded from a ball electrode at the round window niche in response to monopolar or bipolar, 50 µs/phase biphasic pulses of alternating anodic- and cathodic-leading polarity. To exclude the large electrical artifact from the analysis, we focused on the late eCAP component. We systematically isolated the eCAP parameter that showed local pre- versus post-lesion changes and lesion-target specificity. Histological evaluation of the cleared cochleae revealed focal damage of an average size of 0.0036 mm³ with an apical-basal span of maximal 440 µm. We found that the threshold of the late N2P2 eCAP component was significantly elevated after lesioning when stimulating at basal (near the lesion), but not apical (distant to the lesion) CI contacts. To circumvent the potentially conflicting influence of the apical-basal gradient in eCAP thresholds, we used the polarity effect (PE=cathodic-anodic) as a relative measure. During monopolar stimulation, but not bipolar stimulation, the PE was sensitive to the lesion target and showed significantly better cathodic than anodic thresholds after soma lesions. We conclude that the difference in N2P2 thresholds in response to cathodic versus anodic-leading monopolar stimulation corresponds to the presence of SGN soma damage, and may therefore be a marker for SGN loss. We consider this electrophysiological estimate of local neural health a potentially relevant tool for human applications because of the temporal separation from the stimulation artifact and possible implementation into common eCAP measurements.

Relationships between Intrascalar Tissue, Neuron Survival, and Cochlear Implant Function

Fibrous tissue and/or new bone are often found surrounding a cochlear implant in the cochlear scalae. This new intrascalar tissue could potentially limit cochlear implant function by increasing impedance and altering signaling pathways between the implant and the auditory nerve. In this study, we investigated the relationship between intrascalar tissue and 5 measures of implant function in guinea pigs. Variation in both spiral ganglion neuron (SGN) survival and intrascalar tissue was produced by implanting hearing ears, ears deafened with neomycin, and neomycin-deafened ears treated with a neurotrophin. We found significant effects of SGN density on 4 functional measures but adding intrascalar tissue level to the analysis did not explain more variation in any measure than was explained by SGN density alone. These results suggest that effects of intrascalar tissue on electrical hearing are relatively unimportant in comparison to degeneration of the auditory nerve, although additional studies in human implant recipients are still needed to assess the effects of this tissue on complex hearing tasks like speech perception. The results also suggest that efforts to minimize the trauma that aggravates both tissue development and SGN loss could be beneficial.

Evaluation of artifacts of cochlear implant electrodes in cone beam computed tomography

Purpose

Cone Beam Computed Tomography (CBCT) offers a valid alternative to conventional Computed Tomography (CT). A possible radiation dose reduction with the use of CBCT in postoperative imaging of CIs is of great importance. Whether the visualization of Cochlear Implant (CI) electrodes in CBCT correlates with the radiation dose applied was investigated in this study.

Methods

We compared the visualization quality of Contour Advance CIs to Straight CIs from Cochlear using CBCT with varying tube parameters on whole-head specimen.

Results

The internal diameter of the cochlea decreases from base to apex, resulting in a significantly different intracochlear positioning of the two tested CI models. While electrodes of the Contour Advance series are located close to the modiolus, thus closer to the spiral ganglion neurons, those of the Straight series are located further away. The artifact portion of the electrode amounts to 50–70% of the radiological diameter of the electrode. An increase in artifact portion from the base (electrode #1 approx. 50%) to the apex (electrode #20 approx. 70%) of the cochlea was observed. The visualization of electrodes in the medial and apical part of the cochlea is limited due to artifact overlapping. There was no correlation between the artifact size and the applied radiation dose.

Conclusion

The results indicate that a reduction of the radiation dose by up to 45% of the currently applied radiation dose of standard protocols would be possible. Investigations of the effects on subjective image quality still need to be performed.

Comparison of Perimodiolar Electrodes: Imaging and Electrophysiological Outcomes

BACKGROUND

The perimodiolar CI532 Slim Modiolar electrode has been designed to bring the electrode contacts close to auditory nerve while reducing cochlear trauma during its insertion. It is currently unknown to what extent the electrode position and electrophysiological outcomes of the Slim Modiolar electrode differ from other perimodiolar electrodes.

OBJECTIVES

The objective was to compare the electrode position and electrophysiological outcomes between the CI532 Slim Modiolar and CI512 Contour Advance electrode.

METHOD

Forty-six adult patients received a Slim Modiolar or Contour Advance electrode. Electrode types were compared using intraoperative electrode impedances, evoked compound action potential (ECAP) and stapedius reflex thresholds, as well as position parameters from postoperative computed tomography or digital volume tomography images (medial-lateral position, electrode-to-modiolus distance, insertion angle).

RESULTS

The medial-lateral position indicates a closer modiolar placement of the Slim Modiolar compared with the Contour Advance. Individual electrode contact measurements, however, showed significantly larger electrode-to-modiolus distances and higher ECAP thresholds for the Slim Modiolar in the basal region. On contacts E20-22 the Slim Modiolar is slightly closer to the modiolus compared with the Contour Advance, but this did not result in lower ECAP thresholds.

CONCLUSIONS

Perimodiolar electrodes can vary in their intracochlear position, leading to divergent electrophysiological outcomes. To detect these differences, investigations must be done for each electrode contact rather than using a global factor for the whole electrode array. While the electrode dislocation rate is lower with the Slim Modiolar than with the Contour Advance, electrode-to-modiolus proximity is smaller and ECAP thresholds are lower with the Contour Advance in the basal cochlear region.

Effect of age, electrode array, and time on cochlear implant impedances

Objectives: To determine the impact of age, electrode array, and time on impedance patterns in cochlear implant (CI) patients. Methods: A retrospective case review was performed on 98 patients implanted with the CI24RE perimodiolar (PM) and CI422 lateral wall (LW) arrays between 2010 and 2014 to assess impedances at the 1 week and 3-6 month visit after initial stimulation (IS). Results: With respect to age, impedances were higher in young patients compared to older patients in the middle and apical turns. With time, there were significant reductions in impedances across most electrodes. Electrode array type also had a significant impact on impedance measurements with PM and LW arrays having higher impedances in the basal turn and apical turns, respectively. Furthermore, PM arrays demonstrated significantly lower impedances in the middle and apical turn with time, when compared to LW arrays. Conclusions: Age, electrode array, and time can independently affect CI impedances. Moreover, we show that PM arrays may be advantageous to LW arrays, due to demonstrated lower impedances in the middle and apical turns long term. Understanding the impact of impedance on speech discrimination and determining the intracochlear processes that contribute to differences in impedance are future research directions.

Electrochemical and biological characterization of thin-film platinum-iridium alloy electrode coatings: a chronic in vivo study

OBJECTIVE

To evaluate the electrochemical properties, biological response, and surface characterization of an electrodeposited Platinum-Iridium (Pt-Ir) electrode coating on cochlear implants subjected to chronic stimulation in vivo.

APPROACH

Electrochemical impedance spectroscopy (EIS), charge storage capacity (CSC), charge injection limit (CIL), and voltage transient (VT) impedance were measured bench-top before and after implant and in vivo. Coated Pt-Ir and uncoated Pt electrode arrays were implanted into cochlea of normal hearing rats and stimulated for ∼4 h d, 5 d week^-1 for 5 weeks at levels within the normal clinical range. Neural function was monitored using electrically-evoked auditory brainstem responses. After explant, the electrode surfaces were assessed, and cochleae examined histologically.

MAIN RESULTS

When measured on bench-top before and after stimulation, Pt-Ir coated electrodes had significantly lower VT impedance (p < 0.001) and significantly higher CSC (p < 0.001) and CIL (p < 0.001) compared to uncoated Pt electrodes. In vivo, the CSC and CIL of Pt-Ir were significantly higher than Pt throughout the implantation period (p= 0.047 and p< 0.001, respectively); however, the VT impedance (p= 0.3) was not. There was no difference in foreign body response between material cohorts, although cochleae implanted with coated electrodes contained small deposits of Pt-Ir. There was no evidence of increased neural loss or loss of neural function in either group. Surface examination revealed no Pt corrosion on any electrodes.

SIGNIFICANCE

Electrodeposited Pt-Ir electrodes demonstrated significant improvements in electrochemical performance on the bench-top and in vivo compared to uncoated Pt. Neural function and tissue response to Pt-Ir electrodes were not different from uncoated Pt, despite small deposits of Pt-Ir in the tissue capsule. Electrodeposited Pt-Ir coatings offer promise as an improved electrode coating for active neural prostheses.

Matched Cohort Comparison Indicates Superiority of Precurved Electrode Arrays

OBJECTIVE

Characterize differences in adult cochlear implant outcomes and programming parameters for a straight (CI422/522) and a precurved (CI532) electrode array.

SETTING

Cochlear implant (CI) program at a tertiary otologic center.

PATIENTS

Fifty-eight adults were included in the study; 29 were implanted with CI422 or CI522 and 29 were implanted with CI532. Each CI532 recipient was matched to a CI422/522 recipient in terms of age and preoperative hearing thresholds for comparison purposes.

MAIN OUTCOME MEASURES

Consonant-Nucleus-Consonant (CNC) words, AzBio sentences, residual audiometric thresholds, and Speech Spatial Qualities (SSQ) questionnaire collected 6 months postoperatively were used to characterize outcomes. Pulse duration, maxima, impedances, and overall charge measurements were used to characterize programming parameters.

RESULTS

Postoperative unaided low frequency pure-tone average (LFPTA) was significantly better for the CI532 group. CNC scores were significantly better for the CI532 group. Impedances and pulse duration were significantly lower for the CI532 group, but there was no difference in overall charge between the groups.

CONCLUSION

The CI532 group showed either similar or statistically superior results on all measures when compared with the CI422/522 suggesting that the CI532 electrode may be an advantageous substitute for the CI522.

Effects of Electrode Location on Estimates of Neural Health in Humans with Cochlear Implants

There are a number of psychophysical and electrophysiological measures that are correlated with SGN density in animal models, and these same measures can be performed in humans with cochlear implants (CIs). Thus, these measures are potentially applicable in humans for estimating the condition of the neural population (so called "neural health" or "cochlear health") at individual sites along the electrode array and possibly adjusting the stimulation strategy in the CI sound processor accordingly. Some measures used to estimate neural health in animals have included the electrically evoked compound potential (ECAP), psychophysical detection thresholds, and multipulse integration (MPI). With regard to ECAP measures, it has been shown that the change in the ECAP response as a function of increasing the stimulus interphase gap ("IPG Effect") also reflects neural density in implanted animals. These animal studies have typically been conducted using preparations in which the electrode was in a fixed position with respect to the neural population, whereas in human cochlear implant users, the position of individual electrodes varies widely within an electrode array and also across subjects. The current study evaluated the effects of electrode location in the implanted cochlea (specifically medial-lateral location) on various electrophysiological and psychophysical measures in eleven human subjects. The results demonstrated that some measures of interest, specifically ECAP thresholds, psychophysical detection thresholds, and ECAP amplitude-growth function (AGF) linear slope, were significantly related to the distances between the electrode and mid-modiolar axis (MMA). These same measures were less strongly related or not significantly related to the electrode to medial wall (MW) distance. In contrast, neither the IPG Effect for the ECAP AGF slope or threshold, nor the MPI slopes were significantly related to MMA or MW distance from the electrodes. These results suggest that "within-channel" estimates of neural health such as the IPG Effect and MPI slope might be more suitable for estimating nerve condition in humans for clinical application since they appear to be relatively independent of electrode position.

First Experience With a New Thin Lateral Wall Electrode in Human Temporal Bones

INTRODUCTION

A modern cochlear implant electrode array design must combine: improved surgical ease of use, structure preservation, particularly important for pediatric application, stable position within the cochlea over time, and a meaningful balance between hearing preservation against addressing sufficient cochlear tissue to support electrical-only hearing. The aim of this study was to investigate a new lateral wall electrode array design from Advanced Bionics on human temporal bones (TBs).

METHODS

Ten fresh-frozen TBs were implanted with the SlimJ electrode array via the round window. The electrode array is 23 mm long, with a cross-section varying from 0.25 × 0.55 mm at the most apical contact to 0.6 × 0.8 mm at the proximal marker contact. To assess location of the electrode array, the TBs were postoperatively scanned using cone beam computed tomography, and histology was performed to assess intracochlear trauma (Grades 0-4).

RESULTS

All electrode arrays were considered easy to insert. The average insertion depth was 432 degrees measured from the round window with a range from 411 to 450 degrees azimuth. Nine out of 10 electrode arrays were inserted fully (<0.5 mm out of the cochlea), one electrode array was left 1.5 mm out of the cochlea. No translocations were observed in all 10 cochleae, slight touching of the basilar membrane at the distal portion of the array was observed in 50% of the cases.

CONCLUSION

The results from the new thin lateral wall electrode array from Advanced Bionics provided consistent scala tympani locations. No translocations were observed and almost all electrode arrays were fully inserted. These results are promising and the new electrode array will be further studied in clinical practice investigating hearing preservation capabilities and speech performance.

Ramped pulse shapes are more efficient for cochlear implant stimulation in an animal model

In all commercial cochlear implant (CI) devices, the electric stimulation is performed with a rectangular pulse that generally has two phases of opposite polarity. To date, developing new stimulation strategies has relied on the efficacy of this shape. Here, we investigate the potential of a novel stimulation paradigm that uses biophysically-inspired electrical ramped pulses. Using electrically-evoked auditory brainstem response (eABR) recordings in mice, we found that less charge, but higher current level amplitude, is needed to evoke responses with ramped shapes that are similar in amplitude to responses obtained with rectangular shapes. The most charge-efficient pulse shape had a rising ramp over both phases, supporting findings from previous in vitro studies. This was also true for longer phase durations. Our study presents the first physiological data on CI-stimulation with ramped pulse shapes. By reducing charge consumption ramped pulses have the potential to produce more battery-efficient CIs and may open new perspectives for designing other efficient neural implants in the future.

Ruthenium oxide based microelectrode arrays for in vitro and in vivo neural recording and stimulation

We have characterized the in vitro and in vivo extracellular neural recording and stimulation properties of ruthenium oxide (RuOx) based microelectrodes. Cytotoxicity and functional neurotoxicity assays were carried out to confirm the in vitro biocompatibility of RuOx. Material extract assays, in accordance to ISO protocol "10993-5: Biological evaluation of medical devices", revealed no significant effect on neuronal cell viability or the functional activity of cortical networks. In vitro microelectrode arrays (MEAs), with indium tin oxide (ITO) sites modified with sputtered iridium oxide (IrOx) and RuOx in a single array, were developed for a direct comparison of electrochemical and recording performance of RuOx to ITO and IrOx deposited microelectrode sites. The impedance of the RuOx-coated electrodes measured by electrochemical impedance spectroscopy was notably lower than that of ITO electrodes, resulting in robust extracellular recordings from cortical networks in vitro. We found comparable signal-to-noise ratios (SNRs) for RuOx and IrOx, both significantly higher than the SNR for ITO. RuOx-based MEAs were also fabricated and implanted in the rat motor cortex to demonstrate manufacturability of the RuOx processing and acute recording capabilities in vivo. We observed single-unit extracellular action potentials with a SNR >22, representing a first step for neurophysiological recordings in vivo with RuOx based microelectrodes. STATEMENT OF SIGNIFICANCE: A critical challenge in neural interface technology is the development of microelectrodes that have recording and electrical stimulation capabilities suitable for bidirectional communication between the external electronic device and the nervous system. The present study explores the feasibility and functional capabilities of ruthenium oxide microelectrodes as a neural interface. Significant improvement in electrochemical properties and neuronal recordings are reported when compared to commercially available indium tin oxide and was similar to that of iridium oxide electrodes. The data demonstrate the potential for future development of chronic neural interfaces using ruthenium oxide based microelectrodes for recording and stimulation.

Cochlear Implantation with the CI512 and CI532 Precurved Electrode Arrays: One-Year Speech Recognition and Intraoperative Thresholds of Electrically Evoked Compound Action Potentials

INTRODUCTION

Precurved cochlear implant (CI) electrode arrays were developed in an attempt to improve the auditory outcome of cochlear implantation, which varies greatly. The recent CI532 (Cochlear Corp., Sydney, Australia) may offer further advantages as its electrode array is thinner than previous precurved CI electrode arrays. The aims here were to investigate 1-year postoperative speech recognition, intraoperative electrically evoked compound action potentials (ECAPs), and their possible relation in patients implanted with a CI532 or its predecessor CI512.

METHODS

A retrospective analysis of data from 63 patients subjected to cochlear implantation at the Karolinska University Hospital, Sweden, was performed. Speech recognition of the implanted ear was evaluated using phonemically balanced monosyllabic Swedish words at 65 dB SPL. ECAPs were evaluated using the intraoperative ECAP threshold across ≥8 electrodes generated by the automated neural response telemetry of the CI.

RESULTS

The median aided speech recognition score (SRS) 1 year after implantation was 52% (quartile 1 = 40%, quartile 3 = 60%, n = 63) and did not differ statistically significantly between patients with CI512 (n = 38) and CI532 (n = 25). The mean ECAP threshold was 188 CL (current level; SD = 15 CL, n = 54) intraoperatively and did not differ statistically significantly between patients with CI512 (n = 32) and CI532 (n = 22), but the threshold for each electrode varied more between patients with a CI512 (p < 0.0001). A higher mean ECAP threshold was associated with a worse SRS (Spearman's ρ = -0.46, p = 0.0004, n = 54). The association remained among those with a CI512 (Spearman's ρ = -0.62, p = 0.0001, n = 32) when stratified by CI electrode array.

CONCLUSION

No statistically significant difference in speech recognition 1 year after cochlear implantation or in mean threshold of ECAP intraoperatively was found between patients with a CI512 and the more recent, slim CI532, but the ECAP thresholds varied more between those with a CI512. A statistically significant association between SRS and mean ECAP threshold was found, but stratified analysis suggests that the association may be true only for patients with a CI512.

The Insertion Results of a Mid-scala Electrode Assessed by MRI and CBCT Image Fusion

OBJECTIVES

To investigate the results of clinical surgical insertions with a Mid-scala array (HIFocus Mid-Scala Electrode, HFms).

STUDY DESIGN

Consecutive retrospective case study.

SETTINGS

Tertiary referral center.

PATIENTS

Analyses of imaging data of 26 consecutive patients (31 insertions) implanted with the HFms.

INTERVENTION (S)

The evaluation of insertion trauma evoked by a previously validated image fusion technique. Electrode reconstructions from postoperative cone-beam computed tomography (CBCT) were overlaid onto preoperative magnetic resonance imaging (MRI) scans to create artifact-free images.

MAIN OUTCOME MEASURES

The electrode position was quantified in relation to the basilar membrane. Trauma scaling adopted from Eshraghi was used for evaluating insertion trauma. The results of the visual assessment of the postoperative CBCT were compared to those obtained with the fusion technique.

RESULTS

Three insertions had to be excluded due to incompatibility of the imaging data with the fusion software. We found consistent peri- to mid-modiolar placement of the HFms with a mean insertion depth angle of 376°. According to the medical records, a visual examination of the postoperative CBCT indicated that there had been no scala dislocations but when assessed by the image fusion technique, five scala dislocations (17.8%) were found. Additionally, one tip fold-over was detected in the postoperative CBCT even though this was not evident in any intraoperative measurements.

CONCLUSION

HFms showed atraumatic surgical insertion results with consistent mid-modiolar placement. Image fusion enhances the accuracy of the insertion trauma assessment. Routine postoperative imaging is recommended for identifying tip fold-over as well as for quality control and documentation.