Deafness-induced changes in the auditory pathway: implications for cochlear implants

Spiral ganglion neuron degeneration in aminoglycoside-deafened rats involves innate and adaptive immune responses not requiring complement

Spiral ganglion neurons (SGNs) transmit auditory information from cochlear hair cells to the brain. SGNs are thus not only important for normal hearing, but also for effective functioning of cochlear implants, which stimulate SGNs when hair cells are missing. SGNs slowly degenerate following aminoglycoside-induced hair cell loss, a process thought to involve an immune response. However, the specific immune response pathways involved remain unknown. We used RNAseq to gain a deeper understanding immune-related and other transcriptomic changes that occur in the rat spiral ganglion after kanamycin-induced deafening. Among the immune and inflammatory genes that were selectively upregulated in deafened spiral ganglia, the complement cascade genes were prominent. We then assessed SGN survival, as well as immune cell numbers and activation, in the spiral ganglia of rats with a CRISPR-Cas9-mediated knockout of complement component 3 (C3). Similar to previous findings in our lab and other deafened rodent models, we observed an increase in macrophage number and increased expression of CD68, a marker of phagocytic activity and cell activation, in macrophages in the deafened ganglia. Moreover, we found an increase in MHCII expression on spiral ganglion macrophages and an increase in lymphocyte number in the deafened ganglia, suggestive of an adaptive immune response. However, C3 knockout did not affect SGN survival or increase in macrophage number/activation, implying that complement activation does not play a role in SGN death after deafening. Together, these data suggest that both innate and adaptive immune responses are activated in the deafened spiral ganglion, with the adaptive response directly contributing to cochlear neurodegeneration.

Peripheral Neural Synchrony in Postlingually Deafened Adult Cochlear Implant Users

OBJECTIVES

This paper reports a noninvasive method for quantifying neural synchrony in the cochlear nerve (i.e., peripheral neural synchrony) in cochlear implant (CI) users, which allows for evaluating this physiological phenomenon in human CI users for the first time in the literature. In addition, this study assessed how peripheral neural synchrony was correlated with temporal resolution acuity and speech perception outcomes measured in quiet and in noise in postlingually deafened adult CI users. It tested the hypothesis that peripheral neural synchrony was an important factor for temporal resolution acuity and speech perception outcomes in noise in postlingually deafened adult CI users.

DESIGN

Study participants included 24 postlingually deafened adult CI users with a Cochlear™ Nucleus® device. Three study participants were implanted bilaterally, and each ear was tested separately. For each of the 27 implanted ears tested in this study, 400 sweeps of the electrically evoked compound action potential (eCAP) were measured at four electrode locations across the electrode array. Peripheral neural synchrony was quantified at each electrode location using the phase-locking value (PLV), which is a measure of trial-by-trial phase coherence among eCAP sweeps/trials. Temporal resolution acuity was evaluated by measuring the within-channel gap detection threshold (GDT) using a three-alternative, forced-choice procedure in a subgroup of 20 participants (23 implanted ears). For each ear tested in these participants, GDTs were measured at two electrode locations with a large difference in PLVs. For 26 implanted ears tested in 23 participants, speech perception performance was evaluated using consonant-nucleus-consonant (CNC) word lists presented in quiet and in noise at signal to noise ratios (SNRs) of +10 and +5 dB. Linear Mixed effect Models were used to evaluate the effect of electrode location on the PLV and the effect of the PLV on GDT after controlling for the stimulation level effects. Pearson product-moment correlation tests were used to assess the correlations between PLVs, CNC word scores measured in different conditions, and the degree of noise effect on CNC word scores.

RESULTS

There was a significant effect of electrode location on the PLV after controlling for the effect of stimulation level. There was a significant effect of the PLV on GDT after controlling for the effects of stimulation level, where higher PLVs (greater synchrony) led to lower GDTs (better temporal resolution acuity). PLVs were not significantly correlated with CNC word scores measured in any listening condition or the effect of competing background noise presented at an SNR of +10 dB on CNC word scores. In contrast, there was a significant negative correlation between the PLV and the degree of noise effect on CNC word scores for a competing background noise presented at an SNR of +5 dB, where higher PLVs (greater synchrony) correlated with smaller noise effects on CNC word scores.

CONCLUSIONS

This newly developed method can be used to assess peripheral neural synchrony in CI users, a physiological phenomenon that has not been systematically evaluated in electrical hearing. Poorer peripheral neural synchrony leads to lower temporal resolution acuity and is correlated with a larger detrimental effect of competing background noise presented at an SNR of 5 dB on speech perception performance in postlingually deafened adult CI users.

Improving rehabilitation of deaf patients by advanced imaging before cochlear implantation

INTRODUCTION

Cochlear implants have advanced the management of severe to profound deafness. However, there is a strong disparity in hearing performance after implantation from one patient to another. Moreover, there are several advanced kinds of imaging assessment before cochlear implantation. Microstructural white fiber degeneration can be studied with Diffusion weighted MRI (DWI) or tractography of the central auditory pathways. Functional MRI (fMRI) allows us to evaluate brain function, and CT or MRI segmentation to better detect inner ear anomalies.

OBJECTIVE

This literature review aims to evaluate how helpful pre-implantation anatomic imaging can be to predict hearing rehabilitation outcomes in deaf patients. These techniques include DWI and fMRI of the central auditory pathways, and automated labyrinth segmentation by CT scan, cone beam CT and MRI.

DESIGN

This systematic review was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Studies were selected by searching in PubMed and by checking the reference lists of relevant articles. Inclusion criteria were adults over 18, with unilateral or bilateral hearing loss, who had DWI acquisition or fMRI or CT/ Cone Beam CT/ MRI image segmentation.

RESULTS

After reviewing 172 articles, we finally included 51. Studies on DWI showed changes in the central auditory pathways affecting the white matter, extending to the primary and non-primary auditory cortices, even in sudden and mild hearing impairment. Hearing loss patients show a reorganization of brain activity in various areas, such as the auditory and visual cortices, as well as regions involved in language and emotions, according to fMRI studies. Deep Learning's automatic segmentation produces the best CT segmentation in just a few seconds. MRI segmentation is mainly used to evaluate fluid space of the inner ear and determine the presence of an endolymphatic hydrops.

CONCLUSION

Before cochlear implantation, a DWI with tractography can evaluate the central auditory pathways up to the primary and non-primary auditory cortices. This data is then used to generate predictions on the auditory rehabilitation of patients. A CT segmentation with systematic 3D reconstruction allow a better evaluation of cochlear malformations and predictable difficulties during surgery.

Adult Neurogenesis of the Medial Geniculate Body: In Vitro and Molecular Genetic Analyses Reflect the Neural Stem Cell Capacity of the Rat Auditory Thalamus over Time

Neural stem cells (NSCs) have been recently identified in the neonatal rat medial geniculate body (MGB). NSCs are characterized by three cardinal features: mitotic self-renewal, formation of progenitors, and differentiation into all neuroectodermal cell lineages. NSCs and the molecular factors affecting them are particularly interesting, as they present a potential target for treating neurologically based hearing disorders. It is unclear whether an NSC niche exists in the rat MGB up to the adult stage and which neurogenic factors are essential during maturation. The rat MGB was examined on postnatal days 8, 12, and 16, and at the adult stadium. The cardinal features of NSCs were detected in MGB cells of all age groups examined by neurosphere, passage, and differentiation assays. In addition, real-time quantitative polymerase chain reaction arrays were used to compare the mRNA levels of 84 genes relevant to NSCs and neurogenesis. In summary, cells of the MGB display the cardinal features of NSCs up to the adult stage with a decreasing NSC potential over time. Neurogenic factors with high importance for MGB neurogenesis were identified on the mRNA level. These findings should contribute to a better understanding of MGB neurogenesis and its regenerative capacity.

Peripheral neural synchrony in post-lingually deafened adult cochlear implant users

Objective

This paper reports a noninvasive method for quantifying neural synchrony in the cochlear nerve (i.e., peripheral neural synchrony) in cochlear implant (CI) users, which allows for evaluating this physiological phenomenon in human CI users for the first time in the literature. In addition, this study assessed how peripheral neural synchrony was correlated with temporal resolution acuity and speech perception outcomes measured in quiet and in noise in post-lingually deafened adult CI users. It tested the hypothesis that peripheral neural synchrony was an important factor for temporal resolution acuity and speech perception outcomes in noise in post-lingually deafened adult CI users.

Design

Study participants included 24 post-lingually deafened adult CI users with a Cochlear™ Nucleus® device. Three study participants were implanted bilaterally, and each ear was tested separately. For each of the 27 implanted ears tested in this study, 400 sweeps of the electrically evoked compound action potential (eCAP) were measured at four electrode locations across the electrode array. Peripheral neural synchrony was quantified at each electrode location using the phase locking value (PLV), which is a measure of trial-by-trial phase coherence among eCAP sweeps/trials. Temporal resolution acuity was evaluated by measuring the within-channel gap detection threshold (GDT) using a three-alternative, forced-choice procedure in a subgroup of 20 participants (23 implanted ears). For each ear tested in these participants, GDTs were measured at two electrode locations with a large difference in PLVs. For 26 implanted ears tested in 23 participants, speech perception performance was evaluated using Consonant-Nucleus-Consonant (CNC) word lists presented in quiet and in noise at signal-to-noise ratios (SNRs) of +10 and +5 dB. Linear Mixed effect Models were used to evaluate the effect of electrode location on the PLV and the effect of the PLV on GDT after controlling for the stimulation level effects. Pearson product-moment correlation tests were used to assess the correlations between PLVs, CNC word scores measured in different conditions, and the degree of noise effect on CNC word scores.

Results

There was a significant effect of electrode location on the PLV after controlling for the effect of stimulation level. There was a significant effect of the PLV on GDT after controlling for the effects of stimulation level, where higher PLVs (greater synchrony) led to lower GDTs (better temporal resolution acuity). PLVs were not significantly correlated with CNC word scores measured in any listening condition or the effect of competing background noise presented at a SNR of +10 dB on CNC word scores. In contrast, there was a significant negative correlation between the PLV and the degree of noise effect on CNC word scores for a competing background noise presented at a SNR of +5 dB, where higher PLVs (greater synchrony) correlated with smaller noise effects on CNC word scores.

Conclusions

This newly developed method can be used to assess peripheral neural synchrony in CI users, a physiological phenomenon that has not been systematically evaluated in electrical hearing. Poorer peripheral neural synchrony leads to lower temporal resolution acuity and is correlated with a larger detrimental effect of competing background noise presented at a SNR of 5 dB on speech perception performance in post-lingually deafened adult CI users.

Review of Binaural Processing With Asymmetrical Hearing Outcomes in Patients With Bilateral Cochlear Implants

Bilateral cochlear implants (BiCIs) result in several benefits, including improvements in speech understanding in noise and sound source localization. However, the benefit bilateral implants provide among recipients varies considerably across individuals. Here we consider one of the reasons for this variability: difference in hearing function between the two ears, that is, interaural asymmetry. Thus far, investigations of interaural asymmetry have been highly specialized within various research areas. The goal of this review is to integrate these studies in one place, motivating future research in the area of interaural asymmetry. We first consider bottom-up processing, where binaural cues are represented using excitation-inhibition of signals from the left ear and right ear, varying with the location of the sound in space, and represented by the lateral superior olive in the auditory brainstem. We then consider top-down processing via predictive coding, which assumes that perception stems from expectations based on context and prior sensory experience, represented by cascading series of cortical circuits. An internal, perceptual model is maintained and updated in light of incoming sensory input. Together, we hope that this amalgamation of physiological, behavioral, and modeling studies will help bridge gaps in the field of binaural hearing and promote a clearer understanding of the implications of interaural asymmetry for future research on optimal patient interventions.

Influence of listening environment on usage patterns in cochlear implant patients with single-sided deafness

OBJECTIVE

To compare cochlear implant (CI) data logging of patients with single-sided deafness (SSD) and bilateral sensorineural hearing loss (biSNHL) in various acoustic environments and study the implications of data logging on auditory performance.

STUDY DESIGN

Retrospective case control study.

METHODS

Adult CI patients with SSD or biSNHL from 2010 to 2021 with usage data collected at 3-, 6-, and 12-months following device activation were identified. The CI listening environment was defined as speech in noise, speech in quiet, quiet, music or noise. Auditory performance was measured using the CNC word, AzBio sentence tests and the Tinnitus Handicap Index (THI).

RESULTS

60 adults with SSD or biSNHL were included. CI patients with biSNHL wore their devices more than those with SSD at 3-months post-activation (11.18 versus 8.97 hours/day, p = 0.04), though there were no significant differences at 6-12 months. Device usage was highest in the speech in quiet environment. In SSD CI users, there was a positive correlation (p = 0.03) between device use and CNC scores at 12-months and an improvement in THI scores at 12-months (p = 0.0004).

CONCLUSIONS

CI users with SSD and biSNHL have comparable duration of device usage at longer follow-up periods with greatest device usage recorded in speech in quiet environments.

The efficacy of a TrkB monoclonal antibody agonist in preserving the auditory nerve in deafened guinea pigs

The auditory nerve typically degenerates following loss of cochlear hair cells or synapses. In the case of hair cell loss neural degeneration hinders restoration of hearing through a cochlear implant, and in the case of synaptopathy suprathreshold hearing is affected, potentially degrading speech perception in noise. It has been established that neurotrophins such as brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) can mitigate auditory nerve degeneration. Several potential BDNF mimetics have also been investigated for neurotrophic effects in the cochlea. A recent in vitro study showed favorable effects of M3, a TrkB monoclonal antibody agonist, when compared with BDNF. In the present study we set out to examine the effect of M3 on auditory nerve preservation in vivo. Thirty-one guinea pigs were bilaterally deafened, and unilaterally treated with a single 3-µl dose of 7 mg/ml, 0.7 mg/ml M3 or vehicle-only by means of a small gelatin sponge two weeks later. During the experiment and analyses the experimenters were blinded to the three treatment groups. Four weeks after treatment, we assessed the treatment effect (1) histologically, by quantifying survival of SGCs and their peripheral processes (PPs); and (2) electrophysiologically, with two different paradigms of electrically evoked compound action potential (eCAP) recordings shown to be indicative of neural health: single-pulse stimulation with varying inter-phase gap (IPG), and pulse-train stimulation with varying inter-pulse interval. We observed a consistent and significant preservative effect of M3 on SGC survival in the lower basal turn (approximately 40% more survival than in the untreated contralateral cochlea), but also in the upper middle and lower apical turn of the cochlea. This effect was similar for the two treatment groups. Survival of PPs showed a trend similar to that of the SGCs, but was only significantly higher for the highest dose of M3. The protective effect of M3 on SGCs was not reflected in any of the eCAP measures: no statistically significant differences were observed between groups in IPG effect nor between the M3 treatment groups and the control group using the pulse-train stimulation paradigm. In short, while a clear effect of M3 was observed on SGC survival, this was not clearly translated into functional preservation.

Advanced Omics Techniques for Understanding Cochlear Genome, Epigenome, and Transcriptome in Health and Disease

Advanced genomics, transcriptomics, and epigenomics techniques are providing unprecedented insights into the understanding of the molecular underpinnings of the central nervous system, including the neuro-sensory cochlea of the inner ear. Here, we report for the first time a comprehensive and updated overview of the most advanced omics techniques for the study of nucleic acids and their applications in cochlear research. We describe the available in vitro and in vivo models for hearing research and the principles of genomics, transcriptomics, and epigenomics, alongside their most advanced technologies (like single-cell omics and spatial omics), which allow for the investigation of the molecular events that occur at a single-cell resolution while retaining the spatial information.

Interaural asymmetry of dynamic range: Abnormal fusion, bilateral interference, and shifts in attention

Speech information in the better ear interferes with the poorer ear in patients with bilateral cochlear implants (BiCIs) who have large asymmetries in speech intelligibility between ears. The goal of the present study was to assess how each ear impacts, and whether one dominates, speech perception using simulated CI processing in older and younger normal-hearing (ONH and YNH) listeners. Dynamic range (DR) was manipulated symmetrically or asymmetrically across spectral bands in a vocoder. We hypothesized that if abnormal integration of speech information occurs with asymmetrical speech understanding, listeners would demonstrate an atypical preference in accuracy when reporting speech presented to the better ear and fusion of speech between the ears (i.e., an increased number of one-word responses when two words were presented). Results from three speech conditions showed that: (1) When the same word was presented to both ears, speech identification accuracy decreased if one or both ears decreased in DR, but listeners usually reported hearing one word. (2) When two words with different vowels were presented to both ears, speech identification accuracy and percentage of two-word responses decreased consistently as DR decreased in one or both ears. (3) When two rhyming words (e.g., bed and led) previously shown to phonologically fuse between ears (e.g., bled) were presented, listeners instead demonstrated interference as DR decreased. The word responded in (2) and (3) came from the right (symmetric) or better (asymmetric) ear, especially in (3) and for ONH listeners in (2). These results suggest that the ear with poorer dynamic range is downweighted by the auditory system, resulting in abnormal fusion and interference, especially for older listeners.

Linking Cerebrovascular Dysfunction to Age-Related Hearing Loss and Alzheimer’s Disease—Are Systemic Approaches for Diagnosis and Therapy Required?

Alzheimer’s disease (AD), the most common cause of dementia in the elderly, is a neurodegenerative disorder associated with neurovascular dysfunction, cognitive decline, and the accumulation of amyloid β peptide (Aβ) in the brain and tau-related lesions in neurons termed neurofibrillary tangles (NFTs). Aβ deposits and NFT formation are the central pathological hallmarks in AD brains, and the majority of AD cases have been shown to exhibit a complex combination of systemic comorbidities. While AD is the foremost common cause of dementia in the elderly, age-related hearing loss (ARHL) is the most predominant sensory deficit in the elderly. During aging, chronic inflammation and resulting endothelial dysfunction have been described and might be key contributors to AD; we discuss an intriguing possible link between inner ear strial microvascular pathology and blood–brain barrier pathology and present ARHL as a potentially modifiable and treatable risk factor for AD development. We present compelling evidence that ARHL might well be seen as an important risk factor in AD development: progressive hearing impairment, leading to social isolation, and its comorbidities, such as frailty, falls, and late-onset depression, link ARHL with cognitive decline and increased risk of dementia, rendering it tempting to speculate that ARHL might be a potential common molecular and pathological trigger for AD. Additionally, one could speculate that amyloid-beta might damage the blood–labyrinth barrier as it does to the blood–brain barrier, leading to ARHL pathology. Finally, there are options for the treatment of ARHL by targeted neurotrophic factor supplementation to the cochlea to improve cognitive outcomes; they can also prevent AD development and AD-related comorbidity in the future.

Sound source localization patterns and bilateral cochlear implants: Age at onset of deafness effects

The ability to determine a sound’s location is critical in everyday life. However, sound source localization is severely compromised for patients with hearing loss who receive bilateral cochlear implants (BiCIs). Several patient factors relate to poorer performance in listeners with BiCIs, associated with auditory deprivation, experience, and age. Critically, characteristic errors are made by patients with BiCIs (e.g., medial responses at lateral target locations), and the relationship between patient factors and the type of errors made by patients has seldom been investigated across individuals. In the present study, several different types of analysis were used to understand localization errors and their relationship with patient-dependent factors (selected based on their robustness of prediction). Binaural hearing experience is required for developing accurate localization skills, auditory deprivation is associated with degradation of the auditory periphery, and aging leads to poorer temporal resolution. Therefore, it was hypothesized that earlier onsets of deafness would be associated with poorer localization acuity and longer periods without BiCI stimulation or older age would lead to greater amounts of variability in localization responses. A novel machine learning approach was introduced to characterize the types of errors made by listeners with BiCIs, making them simple to interpret and generalizable to everyday experience. Sound localization performance was measured in 48 listeners with BiCIs using pink noise trains presented in free-field. Our results suggest that older age at testing and earlier onset of deafness are associated with greater average error, particularly for sound sources near the center of the head, consistent with previous research. The machine learning analysis revealed that variability of localization responses tended to be greater for individuals with earlier compared to later onsets of deafness. These results suggest that early bilateral hearing is essential for best sound source localization outcomes in listeners with BiCIs.

Estimating health of the implanted cochlea using psychophysical strength-duration functions and electrode configuration

It is generally believed that the efficacy of cochlear implants is partly dependent on the condition of the stimulated neural population. Cochlear pathology is likely to affect the manner in which neurons respond to electrical stimulation, potentially resulting in differences in perception of electrical stimuli across cochlear implant recipients and across the electrode array in individual cochlear implant users. Several psychophysical and electrophysiological measures have been shown to predict cochlear health in animals and were used to assess conditions near individual stimulation sites in humans. In this study, we examined the relationship between psychophysical strength-duration functions and spiral ganglion neuron density in two groups of guinea pigs with cochlear implants who had minimally-overlapping cochlear health profiles. One group was implanted in a hearing ear (N = 10) and the other group was deafened by cochlear perfusion of neomycin, inoculated with an adeno-associated viral vector with an Ntf3-gene insert (AAV.Ntf3) and implanted (N = 14). Psychophysically measured strength-duration functions for both monopolar and tripolar electrode configurations were then compared for the two treatment groups. Results were also compared to their histological outcomes. Overall, there were considerable differences between the two treatment groups in terms of their psychophysical performance as well as the relation between their functional performance and histological data. Animals in the neomycin-deafened, neurotrophin-treated, and implanted group (NNI) exhibited steeper strength-duration function slopes; slopes were positively correlated with SGN density (steeper slopes in animals that had higher SGN densities). In comparison, the implanted hearing (IH) group had shallower slopes and there was no relation between slopes and spiral ganglion density. Across all animals, slopes were negatively correlated with ensemble spontaneous activity levels (shallower slopes with higher ensemble spontaneous activity levels). We hypothesize that differences in strength-duration function slopes between the two treatment groups were related to the condition of the inner hair cells, which generate spontaneous activity that could affect the across-fiber synchrony and/or the size of the population of neural elements responding to electrical stimulation. In addition, it is likely that spiral ganglion neuron peripheral processes were present in the IH group, which could affect membrane properties of the stimulated neurons. Results suggest that the two treatment groups exhibited distinct patterns of variation in conditions near the stimulating electrodes that altered detection thresholds. Overall, the results of this study suggest a complex relationship between psychophysical detection thresholds for cochlear implant stimulation and nerve survival in the implanted cochlea. This relationship seems to depend on the characteristics of the electrical stimulus, the electrode configuration, and other biological features of the implanted cochlea such as the condition of the inner hair cells and the peripheral processes.

Correlation Between Cochlear Length, Insertion Angle, and Tonotopic Mismatch for MED-EL FLEX28 Electrode Arrays

OBJECTIVE

To investigate the relationship between cochlear length, insertion angle, and tonotopic mismatch and to compare the tonotopic mismatches with respect to the spiral ganglion and the organ of Corti.

STUDY DESIGN

Retrospective.

SETTING

Tertiary referral center with cochlear implant program.

PATIENTS

Analyses of patients' computed tomography images after cochlear implant surgery.

INTERVENTION

Cochlear implantation with 28-mm-long straight lateral wall electrode arrays.

MAIN OUTCOME MEASURE

Cochlear length, insertion angle, and insertion depth were assessed using the OTOPLAN software. Tonotopic mismatch for each electrode contact was estimated using the Greenwood (organ of Corti) and the Stakhovskaya (spiral ganglion) maps and compared.

RESULTS

106 cochleae were analyzed. 99% of the electrode arrays were located in the tympanic ramp. The insertion was complete in 96% of cases. The mean cochlear length was 34.5 mm and the mean insertion angle of the apical electrode was 545°. Cochlear length was negatively correlated with the insertion angle of the contacts E1 to E9 (all p < 0.004). The tonotopic mismatch was greater at the organ of Corti than at the spiral ganglion. It was also greater at the organ of Corti in larger cochleae (correlation with mismatch for E1 r = 0.421, p < 0.0001) and in the apical than in the middle and basal regions of the cochlea.

CONCLUSION

Small cochlea size corresponded to higher insertion angle and reduction of tonotopic mismatch on a 28-mm-long straight lateral wall electrode array. Tonotopic mismatch could be minimized preoperatively by choosing electrode arrays according to the individual cochlear morphology and postoperatively by appropriate frequency fitting.

Impact of Duration of Deafness on Speech Perception in Single-Sided Deafness Cochlear Implantation in Adults

OBJECTIVE

To evaluate the impact of prolonged auditory deprivation on speech perception outcomes in adult acquired single-sided deafness (SSD) cochlear implant (CI) recipients.

STUDY DESIGN

Retrospective case series.

SETTING

Tertiary care academic center.

PATIENTS

Acquired SSD in adults with and without prolonged duration of deafness (defined as >10 yr) who underwent CI between 2014 and 2019.

INTERVENTIONS

CI.

MAIN OUTCOME MEASURES

Consonant-nucleus-consonant (CNC) and AzBio in quiet scores within first year of follow-up.

RESULTS

A total of 35 adult patients with SSD were evaluated, with a median overall duration of deafness of 2.4 years (interquartile range [IQR] 1.2-6.0 yr): seven patients with prolonged auditory deprivation (median 18 yr, IQR 15-28) were compared with 28 SSD patients with duration of deafness less than 10 years (median 1.7 yr, IQR 1.1-3.2). At last follow-up, the median CNC scores were 39% (IQR 31-64) and 54% (IQR 46-64) for the prolonged and shorter duration of deafness cohorts, respectively (p = 0.3). The median AzBio scores were 66% (IQR 65-68) and 72% (IQR 60-82) for the prolonged and shorter duration of deafness cohorts, respectively (p = 0.6). In a separate analysis evaluating duration of deafness as a continuous variable across all 35 patients, Spearman correlation coefficients for associations of duration of deafness with most recent CNC and AzBio scores were -0.02 (p = 0.9) and 0.02 (p = 0.9), respectively.

CONCLUSIONS

Adult CI recipients with acquired SSD, with and without prolonged auditory deprivation, demonstrated comparable speech perception scores. Prolonged duration of deafness alone should not preclude a motivated SSD patient from undergoing cochlear implantation.

No Protective Effects of Hair Cells or Supporting Cells in Ototoxically Deafened Guinea Pigs upon Administration of BDNF

We investigated whether treatment with brain-derived neurotrophic factor (BDNF), which is known to protect spiral ganglion cells (SGCs), could also protect hair cells (HCs) and supporting cells (SCs) in the organ of Corti of a guinea pig model of sensorineural hearing loss. Hearing loss was induced by administration of kanamycin/furosemide and two BDNF treatments were performed: (1) by gelatin sponge (BDNF-GS) with acute cochlear implantation (CI), and (2) through a mini-osmotic pump (BDNF-OP) with chronic CI. Outer HCs (OHCs), inner HCs (IHCs), Border, Phalangeal, Pillar, Deiters', and Hensen's cells were counted. The BDNF-GS cochleas had significantly fewer OHCs compared to the untreated ones, while the IHC and SC numbers did not differ between treated and untreated cochleas. The BDNF-OP group showed similar cell numbers to the untreated group. SGC packing density was not correlated with the total number of SCs for either BDNF group. Our data suggest that: (1) BDNF does not prevent cell death in the organ of Corti, and that the protection of SGCs could result from a direct targeting by BDNF; (2) BDNF might induce a different function/activity of the remaining cells in the organ of Corti (independently from cell number).

Aging Effects on Cortical Responses to Tones and Speech in Adult Cochlear-Implant Users

Age-related declines in auditory temporal processing contribute to speech understanding difficulties of older adults. These temporal processing deficits have been established primarily among acoustic-hearing listeners, but the peripheral and central contributions are difficult to separate. This study recorded cortical auditory evoked potentials from younger to middle-aged (< 65 years) and older (≥ 65 years) cochlear-implant (CI) listeners to assess age-related changes in temporal processing, where cochlear processing is bypassed in this population. Aging effects were compared to age-matched normal-hearing (NH) listeners. Advancing age was associated with prolonged P2 latencies in both CI and NH listeners in response to a 1000-Hz tone or a syllable /da/, and with prolonged N1 latencies in CI listeners in response to the syllable. Advancing age was associated with larger N1 amplitudes in NH listeners. These age-related changes in latency and amplitude were independent of stimulus presentation rate. Further, CI listeners exhibited prolonged N1 and P2 latencies and smaller P2 amplitudes than NH listeners. Thus, aging appears to degrade some aspects of auditory temporal processing when peripheral-cochlear contributions are largely removed, suggesting that changes beyond the cochlea may contribute to age-related temporal processing deficits.

Effect of Stimulation Rate on Speech Understanding in Older Cochlear-Implant Users

OBJECTIVES

Cochlear implants (CIs) are considered a safe and effective intervention for more severe degrees of hearing loss in adults of all ages. Although older CI users ≥65 years of age can obtain large benefits in speech understanding from a CI, there is a growing body of literature suggesting that older CI users may not perform as well as younger CI users. One reason for this potential age-related limitation could be that default CI stimulation settings are not optimal for older CI users. The goal of this study was to determine whether improvements in speech understanding were possible when CI users were programmed with nondefault stimulation rates and to determine whether lower-than-default stimulation rates improved older CI users' speech understanding.

DESIGN

Sentence recognition was measured acutely using different stimulation rates in 37 CI users ranging in age from 22 to 87 years. Maps were created using rates of 500, 720, 900, and 1200 pulses per second (pps) for each subject. An additional map using a rate higher than 1200 pps was also created for individuals who used a higher rate in their clinical processors. Thus, the clinical rate of each subject was also tested, including non-default rates above 1200 pps for Cochlear users and higher rates consistent with the manufacturer defaults for subjects implanted with Advanced Bionics and Med-El devices. Speech understanding performance was evaluated at each stimulation rate using AzBio and Perceptually Robust English Sentence Test Open-set (PRESTO) sentence materials tested in quiet and in noise.

RESULTS

For Cochlear-brand users, speech understanding performance using non-default rates was slightly poorer when compared with the default rate (900 pps). However, this effect was offset somewhat by age, in which older subjects were able to maintain comparable performance using a 500-pps map compared with the default rate map when listening to the more difficult PRESTO sentence material. Advanced Bionics and Med-El users showed modest improvements in their overall performance using 720 pps compared with the default rate (>1200 pps). On the individual-subject level, 10 subjects (11 ears) showed a significant effect of stimulation rate, with 8 of those ears performing best with a lower-than-default rate.

CONCLUSIONS

Results suggest that default stimulation rates are likely sufficient for many CI users, but some CI users at any age can benefit from a lower-than-default rate. Future work that provides experience with novel rates in everyday life has the potential to identify more individuals whose performance could be improved with changes to stimulation rate.

Neurite Extension and Orientation of Spiral Ganglion Neurons Can Be Directed by Superparamagnetic Iron Oxide Nanoparticles in a Magnetic Field

Introduction

Neuroregeneration is a major challenge in neuroscience for treating degenerative diseases and for repairing injured nerves. Numerous studies have shown the importance of physical stimulation for neuronal growth and development, and here we report an approach for the physical guidance of neuron orientation and neurite growth using superparamagnetic iron oxide (SPIO) nanoparticles and magnetic fields (MFs).

Methods

SPIO nanoparticles were synthesized by classic chemical co-precipitation methods and then characterized by transmission electron microscope, dynamic light scattering, and vibrating sample magnetometer. The cytotoxicity of the prepared SPIO nanoparticles and MF was determined using CCK-8 assay and LIVE/DEAD assay. The immunofluorescence images were captured by a laser scanning confocal microscopy. Cell migration was evaluated using the wound healing assay.

Results

The prepared SPIO nanoparticles showed a narrow size distribution, low cytotoxicity, and superparamagnetism. SPIO nanoparticles coated with poly-L-lysine could be internalized by spiral ganglion neurons (SGNs) and showed no cytotoxicity at concentrations less than 300 µg/mL. The neurite extension of SGNs was promoted after internalizing SPIO nanoparticles with or without an external MF, and this might be due to the promotion of growth cone development. It was also confirmed that SPIO can regulate cell migration and can direct neurite outgrowth in SGNs preferentially along the direction imposed by an external MF.

Conclusion

Our results provide a fundamental understanding of the regulation of cell behaviors under physical cues and suggest alternative treatments for sensorineural hearing loss caused by the degeneration of SGNs.

Access to the Apical Cochlear Modiolus for Possible Stem Cell-based and Gene Therapy of the Auditory Nerve

OBJECTIVE

Loss of spiral ganglion neurons (SGN) is permanent and responsible for a substantial number of patients suffering from hearing impairment. It can derive from the degeneration of SGNs due to the death of sensory hair cells as well as from auditory neuropathy. Utilizing stem cells to recover lost SGNs increasingly emerges as a possible therapeutic option, but access to human SGNs is difficult due to their protected location within the bony impacted cochlea. Aim of this study was to establish a reliable and practicable approach to access SGNs in the human temporal bone for possible stem cell and gene therapies.

METHODS

In seven human temporal bone specimen a transcanal approach was used to carefully drill a cochleostomy in the lateral second turn followed by insertion of a tungsten needle into the apical modiolus to indicate the spot for intramodiolar injections. Subsequent cone beam computed tomography (CBCT) served as evaluation for positioning of the marker and cochleostomy size.

RESULTS

The apical modiolus could be exposed in all cases by a cochleostomy (1.6 mm2, standard deviation ±0.23 mm2) in the lateral second turn. 3D reconstructions and analysis of CBCT revealed reliable positioning of the marker in the apical modiolus, deviating on average 0.9 mm (standard deviation ±0.49 mm) from the targeted center of the second cochlear turn.

CONCLUSION

We established a reliable, minimally invasive, transcanal surgical approach to the apical cochlear modiolus in the human temporal bone in foresight to stem cell-based and gene therapy of the auditory nerve.

Cell Transplantation to Restore Lost Auditory Nerve Function is a Realistic Clinical Opportunity

Hearing is one of our most important means of communication. Disabling hearing loss (DHL) is a long-standing, unmet problem in medicine, and in many elderly people, it leads to social isolation, depression, and even dementia. Traditionally, major efforts to cure DHL have focused on hair cells (HCs). However, the auditory nerve is also important because it transmits electrical signals generated by HCs to the brainstem. Its function is critical for the success of cochlear implants as well as for future therapies for HC regeneration. Over the past two decades, cell transplantation has emerged as a promising therapeutic option for restoring lost auditory nerve function, and two independent studies on animal models show that cell transplantation can lead to functional recovery. In this article, we consider the approaches most likely to achieve success in the clinic. We conclude that the structure and biochemical integrity of the auditory nerve is critical and that it is important to preserve the remaining neural scaffold, and in particular the glial scar, for the functional integration of donor cells. To exploit the natural, autologous cell scaffold and to minimize the deleterious effects of surgery, donor cells can be placed relatively easily on the surface of the nerve endoscopically. In this context, the selection of donor cells is a critical issue. Nevertheless, there is now a very realistic possibility for clinical application of cell transplantation for several different types of hearing loss.

BDNF-mediated preservation of spiral ganglion cell peripheral processes and axons in comparison to that of their cell bodies

Treatment with neurotrophins prevents degeneration of spiral ganglion cells (SGCs) after severe hair cell loss. In a previous study we demonstrated a long-lasting effect with brain-derived neurotrophic factor (BDNF) after cessation of treatment. In that study the survival of the SGC cell bodies was examined. Here we address the question whether their peripheral processes and central processes (axons) were protected by this treatment as well in the cochleas of the aforementioned study. Guinea pigs were deafened by co-administration of kanamycin and furosemide. Two weeks after deafening the right cochleas were implanted with an intracochlear electrode array combined with a cannula connected to an osmotic pump filled with BDNF solution. Four weeks later the treatment was stopped by surgically removing the osmotic pump. At that point, or another four or eight weeks later, the animals were sacrificed for histological analysis. Control groups consisted of normal-hearing animals, and three groups of deafened animals: two-weeks-deaf untreated animals, and six- and fourteen-weeks-deaf sham-treated animals. Cochleas were processed for analysis of: (1) the myelinated portion of peripheral processes in the osseous spiral lamina, (2) the cell bodies in Rosenthal's canal, and (3) axons in the internal acoustic meatus. Packing densities and cross-sectional areas were determined using light microscopy. Up to eight weeks after treatment cessation the numbers of peripheral processes and axons were significantly higher than in untreated cochleas of control animals. Whereas the numbers of cell bodies and axons were similar to those at the start of treatment, the peripheral processes were significantly less well preserved. This smaller protective effect was found mainly in the apical turns. Strategies to prevent SGC degeneration after hair cell loss should consider the differential effects on the various neural elements.

BDNF Outperforms TrkB Agonist 7,8,3'-THF in Preserving the Auditory Nerve in Deafened Guinea Pigs

In deaf subjects using a cochlear implant (CI) for hearing restoration, the auditory nerve is subject to degeneration, which may negatively impact CI effectiveness. This nerve degeneration can be reduced by neurotrophic treatment. Here, we compare the preservative effects of the naturally occurring tyrosine receptor kinase B (TrkB) agonist brain-derived neurotrophic factor (BDNF) and the small-molecule TrkB agonist 7,8,3′-trihydroxyflavone (THF) on the auditory nerve in deafened guinea pigs. THF may be more effective than BDNF throughout the cochlea because of better pharmacokinetic properties. The neurotrophic compounds were delivered by placement of a gelatin sponge on the perforated round window membrane. To complement the histology of spiral ganglion cells (SGCs), electrically evoked compound action potential (eCAP) recordings were performed four weeks after treatment initiation. We analyzed the eCAP inter-phase gap (IPG) effect and measures derived from pulse-train evoked eCAPs, both indicative of SGC healthiness. BDNF but not THF yielded a significantly higher survival of SGCs in the basal cochlear turn than untreated controls. Regarding IPG effect and pulse-train responses, the BDNF-treated animals exhibited more normal responses than both untreated and THF-treated animals. We have thus confirmed the protective effect of BDNF, but we have not confirmed previously reported protective effects of THF with our clinically applicable delivery method.

Age-Related Temporal Processing Deficits in Word Segments in Adult Cochlear-Implant Users

Aging may limit speech understanding outcomes in cochlear-implant (CI) users. Here, we examined age-related declines in auditory temporal processing as a potential mechanism that underlies speech understanding deficits associated with aging in CI users. Auditory temporal processing was assessed with a categorization task for the words dish and ditch (i.e., identify each token as the word dish or ditch) on a continuum of speech tokens with varying silence duration (0 to 60 ms) prior to the final fricative. In Experiments 1 and 2, younger CI (YCI), middle-aged CI (MCI), and older CI (OCI) users participated in the categorization task across a range of presentation levels (25 to 85 dB). Relative to YCI, OCI required longer silence durations to identify ditch and exhibited reduced ability to distinguish the words dish and ditch (shallower slopes in the categorization function). Critically, we observed age-related performance differences only at higher presentation levels. This contrasted with findings from normal-hearing listeners in Experiment 3 that demonstrated age-related performance differences independent of presentation level. In summary, aging in CI users appears to degrade the ability to utilize brief temporal cues in word identification, particularly at high levels. Age-specific CI programming may potentially improve clinical outcomes for speech understanding performance by older CI listeners.

Atrial Natriuretic Peptide Improves Neurite Outgrowth from Spiral Ganglion Neurons In Vitro through a cGMP-Dependent Manner

The spiral ganglion neurons (SGNs) are the primary afferent neurons in the spiral ganglion (SG), while their degeneration or loss would cause sensorineural hearing loss. As a cardiac-derived hormone, atrial natriuretic peptide (ANP) plays a critical role in cardiovascular homeostasis through binding to its functional receptors (NPR-A and NPR-C). ANP and its receptors are widely expressed in the mammalian nervous system where they could be implicated in the regulation of multiple neural functions. Although previous studies have provided direct evidence for the presence of ANP and its functional receptors in the inner ear, their presence within the cochlear SG and their regulatory roles during auditory neurotransmission and development remain largely unknown. Based on our previous findings, we investigated the expression patterns of ANP and its receptors in the cochlear SG and dissociated SGNs and determined the influence of ANP on neurite outgrowth in vitro by using organotypic SG explants and dissociated SGN cultures from postnatal rats. We have demonstrated that ANP and its receptors are expressed in neurons within the cochlear SG of postnatal rat, while ANP may promote neurite outgrowth of SGNs via the NPR-A/cGMP/PKG pathway in a dose-dependent manner. These results indicate that ANP would play a role in normal neuritogenesis of SGN during cochlear development and represents a potential therapeutic candidate to enhance regeneration and regrowth of SGN neurites.

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.

Impact of Universal Newborn Hearing Screening on cochlear implanted children in Ireland

OBJECTIVES

Cochlear Implant (CI) is an established treatment for severe to profound hearing loss (HL). Early diagnosis and intervention in HL are crucial in order to provide access to sound and increase the likelihood of spoken language development in pre-lingually deaf children. In April 2011, the Health Service Executive (HSE) implemented the Universal Newborn Hearing Screening (UNHS) in a phased regional basis in Ireland. This study aimed to investigate the general clinical pathway for UNHS referrals to the CI service and to evaluate the impact of earlier referrals via UNHS on functional outcomes in children.

METHODS

The first part of this study constituted a retrospective review of 100 children referred to the National Hearing Implant and Research Centre (NHIRC) via UNHS from November 2011 to December 2016. Implanted children referred via UNHS were categorised into three groups according to their medical status. Their clinical pathway to cochlear implantation was evaluated. Functional outcomes were investigated based on medical and developmental status, respectively. In the second part of this study, developmentally healthy implanted children referred post-UNHS were compared with medically healthy children referred pre-UNHS under the age of four, from January 2005 to June 2011. Current implant status of children, age at referral and functional outcomes were investigated.

RESULTS

Medically healthy children were referred to the NHIRC at an earlier age than the medically complex children (2.8 months vs 5.2 months, p < 0.01) and the children presenting with auditory neuropathy spectrum disorder (ANSD) (2.8 months vs 5.3 months, p < 0.01). On average they attended their first appointment and were implanted at a younger age than the ANSD group (6.1 months vs 10.1 months, p < 0.01; 16.3 months vs 29.4 months, p < 0.001, respectively). Developmentally healthy children had significantly better functional outcomes than children with developmental delays. Children referred via UNHS were referred and implanted at a younger age than those referred pre-UNHS. The former group achieved better Categories of Auditory Performance (CAP) and Speech Intelligibility Rating (SIR) scores 2 years post-implantation.

CONCLUSION

UNHS in Ireland is an important platform for earlier diagnosis and management of congenital HL and our results show that early intervention has a positive impact on functional outcomes in children.

Assessment of temporal processing functions in early period cochlear implantation

PURPOSE

The purpose of this study is to compare the temporal processing performance of children with cochlear implant (CI) according to the age of implantation and to determine their relation with auditory perception scores.

METHODS

In this study, 30 cochlear implant users and ten normal hearing children at 9 and 10 years were included. Children with cochlear implants are divided into two groups according to the age of implantation: group I includes participants whose implantation age is between 13 and 35 months (20 children), group II includes participants whose implantation age is between 36 and 45 months (10 children). Individuals were evaluated with random gap detection test (RGDT), duration pattern test (DPT), frequency pattern test (FPT), the Mr. Potato Head task, word recognition, and sentence recognition test.

RESULTS

A significant difference was found between the control and CI groups in temporal processing performance. The temporal processing ability of CI groups was significantly worse than those of normal hearing. Although there was no significant difference among the groups with cochlear implant in terms of temporal processing performance, children who started to use CI at an earlier age showed a tendency of better performance on temporal processing tasks. There was a significant relationship between Daily Sentence Test and FPT, and the Mr. Potato Head task and FPT rev (the score calculated by accepting the reverse patterns correctly). There was a significant relationship between duration of implant use and temporal ordering performance CONCLUSION: In this study, children with CI cannot perform as well as normal-hearing peers on temporal processing tasks, even if they had started to use their CIs at an early age. It is important to evaluate temporal processing in implanted individuals and to guide auditory training considering the evaluation results.

Emerging approaches for restoration of hearing and vision

Impairments of vision and hearing are highly prevalent conditions limiting the quality of life and presenting a major socioeconomic burden. For long, retinal and cochlear disorders have remained intractable for causal therapies, with sensory rehabilitation limited to glasses, hearing aids, and electrical cochlear or retinal implants. Recently, the application of gene therapy and optogenetics to eye and ear has generated hope for a fundamental improvement of vision and hearing restoration. To date, one gene therapy for the restoration of vision has been approved and undergoing clinical trials will broaden its application including gene replacement, genome editing, and regenerative approaches. Moreover, optogenetics, i.e. controlling the activity of cells by light, offers a more general alternative strategy. Over little more than a decade, optogenetic approaches have been developed and applied to better understand the function of biological systems, while protein engineers have identified and designed new opsin variants with desired physiological features. Considering potential clinical applications of optogenetics, the spotlight is on the sensory systems. Multiple efforts have been undertaken to restore lost or hampered function in eye and ear. Optogenetic stimulation promises to overcome fundamental shortcomings of electrical stimulation, namely poor spatial resolution and cellular specificity, and accordingly to deliver more detailed sensory information. This review aims at providing a comprehensive reference on current gene therapeutic and optogenetic research relevant to the restoration of hearing and vision. We will introduce gene-therapeutic approaches and discuss the biotechnological and optoelectronic aspects of optogenetic hearing and vision restoration.

Development and Application of Cochlear Implant-Based Electric-Acoustic Stimulation of Spiral Ganglion Neurons

Cochlear implants are currently the most effective treatment for profound sensorineural hearing loss. However, their therapeutic effect is limited by the survival and proper physiological function of spiral ganglion neurons (SGNs), which are targeted by the cochlear implant. It is therefore critical to explore the mechanism behind the effect of electric-acoustic stimulation (EAS) on the targeted SGNs. In this work, a biocompatible cochlear implant/graphene EAS system was created by combining a cochlear implant to provide the electrically transformed sound stimulation with graphene as the conductive neural interface. SGNs were cultured on the graphene and exposed to EAS from the cochlear implant. Neurite extension of SGNs was accelerated with long-term stimulation, which might contribute to the development of growth cones. Our system allows us to study the effects of cochlear implants on SGNs in a low-cost and time-saving way, and this might provide profound insights into the use of cochlear implants and thus be of benefit to the populations suffering from sensorineural hearing loss.

Guided growth with aligned neurites in adult spiral ganglion neurons cultured in vitro on silicon micro-pillar substrates

OBJECTIVE

Assessment of the relationship between the topographical organization of silicon micro-pillar surfaces (MPS) on guidance and neural alignment of adult spiral ganglion neurons (SGN) and use of the otosurgical approach as an alternative for the extraction and isolation of SGNs from adult guinea pigs.

APPROACH

SGNs from adult guinea pigs were isolated using conventional and otosurgical approach for in vitro cell culturing on MPS of various micro-pillar widths (1-5.6 µm) and spacing (0.6-15 µm). Cell cultures were compared morphologically with neuronal cultures on control glass coverslips.

MAIN RESULTS

We found enhanced SGN in vitro cultures in MPS areas with small and intermediate inter-pillar spacing (from 0.6 µm to 3.2 µm) as well as in MPS areas with wider pillars (from 1.8 µm to 4 µm) compared to MPS flat zones and control glass coverslips. Scanning electron microscopy (SEM) images highlighted how neurites of SGNs follow straight lines by growing on top and between micro-pillars. Only micro-pillars with small and intermediate pillar spacings favor neurite alignment along preferred angles (30°, 90°, and 150°), while pillars with wider spacing produced less aligned neurites. We found propensity of adult SGNs grown on MPSs to attain more bipolar and multipolar morphologies. Additionally, we observed reduced interaction between neuronal and glial cells compared to control glass coverslips. Finally, we found that the otosurgical approach was more beneficial for SGN survival on glass coverslips and MPS flat surfaces than the conventional method.

SIGNIFICANCE

MPS with specific architecture supports the guided growth of adult SGNs in vitro and controls adult SGN development and behavior.

Effects of rate and age in processing interaural time and level differences in normal-hearing and bilateral cochlear-implant listeners

Bilateral cochlear implants (BICIs) provide improved sound localization and speech understanding in noise compared to unilateral CIs. However, normal-hearing (NH) listeners demonstrate superior binaural processing abilities compared to BICI listeners. This investigation sought to understand differences between NH and BICI listeners' processing of interaural time differences (ITDs) and interaural level differences (ILDs) as a function of fine-structure and envelope rate using an intracranial lateralization task. The NH listeners were presented band-limited acoustical pulse trains and sinusoidally amplitude-modulated tones using headphones, and the BICI listeners were presented single-electrode electrical pulse trains using direct stimulation. Lateralization range increased as fine-structure rate increased for ILDs in BICI listeners. Lateralization range decreased for rates above 100 Hz for fine-structure ITDs, but decreased for rates lower or higher than 100 Hz for envelope ITDs in both groups. Lateralization ranges for ITDs were smaller for BICI listeners on average. After controlling for age, older listeners showed smaller lateralization ranges and BICI listeners had a more rapid decline for ITD sensitivity at 300 pulses per second. This work suggests that age confounds comparisons between NH and BICI listeners in temporal processing tasks and that some NH-BICI binaural processing differences persist even when age differences are adequately addressed.

Functional brain alterations following mild-to-moderate sensorineural hearing loss in children

Auditory deprivation in the form of deafness during development leads to lasting changes in central auditory system function. However, less is known about the effects of mild-to-moderate sensorineural hearing loss (MMHL) during development. Here, we used a longitudinal design to examine late auditory evoked responses and mismatch responses to nonspeech and speech sounds for children with MMHL. At Time 1, younger children with MMHL (8-12 years; n = 23) showed age-appropriate mismatch negativities (MMNs) to sounds, but older children (12-16 years; n = 23) did not. Six years later, we re-tested a subset of the younger (now older) children with MMHL (n = 13). Children who had shown significant MMNs at Time 1 showed MMNs that were reduced and, for nonspeech, absent at Time 2. Our findings demonstrate that even a mild-to-moderate hearing loss during early-to-mid childhood can lead to changes in the neural processing of sounds in late childhood/adolescence.

Asymmetric temporal envelope encoding: Implications for within- and across-ear envelope comparison

Separating sound sources in acoustic environments relies on making ongoing, highly accurate spectro-temporal comparisons. However, listeners with hearing impairment may have varying quality of temporal encoding within or across ears, which may limit the listeners' ability to make spectro-temporal comparisons between places-of-stimulation. In this study in normal hearing listeners, depth of amplitude modulation (AM) for sinusoidally amplitude modulated (SAM) tones was manipulated in an effort to reduce the coding of periodicity in the auditory nerve. The ability to judge differences in AM rates was studied for stimuli presented to different cochlear places-of-stimulation, within- or across-ears. It was hypothesized that if temporal encoding was poorer for one tone in a pair, then sensitivity to differences in AM rate of the pair would decrease. Results indicated that when the depth of AM was reduced from 50% to 20% for one SAM tone in a pair, sensitivity to differences in AM rate decreased. Sensitivity was greatest for AM rates near 90 Hz and depended upon the places-of-stimulation being compared. These results suggest that degraded temporal representations in the auditory nerve for one place-of-stimulation could lead to deficits comparing that temporal information with other places-of-stimulation.

Systematic Review of Auditory Training in Pediatric Cochlear Implant Recipients

Objective The purpose of this systematic review is to evaluate the published research in auditory training (AT) for pediatric cochlear implant (CI) recipients. This review investigates whether AT in children with CIs leads to improvements in speech and language development, cognition, and/or quality of life and whether improvements, if any, remain over time post AT intervention. Method A systematic search of 7 databases identified 96 review articles published up until January 2017, 9 of which met the inclusion criteria. Data were extracted and independently assessed for risk of bias and quality of study against a PICOS (participants, intervention, control, outcomes, and study) framework. Results All studies reported improvements in trained AT tasks, including speech discrimination/identification and working memory. Retention of improvements over time was found whenever it was assessed. Transfer of learning was measured in 4 of 6 studies, which assessed generalization. Quality of life was not assessed. Overall, evidence for the included studies was deemed to be of low quality. Conclusion Benefits of AT were illustrated through the improvement in trained tasks, and this was observed in all reviewed studies. Transfer of improvement to other domains and also retention of benefits post AT were evident when assessed, although rarely done. However, higher quality evidence to further examine outcomes of AT in pediatric CI recipients is needed.

Auditory motion tracking ability of adults with normal hearing and with bilateral cochlear implants

Adults with bilateral cochlear implants (BiCIs) receive benefits in localizing stationary sounds when listening with two implants compared with one; however, sound localization ability is significantly poorer when compared to normal hearing (NH) listeners. Little is known about localizing sound sources in motion, which occurs in typical everyday listening situations. The authors considered the possibility that sound motion may improve sound localization in BiCI users by providing multiple places of information. Alternatively, the ability to compare multiple spatial locations may be compromised in BiCI users due to degradation of binaural cues, and thus result in poorer performance relative to NH adults. In this study, the authors assessed listeners' abilities to distinguish between sounds that appear to be moving vs stationary, and track the angular range and direction of moving sounds. Stimuli were bandpass-filtered (150-6000 Hz) noise bursts of different durations, panned over an array of loudspeakers. Overall, the results showed that BiCI users were poorer than NH adults in (i) distinguishing between a moving vs stationary sound, (ii) correctly identifying the direction of movement, and (iii) tracking the range of movement. These findings suggest that conventional cochlear implant processors are not able to fully provide the cues necessary for perceiving auditory motion correctly.

Chronic Deafness Degrades Temporal Acuity in the Electrically Stimulated Auditory Pathway

Electrical stimulation of the auditory nerve with a penetrating intraneural (IN) electrode in acutely deafened cats produces much more restricted spread of excitation than is obtained in that preparation with a conventional cochlear implant (CI) as reported by Middlebrooks and Snyder (J Assoc Res Otolaryngol 8:258–279, 2007). That suggests that a future auditory prosthesis employing IN stimulation might offer human patients greater frequency selectivity than is available with a present-day CI. Nevertheless, it is a concern that the electrical field produced by an IN electrode might be too restricted to produce adequate stimulation of the partially depopulated auditory nerve of a deaf patient. We evaluated this by testing responses to IN and CI stimulation in adult-deafened cats. Activation of the auditory pathway was monitored by recording from the central nucleus of the inferior colliculus (ICC). Cats deaf for 153–277 days exhibited a ~ 30 % loss of auditory nerve fibers compared to cats deaf for < 18 h. Contrary to our concern, measures of thresholds and dynamic ranges showed no significant deafness-related impairment of excitation by IN or CN stimulation. Surprisingly, however, temporal acuity decreased dramatically in these adult-deafened cats, as demonstrated by a marked decrease in the maximum rate of electrical cochlear stimulation to which ICC neurons synchronized to IN or CI stimulation. For instance, half of ICC neurons synchronized to IN stimulation up to 203 pulses per second (pps) in acute deafness, whereas that number dropped to 79 pps for chronic deafness. Such a loss of temporal acuity might contribute to the poor sensitivity to temporal fine structure that has been reported in human CI users. Seemingly, the degraded temporal acuity that we observed in cats was even worse than the fine-structure sensitivity of human CI users, suggesting that most patients experience some improvement of temporal acuity resulting from restoration of patterned auditory nerve stimulation by a CI.

Intensity Discrimination and Speech Recognition of Cochlear Implant Users

The relation between speech recognition and within-channel or across-channel (i.e., spectral tilt) intensity discrimination was measured in nine CI users (11 ears). Within-channel intensity difference limens (IDLs) were measured at four electrode locations across the electrode array. Spectral tilt difference limens were measured with (XIDL-J) and without (XIDL) level jitter. Only three subjects could perform the XIDL-J task with the amount of jitter required to limit use of within-channel cues. XIDLs (normalized to %DR) were correlated with speech recognition (r = 0.67, P = 0.019) and were highly correlated with IDLs. XIDLs were on average nearly 3 times larger than IDLs and did not vary consistently with the spatial separation of the two component electrodes. The overall pattern of results was consistent with a common underlying subject-dependent limitation in the two difference limen tasks, hypothesized to be perceptual variance (how the perception of a sound differs on different presentations), which may also underlie the correlation of XIDLs with speech recognition. Evidence that spectral tilt discrimination is more important for speech recognition than within-channel intensity discrimination was not unequivocally shown in this study. However, the results tended to support this proposition, with XIDLs more correlated with speech performance than IDLs, and the ratio XIDL/IDL also being correlated with speech recognition. If supported by further research, the importance of perceptual variance as a limiting factor in speech understanding for CI users has important implications for efforts to improve outcomes for those with poor speech recognition.

Advantageous environment of micro-patterned, high-density complementary metal-oxide-semiconductor electrode array for spiral ganglion neurons cultured in vitro

This study investigated micro-patterned, high-density complementary metal–oxide–semiconductor (CMOS) electrode array to be used as biologically permissive environment for organization, guidance and electrical stimulation of spiral ganglion neurons (SGN). SGNs extracted and isolated from cochleae of P5-P7 rat pups and adult guinea pigs were cultured 1, 4 and 7 days in vitro on glass coverslips (control) and CMOS electrode array. The cultures were analyzed visually and immunohistochemically for SGN presence, outgrowth, neurite alignment, neurite length, neurite asymmetry as well as the contact of a neuronal soma and neurites with the micro-electrodes. Our findings indicate that topographical environment of CMOS chip with micro-patterned pillars enhanced growth, survival, morphology, neural orientation and alignment of SGNs in vitro compared to control. Smaller spacing (0.8–1.6 µm) between protruding pillars on CMOS led SGNs to develop structured and guided neurites oriented along three topographical axes separated by 60°. We found morphological basis for positioning of the micro-electrodes on the chip that was appropriate for direct contact of SGNs with them. This configuration allowed CMOS electrode array to electrically stimulate the SGN whose responses were observed with live Fluo 4 calcium imaging.

Voice Discrimination by Adults with Cochlear Implants: the Benefits of Early Implantation for Vocal-Tract Length Perception

Cochlear implant (CI) users find it extremely difficult to discriminate between talkers, which may partially explain why they struggle to understand speech in a multi-talker environment. Recent studies, based on findings with postlingually deafened CI users, suggest that these difficulties may stem from their limited use of vocal-tract length (VTL) cues due to the degraded spectral resolution transmitted by the CI device. The aim of the present study was to assess the ability of adult CI users who had no prior acoustic experience, i.e., prelingually deafened adults, to discriminate between resynthesized "talkers" based on either fundamental frequency (F0) cues, VTL cues, or both. Performance was compared to individuals with normal hearing (NH), listening either to degraded stimuli, using a noise-excited channel vocoder, or non-degraded stimuli. Results show that (a) age of implantation was associated with VTL but not F0 cues in discriminating between talkers, with improved discrimination for those subjects who were implanted at earlier age; (b) there was a positive relationship for the CI users between VTL discrimination and speech recognition score in quiet and in noise, but not with frequency discrimination or cognitive abilities; (c) early-implanted CI users showed similar voice discrimination ability as the NH adults who listened to vocoded stimuli. These data support the notion that voice discrimination is limited by the speech processing of the CI device. However, they also suggest that early implantation may facilitate sensory-driven tonotopicity and/or improve higher-order auditory functions, enabling better perception of VTL spectral cues for voice discrimination.

Microelectrode array-induced neuronal alignment directs neurite outgrowth: analysis using a fast Fourier transform (FFT)

Many studies have shown that the topography of the substrate on which neurons are cultured can promote neuronal adhesion and guide neurite outgrowth in the same direction as the underlying topography. To investigate this effect, isotropic substrate-complementary metal-oxide-semiconductor (CMOS) chips were used as one example of microelectrode arrays (MEAs) for directing neurite growth of spiral ganglion neurons. Neurons were isolated from 5 to 7-day-old rat pups, cultured 1 day in vitro (DIV) and 4 DIV, and then fixed with 4% paraformaldehyde. For analysis of neurite alignment and orientation, fast Fourier transformation (FFT) was used. Results revealed that on the micro-patterned surface of a CMOS chip, neurons orient their neurites along three directional axes at 30, 90, and 150° and that neurites aligned in straight lines between adjacent pillars and mostly followed a single direction while occasionally branching perpendicularly. We conclude that the CMOS substrate guides neurites towards electrodes by means of their structured pillar organization and can produce electrical stimulation of aligned neurons as well as monitoring their neural activities once neurites are in the vicinity of electrodes. These findings are of particular interest for neural tissue engineering with the ultimate goal of developing a new generation of MEA essential for improved electrical stimulation of auditory neurons.

Binaural sensitivity in children who use bilateral cochlear implants

Children who are deaf and receive bilateral cochlear implants (BiCIs) perform better on spatial hearing tasks using bilateral rather than unilateral inputs; however, they underperform relative to normal-hearing (NH) peers. This gap in performance is multi-factorial, including the inability of speech processors to reliably deliver binaural cues. Although much is known regarding binaural sensitivity of adults with BiCIs, less is known about how the development of binaural sensitivity in children with BiCIs compared to NH children. Sixteen children (ages 9-17 years) were tested using synchronized research processors. Interaural time differences and interaural level differences (ITDs and ILDs, respectively) were presented to pairs of pitch-matched electrodes. Stimuli were 300-ms, 100-pulses-per-second, constant-amplitude pulse trains. In the first and second experiments, discrimination of interaural cues (either ITDs or ILDs) was measured using a two-interval left/right task. In the third experiment, subjects reported the perceived intracranial position of ITDs and ILDs in a lateralization task. All children demonstrated sensitivity to ILDs, possibly due to monaural level cues. Children who were born deaf had weak or absent sensitivity to ITDs; in contrast, ITD sensitivity was noted in children with previous exposure to acoustic hearing. Therefore, factors such as auditory deprivation, in particular, lack of early exposure to consistent timing differences between the ears, may delay the maturation of binaural circuits and cause insensitivity to binaural differences.

The results of newborn hearing screening by means of transient otoacoustic emissions - has anything changed over 10 years?

OBJECTIVES

Universal newborn hearing screening (UNHS) has become the standard of care in many countries. The aim of this study was to evaluate the results of UNHS after ten years of the program in Poland and to compare them with the results of 2003.

METHODS

In the study, we analyze the results of UNHS in the University Hospital in Poznan, Poland. Between 01.01.2013 and 31.12.2013, 6827 children were examined by means of otoacoustic emissions.

RESULTS

Risk factors (RF) were identified in 772 (11.3%) newborns, which is significantly less than 10 years ago (p < 0.05). The most frequent RF were: ototoxic medications, treatment in neonatal intensive care unit (NICU) and prematurity < 33 weeks of gestation. In 2003, the most frequent were ototoxic medications and prematurity, less frequent was treatment in NICU and more common was low Apgar score. In 51 (6.6%) newborns with RF, the result of OAE was positive either unilaterally or bilaterally. In infants without RF the result was positive unilaterally in 22 (0.4%) and bilaterally in 14 (0.2%) patients. These results are significantly lower than in our former study. The relative risk of positive result was the highest in infants with complex congenital anomalies (RR = 44.99), craniofacial anomalies (RR = 17.46) and mechanical ventilation for > 5 days (RR = 10.69). In our previous study, the highest RR of positive test results was in infants with family history, congenital malformations and low Apgar score. We found that most predictive as to the final diagnosis was bilaterally positive OAE test. In most patients, the second check confirmed the diagnosis, independently of RF. The number of false positive tests at the 1st level of screening is significantly lower now than 10 years ago, probably due to better staff training.

CONCLUSIONS

Long term monitoring and the appropriate management of hearing deficit in children is essential. UNHS seems to be the most efficient way of finding children who require treatment of hearing impairment. The prevalence of most risk factors of hearing deficit has significantly changed over the years. The number of false positive results has significantly decreased over the years thanks to better staff training.

Encapsulated cell device approach for combined electrical stimulation and neurotrophic treatment of the deaf cochlea

Profound hearing impairment can be overcome by electrical stimulation (ES) of spiral ganglion neurons (SGNs) via a cochlear implant (CI). Thus, SGN survival is critical for CI efficacy. Application of glial cell line-derived neurotrophic factor (GDNF) has been shown to reduce SGN degeneration following deafness. We tested a novel method for local, continuous GDNF-delivery in combination with ES via a CI. The encapsulated cell (EC) device contained a human ARPE-19 cell-line, genetically engineered for secretion of GDNF. In vitro, GDNF delivery was stable during ES delivered via a CI. In the chronic in vivo part, cats were systemically deafened and unilaterally implanted into the scala tympani with a CI and an EC device, which they wore for six months. The implantation of control devices (same cell-line not producing GDNF) had no negative effect on SGN survival. GDNF application without ES led to an unexpected reduction in SGN survival, however, the combination of GDNF with initial, short-term ES resulted in a significant protection of SGNs. A tight fibrous tissue formation in the scala tympani of the GDNF-only group is thought to be responsible for the increased SGN degeneration, due to mechanisms related to an aggravated foreign body response. Furthermore, the fibrotic encapsulation of the EC device led to cell death or cessation of GDNF release within the EC device during the six months in vivo. In both in vitro and in vivo, fibrosis was reduced by CI stimulation, enabling the neuroprotective effect of the combined treatment. Thus, fibrous tissue growth limits treatment possibilities with an EC device. For a stable and successful long-term neurotrophic treatment of the SGN via EC devices in human CI users, it would be necessary to make changes in the treatment approach (provision of anti-inflammatories), the EC device surface (reduced cell adhesion) and the ES (initiation prior to fibrosis formation).

Validation of the LittlEARS Auditory Questionnaire in cochlear implanted infants and toddlers

OBJECTIVES

The LittlEARS Auditory Questionnaire (LEAQ) has so far been validated to assess auditory development in groups of normal-hearing children in over 20 different languages. Considering the huge variability in auditory development of CI children, especially since candidacy criteria have been relaxed, additional evidence to validate the use of LEAQ scores in this particular population is needed. The aim of this study is to provide evidence for the reliability and validity of LEAQ scores for assessing the auditory development of CI infants and toddlers based on an evaluation of LEAQ's internal structure and its relation to other variables.

METHODS

The study was prospective, with sequential enrolment and within-subject repeated measures. It included 122 children with profound bilateral sensorineural hearing loss implanted at 6-22 months of age. All children were evaluated with the Polish version of LEAQ on the first day of CI activation and at each of four follow-up visits related to sound processor fitting. The study was undertaken in the light of current psychometric thinking about how assessment instruments should be validated. The main aim of the study was to obtain evidence for the validity of interpreting LEAQ measures from CI children in terms of auditory development. First, in order to collect evidence for score reliability and validity based on LEAQ's internal structure, the psychometric properties of LEAQ scores from CI children were determined. A second step was to confirm validity by investigating the effect of concomitant variables on LEAQ scores. Correlations between LEAQ score and duration of hearing aid (HA) use, and between LEAQ score and duration of CI use, were investigated. Additionally, group differences in LEAQ scores between: 1) early and late implanted children; 2) children with long and short HA experience prior to implantation; and 3) children who showed responses over a wide frequency range from using their HAs (prior to implantation) vs those who did not.

RESULTS

On each of the five administrations of LEAQ, the item difficulty indices increased (meaning the items became easier) and over the series they progressively increased with a range of: 0.01-0.62, 0.03-0.92, 0.09-1.00, 0.26-1.00, and 0.52-1.00. At the same time, item-total correlations were in the ranges: 0.09-0.77, 0.26-0.62, 0.00-0.65, 0.00-0.65, and 0.00-0.67. Cronbach's alpha values were above 0.80 for all administrations. A positive correlation between LEAQ score and duration of HA use, and subsequent duration of CI use (hearing experience) was found. When the children were stratified into groups according to age at cochlear implantation, duration of HA use before implantation, and audibility provided by HAs prior to implantation, the differences between the groups were reflected in both their rate of auditory development and their LEAQ score.

CONCLUSION

The interpretation of LEAQ scores from CI children in terms of auditory development was supported by the validity evidence of internal structure and from a logical relationship to other variables. (1) Psychometric properties - item difficulty, item-total correlations, and Cronbach's alpha values - indicate that LEAQ measures are highly consistent and reliably gauge the level of a CI child's auditory development. (2) There was a positive correlation between LEAQ scores and the duration of hearing experience with HAs and a later CI; similarly, there were significant differences between groups of children stratified according to the age at cochlear implantation, duration of HA use before implantation, and audibility provided by HAs prior to implantation, all of which demonstrate the expected relation between LEAQ score and concomitant variables.