Relation of psychophysical data to histopathology in monkeys with cochlear implants

Speech Perception Performance in Cochlear Implant Recipients Correlates to the Number and Synchrony of Excited Auditory Nerve Fibers Derived From Electrically Evoked Compound Action Potentials

OBJECTIVES

Many studies have assessed the performance of individuals with cochlear implants (CIs) with electrically evoked compound action potentials (eCAPs). These eCAP-based studies have focused on the amplitude information of the response, without considering the temporal firing properties of the excited auditory nerve fibers (ANFs), such as neural latency and synchrony. These temporal features have been associated with neural health in animal studies and, consequently, could be of importance to clinical CI outcomes. With a deconvolution method, combined with a unitary response, the eCAP can be mathematically unraveled into the compound discharge latency distribution (CDLD). The CDLD reflects both the number and the temporal firing properties of excited ANFs. The present study aimed to determine to what extent the CDLD derived from intraoperatively recorded eCAPs is related to speech perception in individuals with CIs.

DESIGN

This retrospective study acquired data on monosyllabic word recognition scores and intraoperative eCAP amplitude growth functions from 124 adult patients with postlingual deafness that received the Advanced Bionics HiRes 90K device. The CDLD was determined for each recorded eCAP waveform by deconvolution. Each of the two Gaussian components of the CDLD was described by three parameters: the amplitude, the firing latency (the average latency of each component of the CDLD), and the variance of the CDLD components (an indication of the synchronicity of excited ANFs). Apart from these six CDLD parameters, the area under the CDLD curve (AUCD) and the slope of the AUCD growth function were determined as well. The AUCD was indicative of the total number of excited ANFs over time. The slope of the AUCD growth function indicated the increases in the number of excited ANFs with stimulus level. Associations between speech perception and each of these eight CDLD-related parameters were investigated with linear mixed modeling.

RESULTS

In individuals with CIs, larger amplitudes of the two CDLD components, greater AUCD, and steeper slopes of the AUCD growth function were all significantly associated with better speech perception. In addition, a smaller latency variance in the early CDLD component, but not in the late, was significantly associated with better speech recognition scores. Speech recognition was not significantly dependent on CDLD latencies. The AUCD and the slope of the AUCD growth function provided a similar explanation of the variance in speech perception (R 2 ) as the eCAP amplitude, the slope of the amplitude growth function, the amplitude, and variance of the first CDLD component.

CONCLUSION

The results demonstrate that both the number and the neural synchrony of excited ANFs, as revealed by CDLDs, are indicative of postimplantation speech perception in individuals with a CI. Because the CDLD-based parameters yielded a higher significance than the eCAP amplitude or the AGF slope, the authors conclude that CDLDs can serve as a clinical predictor of the survival of ANFs and that they have predictive value for postoperative speech perception performance. Thus, it would be worthwhile to incorporate the CDLD into eCAP measures in future clinical applications.

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.

Rate modulation detection thresholds for cochlear implant users

The perception of temporal amplitude modulations is critical for speech understanding by cochlear implant (CI) users. The present study compared the ability of CI users to detect sinusoidal modulations of the electrical stimulation rate and current level, at different presentation levels (80% and 40% of the dynamic range) and modulation frequencies (10 and 100 Hz). Rate modulation detection thresholds (RMDTs) and amplitude modulation detection thresholds (AMDTs) were measured and compared to assess whether there was a perceptual advantage to either modulation method. Both RMDTs and AMDTs improved with increasing presentation level and decreasing modulation frequency. RMDTs and AMDTs were correlated, indicating that a common processing mechanism may underlie the perception of rate modulation and amplitude modulation, or that some subject-dependent factors affect both types of modulation detection.

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

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

A physiological and behavioral system for hearing restoration with cochlear implants

Cochlear implants are neuroprosthetic devices that provide hearing to deaf patients, although outcomes are highly variable even with prolonged training and use. The central auditory system must process cochlear implant signals, but it is unclear how neural circuits adapt-or fail to adapt-to such inputs. The knowledge of these mechanisms is required for development of next-generation neuroprosthetics that interface with existing neural circuits and enable synaptic plasticity to improve perceptual outcomes. Here, we describe a new system for cochlear implant insertion, stimulation, and behavioral training in rats. Animals were first ensured to have significant hearing loss via physiological and behavioral criteria. We developed a surgical approach for multichannel (2- or 8-channel) array insertion, comparable with implantation procedures and depth in humans. Peripheral and cortical responses to stimulation were used to program the implant objectively. Animals fitted with implants learned to use them for an auditory-dependent task that assesses frequency detection and recognition in a background of environmentally and self-generated noise and ceased responding appropriately to sounds when the implant was temporarily inactivated. This physiologically calibrated and behaviorally validated system provides a powerful opportunity to study the neural basis of neuroprosthetic device use and plasticity.

Within-subject comparison of word recognition and spiral ganglion cell count in bilateral cochlear implant recipients

OBJECTIVES

Although published reports have not demonstrated a positive correlation between the number of residual spiral ganglion cells (SGCs) and word recognition scores in patients with unilateral multichannel cochlear implants, this study was designed to retest this hypothesis in patients with bilateral multichannel cochlear implants.

MATERIALS AND METHODS

From a pool of 133 temporal bones, all subjects with bilateral multichannel cochlear implants who were deafened bilaterally by the same etiology were studied. A total of 12 temporal bones from 6 subjects were identified and processed after death for histology. The SGCs were counted using standard techniques. The differences between left and right SGC counts as well as the differences in word recognition scores were calculated for each subject. Correlation analysis was performed between the differences of SGC counts and the differences of word recognition scores.

RESULTS

Differences in SGC counts were highly correlated with the differences in word recognition scores (R = 0.934, p = 0.006).

CONCLUSION

This study suggests higher residual SGCs predicted better performance after implantation in a given patient. The results also support attempts to identify factors which may promote survival of SGCs.

Effects of site-specific level adjustments on speech recognition with cochlear implants

OBJECTIVES

Modulation detection thresholds (MDTs) vary across stimulation sites in a cochlear implant (CI) electrode array in a manner that is subject and ear specific. Previous studies have demonstrated that speech recognition with a CI can be improved by site-selection strategies, where selected stimulation sites with poor modulation sensitivity are removed from a subject's processor MAP. Limitations of site-selection strategies are that they can compromise spectral resolution and distort frequency-place mapping because the frequencies assigned to the removed sites are usually reallocated to other sites, and site bandwidths are broadened. The objective of the present study was to test an alternative approach for rehabilitation that aimed at improving the across-site mean MDTs by adjusting stimulation parameters at the poorly performing sites. On the basis of previous findings that modulation detection contributes to speech recognition and improves significantly with stimulus level, the authors hypothesized that modulation sensitivity at the poor sites could be improved by artificially increasing stimulation levels at those sites in the speech processor, which then would lead to improved speech recognition.

DESIGN

Nine postlingually deafened ears implanted with Nucleus CIs were evaluated for MDTs, absolute-detection threshold levels (T levels), and the maximum loudness levels (C levels) on each of the available stimulation sites. For each ear, the minimum stimulation level settings in the speech-processor MAP were raised by 5%, and alternatively by 10%, of the dynamic range (DR) from true thresholds on five stimulation sites with the poorest MDTs. For comparison, a 5% level raise was applied globally to all stimulation sites. The C levels were fixed during these level manipulations. MDTs at the five poorest stimulation sites were compared at 20% DR before and after the level adjustments. Speech-reception thresholds (SRTs), that is, signal to noise ratios required for 50% correct speech recognition, were evaluated for these MAPs using CUNY sentences. The site-specific level-adjusted MAPs were compared with the global-level-adjusted MAP and the MAP without level adjustment. The effects on speech recognition of adjusting the minimal stimulation level settings on the five poorest stimulation sites were also compared with effects of removing these sites from the speech-processor MAP.

RESULTS

The 5% level increase on the five electrodes with the worst MDTs resulted in an improvement in the group mean SRT of 2.36 dB SNR relative to the MAP without level adjustment. The magnitude of level increase that resulted in the greatest SRT improvement for individuals varied across ears. MDTs measured at 20% DR significantly improved on the poor sites after the level adjustment that resulted in the best SRT for that ear was applied. Increasing the minimal stimulation levels on all stimulation sites or removing sites selected for rehabilitation, the parsimonious approaches, did not improve SRTs.

CONCLUSIONS

The site-specific adjustments of the T level settings improved modulation sensitivity at low levels and significantly improved subjects' SRTs. Thus, this site-rehabilitation strategy was an effective alternative to site-selection strategies for improving speech recognition in CI users.

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

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

Psychophysically based site selection coupled with dichotic stimulation improves speech recognition in noise with bilateral cochlear implants

The ability to perceive important features of electrical stimulation varies across stimulation sites within a multichannel implant. The aim of this study was to optimize speech processor MAPs for bilateral implant users by identifying and removing sites with poor psychophysical performance. The psychophysical assessment involved amplitude-modulation detection with and without a masker, and a channel interaction measure quantified as the elevation in modulation detection thresholds in the presence of the masker. Three experimental MAPs were created on an individual-subject basis using data from one of the three psychophysical measures. These experimental MAPs improved the mean psychophysical acuity across the electrode array and provided additional advantages such as increasing spatial separations between electrodes and/or preserving frequency resolution. All 8 subjects showed improved speech recognition in noise with one or more experimental MAPs over their everyday-use clinical MAP. For most subjects, phoneme and sentence recognition in noise were significantly improved by a dichotic experimental MAP that provided better mean psychophysical acuity, a balanced distribution of selected stimulation sites, and preserved frequency resolution. The site-selection strategies serve as useful tools for evaluating the importance of psychophysical acuities needed for good speech recognition in implant users.

Differences between electrically evoked compound action potential (ECAP) and behavioral measures in children with cochlear implants operated in the school age vs. operated in the first years of life

OBJECTIVE

The aim of this study was to identify the differences in the NRT measures, behavioral measures, and their relationship between the group of congenitally deaf children operated in the first years of life and the group of children operated in the school age.

METHODS

The study included 40 congenitally deaf children with cochlear implants divided into two groups. Group 1 was composed of 20 children (mean age at operation 2.3 years, range 1.4-4.6 years) and Group 2 was composed of 20 children (mean age at operation 11.3 years, range 7.0-17.1 years). The ECAP was recorded using the Nucleus 24 neural response telemetry (NRT) system. In each child, the responses were evoked by the apical, middle and basal electrodes. The analyzed parameters were: the ECAP threshold (T-NRT), N1P2 amplitude, N1 latency, slope of the amplitude growth function, response morphology, threshold (T-) level, maximum comfort (C-) level, dynamic range (DR), T-NRT as a percentage of the map DR, the correlation between the T-NRT and the T- and C-levels. The recordings of parameters were performed two years after implantations.

RESULTS

The T-NRT, DR, T-NRT as a percentage of the map DR and the correlation between T-NRT and C-levels were significantly different between both groups of children. There were no statistically significant differences between the groups with respect to the amplitude, latency, slope and morphology recorded using the same electrodes. However, intragroup differences regarding NRT measures and behavioral measures with respect to the position of stimulating electrode were more prominent in Group 2 than in the Group 1.

CONCLUSIONS

Results of this study have also found a great variability of NRT and MAP measures within and across patients in both groups of children, but it was still more pronounced in the group of school children. NRT profile across electrodes follows MAP profiles better in the Group 1 then in the Group 2. Overall findings of NRT and MAP measures are not consistent and unambiguous as we expected, but still suggest potential differences between results in children operated in first years of life, and those operated in school age.

Nerve maintenance and regeneration in the damaged cochlea

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

The relationship between electrically evoked compound action potential and speech perception: a study in cochlear implant users with short electrode array

OBJECTIVES

To determine the extent to which electrically evoked compound action potential (ECAP) measurements were related with speech perception performance in implant users with a short electrode array and to investigate the relationship between ECAP measures and performance according to specific devices.

DESIGN

Prospective study.

SETTING

Tertiary referral center.

PATIENTS

Seventeen Hybrid cochlear implant users were tested in this study. Subjects were divided into 2 groups: 8 using the Nucleus Hybrid M and 9 using the Nucleus Hybrid RE. In addition, 21 Nucleus Freedom long electrode implant (CI24RE) users also were tested to compare with the results of the old device (CI24M).

MAIN OUTCOME MEASURES

ECAP growth functions were recorded using either an interphase gap (IPG) of 8 or 45 mus. We then calculated the slope of the growth function and changes in sensitivity with IPG. For each subject, these measures were compared with performance on tests of word recognition.

RESULTS

The changes in sensitivity using 2 IPGs showed no correlation with the results of word recognition test in Hybrid cochlear implant users. In contrast, relatively strong correlations have been found between the slope of ECAP growth functions and performance on word recognition test. Additionally, when we separate the results of Hybrid M and RE, the slopes of ECAP growth functions from only Hybrid RE CI recipients were significantly correlated with speech performance. The slopes of ECAP growth function in CI24RE users with long electrode also were significantly correlated with performance. However, comparing between 2 independent correlations in RE devices, correlation was higher in Hybrid RE group.

CONCLUSION

The results presented in this article support the view that slope of the ECAP growth can show significant correlation to performance with a cochlear implant. Furthermore, these results suggest that the strength of the correlation may be related to the specific device. These results suggest that ECAP measures may be useful in developing a test to predict outcomes with the implant.

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

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

Stem cell transplantation for auditory nerve replacement

The successful function of cochlear prostheses depends on activation of auditory nerve. The survival of auditory nerve neurons, however, can vary widely in candidates for cochlear implants and influence implant efficacy. Stem cells offer the potential for improving the function of cochlear prostheses and increasing the candidate pool by replacing lost auditory nerve. The first phase of studies for stem cell replacement of auditory nerve has examined the in vitro survival and differentiation as well as in vivo differentiation and survival of exogenous embryonic and tissue stem cells placed into scala tympani and/or modiolus. These studies are reviewed and new results on in vivo placement of B-5 mouse embryonic stem cells into scala tympani of the guinea pig cochleae with differentiation into a glutamatergic neuronal phenotype are presented. Research on the integration and connections of stem cell derived neurons in the cochlea is described. Finally, an alternative approach is considered, based on the use of endogenous progenitors rather than exogenous stem cells, with a review of promising findings that have identified stem cell-like progenitors in cochlear and vestibular tissues to provide the potential for auditory nerve replacement.

Localized cell and drug delivery for auditory prostheses

Localized cell and drug delivery to the cochlea and central auditory pathway can improve the safety and performance of implanted auditory prostheses (APs). While generally successful, these devices have a number of limitations and adverse effects including limited tonal and dynamic ranges, channel interactions, unwanted stimulation of non-auditory nerves, immune rejection, and infections including meningitis. Many of these limitations are associated with the tissue reactions to implanted auditory prosthetic devices and the gradual degeneration of the auditory system following deafness. Strategies to reduce the insertion trauma, degeneration of target neurons, fibrous and bony tissue encapsulation, and immune activation can improve the viability of tissue required for AP function as well as improve the resolution of stimulation for reduced channel interaction and improved place-pitch and level discrimination. Many pharmaceutical compounds have been identified that promote the viability of auditory tissue and prevent inflammation and infection. Cell delivery and gene therapy have provided promising results for treating hearing loss and reversing degeneration. Currently, many clinical and experimental methods can produce extremely localized and sustained drug delivery to address AP limitations. These methods provide better control over drug concentrations while eliminating the adverse effects of systemic delivery. Many of these drug delivery techniques can be integrated into modern auditory prosthetic devices to optimize the tissue response to the implanted device and reduce the risk of infection or rejection. Together, these methods and pharmaceutical agents can be used to optimize the tissue-device interface for improved AP safety and effectiveness.

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

Previous studies have shown large decreases in cochlear implant psychophysical detection thresholds during the weeks following the onset of electrical testing. The current study sought to determine the variables underlying these threshold decreases by examining the effects of four deafening and implantation procedures on detection thresholds and implant impedances. Thirty-two guinea pigs were divided into four matched groups. Group I was deafened and implanted Day 0 and began electrical testing Day 1. Group II was deafened and implanted Day 0 and began electrical testing Day 45. Group III was deafened Day 0, implanted Day 45 and began electrical testing Day 46. Group IV was not predeafened but was implanted Day 0 and began electrical testing Day 1. All groups showed threshold decreases over time but the magnitude of change, time course and final stable threshold levels depended on the type and time course of treatment. Impedances increased over the first two weeks following the onset of electrical testing except in Group II. Results suggest that multiple mechanisms underlie the observed threshold shifts including (1) recovery of the cochlea from a temporary pathology caused by the deafening and/or implantation procedures, (2) effects of electrical stimulation on the auditory pathway, and (3) tissue growth in the implanted cochlea. They also suggest that surviving hair cells influence electrical threshold levels.

Neurotrophic factors and neural prostheses: potential clinical applications based upon findings in the auditory system

Spiral ganglion neurons (SGNs) are the target cells of the cochlear implant, a neural prosthesis designed to provide important auditory cues to severely or profoundly deaf patients. The ongoing degeneration of SGNs that occurs following a sensorineural hearing loss is, therefore, considered a limiting factor in cochlear implant efficacy. We review neurobiological techniques aimed at preventing SGN degeneration using exogenous delivery of neurotrophic factors. Application of these proteins prevents SGN degeneration and can enhance neurite outgrowth. Furthermore, chronic electrical stimulation of SGNs increases neurotrophic factor-induced survival and is correlated with functional benefits. The application of neurotrophic factors has the potential to enhance the benefits that patients can derive from cochlear implants; moreover, these techniques may be relevant for use with neural prostheses in other neurological conditions.

Cochlear implants for DFNA17 deafness

BACKGROUND

Nonsyndromic autosomal-dominant, adult-onset sensorineural hearing loss resulting from DFNA17 was described in a single American kindred in 1997, and the causative gene was subsequently identified as MYH9.

OBJECTIVE

The objective of this study was to report clinical and genetic analyses of an Australian family with nonsyndromic adult-onset sensorineural hearing loss.

METHODS

The clinical presentation of the family was detailed and identification of the causative gene was conducted by SNP genotyping and direct sequencing.

RESULTS

Sequence analysis of the MYH9 gene revealed the same missense mutation as in the original DFNA17 family. We are not aware of a link between the two kindreds, making the present one only the second DFNA17 family to be reported.

CONCLUSIONS

One important point of clinical relevance is the excellent outcome with cochlear implants in the Australian family compared with a "poor" response in the American family. Thus, cochlear implants should be strongly considered for clinical management of patients with DFNA17 deafness.

Is word recognition correlated with the number of surviving spiral ganglion cells and electrode insertion depth in human subjects with cochlear implants?

OBJECTIVES/HYPOTHESIS

Speech perception scores using cochlear implants have ranged widely in all published series. The underlying determinants of success in word recognition are incompletely defined. Although it has been assumed that residual spiral ganglion cell population in the deaf ear may play a critical role, published data from temporal bone specimens from patients have not supported this hypothesis. The depth of insertion of a multichannel cochlear implant has also been suggested as a clinical variable that may be correlated with word recognition. In the current study these correlations were evaluated in 15 human subjects.

STUDY DESIGN

Retrospective review of temporal bone histopathology.

METHODS

Temporal bones were fixed and prepared for histological study by standard techniques. Specimens were then serially sectioned and reconstructed by two-dimensional methods. The spiral ganglion cells were counted, and the depth of insertion of the cochlear implant as measured from the round window was determined. Correlation analyses were then performed between the NU6 word scores and spiral ganglion cell counts and the depth of insertion.

RESULTS

The segmental and total spiral ganglion cell counts were not significantly correlated (P > .50) with NU6 word scores for the 15 subjects. Statistically significant correlations were not achieved by separate analysis of implant types. Similarly, no significant correlation between the depth of insertion of the electrode array and postoperative NU6 word score was identified for the group.

CONCLUSION

Although it is unlikely that the number of residual spiral ganglion cell counts is irrelevant to the determination of word recognition following cochlear implantation, there are, clearly, other clinical variables not yet identified that play an important role in determining success with cochlear implantation.

NGF stimulates extensive neurite outgrowth from implanted dorsal root ganglion neurons following transplantation into the adult rat inner ear

Neuronal tissue transplantation is a potential way to replace degenerated spiral ganglion neurons (SGNs) since these cells cannot regenerate in adult mammals. To investigate whether nerve growth factor (NGF) can stimulate neurite outgrowth from implanted neurons, mouse embryonic dorsal root ganglion (DRG) cells expressing enhanced green fluorescent protein (EGFP) were transplanted into the scala tympani of adult rats with a supplement of NGF or artificial perilymph. DRG neurons were observed in the cochlea for up to 6 weeks postoperatively. A significant difference was identified in the number of DRG neurons between the NGF and non-NGF groups. In the NGF group, extensive neurite projections from DRGs were found penetrating the osseous modiolus towards the spiral ganglion. These results suggest the possibility that embryonic neuronal implants may become integrated within the adult auditory nervous system. In combination with a cochlear prosthesis, a neuronal implantation strategy may provide a possibility for further treatment of profoundly deaf patients.

Modeling the relationship between psychophysical perception and electrically evoked compound action potential threshold in young cochlear implant recipients: clinical implications for implant fitting

OBJECTIVE

In cochlear implant recipients, the threshold of the electrically evoked compound action potential (ECAP) has been shown to correlate with the perceptual detection threshold and maximum comfortable loudness levels (respectively, T- and C-levels) used for implant programming. Our general objective was to model the relationship between ECAP threshold and T/C-levels by taking into account their relative changes within each subject. In particular, we were interested in investigating further the validity of ECAP threshold as a predictor of psychophysical levels, depending on intra-cochlear electrode location and time of testing (from 1 to 18 months post-implantation).

METHODS

A total of 370 ECAP thresholds, measured in 49 children, using a Nucleus 24 cochlear implant, were compared with the corresponding T- and C-levels obtained at the same visit, for the same electrode. Response profiles for the whole group of patients were modeled across four test electrodes spaced equally along the electrode array from base towards apex. A linear regression model was constructed and the quality of the ECAP threshold-based predictions was assessed by testing for correlation between measured and predicted psychophysics. Comparison was made with a more simplistic model (described here as the 'parallel profiles method') stipulating, within each subject, a 1 microA increase in psychophysical levels for every 1 microA increase in ECAP threshold.

RESULTS

Offset between ECAP threshold and psychophysics profiles was found to vary significantly along the electrode array for the T-, but not for the C-level. In contrast with the parallel profiles method, our regression model predicted, within each subject, an average increase of 0.23 microA (95% confidence interval: 0.18-0.28) in T-level for every 1 microA increase in ECAP threshold. This correction improved the quality of T-level prediction when our model was run using measured T-level and ECAP threshold from a reference electrode (r=0.77 vs. r=0.62). The shorter the distance between the electrode for which T-level was predicted and the one used as reference, the stronger the correlation between measured and predicted T-levels. In addition, poorer T-level predictions were obtained at the basal end of the array during the first 3 months post-implantation. In contrast to T-level, individual changes in C-level with ECAP threshold exhibited heterogeneous patterns across subjects so that no common coefficient could account for these changes. However, applying the parallel profiles method led to high-quality C-level prediction.

CONCLUSIONS AND SIGNIFICANCE

The results suggest that covariation between ECAP thresholds and psychophysics plays a decisive role in the relationship of ECAP threshold with T-, but not with C-level. Therefore, our regression model and the parallel profiles method should both be used for predicting, respectively, the T- and the C-levels. Although the predictability of our regression model seems to be better for middle and apical electrodes, its utilization should be extended to basal electrodes after 6 months' implant use.

Survival of neuronal tissue following xenograft implantation into the adult rat inner ear

The poor regenerative capacity of the spiral ganglion neurons of the mammalian inner ear has initiated research on how to assist the functional recovery of the injured auditory system. A possible treatment is to use a biological implant with a potential to establish central or peripheral synaptic contacts to develop into a functional auditory unit. The feasibility of this approach was tested by xenograft implantation of dorsal root ganglion (DRG) neurons from embryonic days 13 to 14, mouse expressing either LacZ or enhanced green fluorescent protein (EGFP) into the scala tympani of the adult rat inner ear. Transplanted DRG neurons survived in the scala tympani for a postoperative period ranging from 3 to 10 weeks, as verified by histochemical detection of LacZ, EGFP fluorescence and immunohistochemical labeling of the neuronal markers neurofilament and Thy 1.2. DRG neurons were found close to structures near the sensory epithelium (the organ of Corti) as well as adjacent to the spiral ganglion neurons with their peripheral dendrites. These results illustrate not only the survival of xenografted DRG neurons in the adult inner ear but also the feasibility of a neuronal transplantation strategy in the degenerated auditory system, thereby creating possibilities to replace spiral ganglion neurons.

Neuronal responses in cat primary auditory cortex to electrical cochlear stimulation: IV. Activation pattern for sinusoidal stimulation

Patterns of threshold distributions for single-cycle sinusoidal electrical stimulation and single pulse electrical stimulation were compared in primary auditory cortex of the adult cat. Furthermore, the effects of auditory deprivation on these distributions were evaluated and compared across three groups of adult cats. Threshold distributions for single and multiple unit responses from the middle cortical layers were obtained on the ectosylvian gyrus in an acutely implanted animal; 2 wk after deafening and implantation (short-term group); and neonatally deafened animals implanted following 2-5 yr of deafness (long-term group). For all three cases, we observed similar patterns of circumscribed regions of low response thresholds in the region of primary auditory cortex (AI). A dorsal and a ventral region of low response thresholds were found separated by a narrow, anterior-posterior strip of elevated thresholds. The ventral low-threshold regions in the short-term group were cochleotopically arranged. By contrast, the dorsal region in the short-term animals and both low-threshold regions in long-term deafened animals maintained only weak cochleotopicity. Analysis of the spatial extent of the low-threshold regions revealed that the activated area for sinusoidal stimulation was smaller and more circumscribed than for pulsatile stimulation for both dorsal and ventral AI. The width of the high-threshold ridge that separated the dorsal and ventral low-threshold regions was greater for sinusoidal stimulation. Sinusoidal and pulsatile threshold behavior differed significantly for electrode configurations with low and high minimum thresholds. Differences in threshold behavior and cortical response distributions between the sinusoidal and pulsatile stimulation suggest that stimulus shape plays a significant role in the activation of cortical activity. Differences in the activation pattern for short-term and long-term deafness reflect deafness-induced reorganizational changes based on factors such as differences in excitatory and inhibitory balance that are affected by the stimulation parameters.

Histopathology of cochlear implants in humans

The insertion of an intrascalar electrode array during cochlear implantation causes immediate damage to the inner ear and may result in delayed onset of additional damage that may interfere with neuronal stimulation. To date, there have been reports on fewer than 50 temporal bone specimens from patients who had undergone implantation during life. The majority of these were single-channel implants, whereas the majority of implants inserted today are multichannel systems. This report presents the histopathologic findings in temporal bones from 8 individuals who in life had undergone multichannel cochlear implantation, with particular attention to the type and location of trauma and to long-term changes within the cochlea. The effect of these changes on spiral ganglion cell counts and the correlation between speech comprehension and spiral ganglion cell counts were calculated. In 4 of the 8 cases, the opposite, unimplanted ear was available for comparison. In 3 of the 4 cases, there was no significant difference between the spiral ganglion cell counts on the implanted and unimplanted sides. In addition, in this series of 8 cases, there was an apparent negative correlation between residual spiral ganglion cell count and hearing performance during life as measured by single-syllable word recognition. This finding suggests that abnormalities in the central auditory pathways are at least as important as spiral ganglion cell loss in limiting the performance of implant users.

Effects of chronic stimulation on auditory nerve survival in ototoxically deafened animals

For almost 10 years, chronic stimulation has been known to affect spiral ganglion cell (SGC) survival in the deaf ear. However, the reported effects of chronic stimulation vary across preparations and studies. In this review, the effects of chronic stimulation on the deafened auditory periphery are examined, and variables that may impact on the efficacy of chronic stimulation are identified. The effects of deafening on the unstimulated peripheral and central auditory system are also described, as the deafened, unstimulated system is the canvas upon which stimulation-mediated effects are imposed. Discrepancies in the effects of chronic stimulation across studies may be attributable in large part to the combined effects of the deafening method and the post-deafening delay prior to chronic stimulation, which vary across studies. Emphasis is placed on the need to consider the natural progression of SGC loss following deafening in the absence of chronic stimulation, as the rate of SGC loss almost certainly affects both the efficacy of stimulation, and the impact of any delay between deafening and initiation of stimulation. The differences across preparations complicate direct comparison of protective efficacy of stimulation. At the same time, these differences can be used to our advantage, aiding characterization of the effects of different factors on the efficacy of chronic stimulation as a neuroprotective intervention.

Electrical cochlear stimulation in the deaf cat: comparisons between psychophysical and central auditory neuronal thresholds

Cochlear prostheses for electrical stimulation of the auditory nerve ("electrical hearing") can provide auditory capacity for profoundly deaf adults and children, including in many cases a restored ability to perceive speech without visual cues. A fundamental challenge in auditory neuroscience is to understand the neural and perceptual mechanisms that make rehabilitation of hearing possible in these deaf humans. We have developed a feline behavioral model that allows us to study behavioral and physiological variables in the same deaf animals. Cats deafened by injection of ototoxic antibiotics were implanted with either a monopolar round window electrode or a multichannel scala tympani electrode array. To evaluate the effects of perceptually significant electrical stimulation of the auditory nerve on the central auditory system, an animal was trained to avoid a mild electrocutaneous shock when biphasic current pulses (0.2 ms/phase) were delivered to its implanted cochlea. Psychophysical detection thresholds and electrical auditory brain stem response (EABR) thresholds were estimated in each cat. At the conclusion of behavioral testing, acute physiological experiments were conducted, and threshold responses were recorded for single neurons and multineuronal clusters in the central nucleus of the inferior colliculus (ICC) and the primary auditory cortex (A1). Behavioral and neurophysiological thresholds were evaluated with reference to cochlear histopathology in the same deaf cats. The results of the present study include: 1) in the cats implanted with a scala tympani electrode array, the lowest ICC and A1 neural thresholds were virtually identical to the behavioral thresholds for intracochlear bipolar stimulation; 2) behavioral thresholds were lower than ICC and A1 neural thresholds in each of the cats implanted with a monopolar round window electrode; 3) EABR thresholds were higher than behavioral thresholds in all of the cats (mean difference = 6.5 dB); and 4) the cumulative number of action potentials for a sample of ICC neurons increased monotonically as a function of the amplitude and the number of stimulating biphasic pulses. This physiological result suggests that the output from the ICC may be integrated spatially across neurons and temporally integrated across pulses when the auditory nerve array is stimulated with a train of biphasic current pulses. Because behavioral thresholds were lower and reaction times were faster at a pulse rate of 30 pps compared with a pulse rate of 2 pps, spatial-temporal integration in the central auditory system was presumably reflected in psychophysical performance.

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

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

Neuronal responses in cat primary auditory cortex to electrical cochlear stimulation. III. Activation patterns in short- and long-term deafness

The effects of auditory deprivation on the spatial distribution of cortical response thresholds to electrical stimulation of the adult cat cochlea were evaluated. Threshold distributions for single- and multiple-unit responses from the middle cortical layers were obtained on the ectosylvian gyrus in three groups of animals: adult, acutely implanted animals ("acute group"); adult animals, 2 wk after deafening and implantation ("short-term group"); adult, neonatally deafened animals ("long-term group") implanted after 2-5 years of deafness. For all three groups, we observed similar patterns of circumscribed regions of low response thresholds in the region of primary auditory cortex (AI). A dorsal and a ventral region of low response thresholds were found separated by a narrow, anterior-posterior strip of elevated thresholds. The two low-threshold regions in the acute and the short-term group were arranged cochleotopically. This was reflected in a systematic shift of the cortical locations with minimum thresholds as a function of cochlear position of the radial and monopolar stimulation electrodes. By contrast, the long-term deafened animals maintained only weak or no signs of cochleotopicity. In some cases of this group, significant deviations from a simple tri-partition of the dorsoventral axis of AI was observed. Analysis of the spatial extent of the low-threshold regions revealed that the activated area in acute cases was significantly smaller than the long- and the short-term cases for both dorsal and ventral AI. There were no significant differences in the rostrocaudal extent of activation between long- and short-term deafening, although the total activated area in the short-term cases was larger than in long-term deafened animals. The width of the narrow high-threshold ridge that separated the dorsal and ventral low-threshold regions was the widest for the acute cases and the narrowest for the short-term deafened animals. The findings of relative large differences in cortical response distributions between the acute and short-term animals suggests that the effects observed in long-term deafened animals are not solely a consequence of loss of peripheral innervation density. The effects may reflect electrode-specific effects or reorganizational changes based on factors such as differences in excitatory and inhibitory balance.

Correlation of acoustic threshold measures and spiral ganglion cell survival in severe to profound sensorineural hearing loss: implications for cochlear implantation

In a temporal bone study of 26 ears from 13 patients who, in life, had severe sensorineural hearing loss, the segmental and total spiral ganglion cell (SGC) counts were correlated with hearing thresholds and with the difference between hearing thresholds in the two ears, the age at death, the duration of deafness, and the duration of hearing loss. A statistically significant correlation was found between the interaural differences in total SGC counts and the interaural difference in pure tone averages for 3, 4, and 5 frequencies. The total SGC count was higher in the ear with the better residual hearing in 11 of 12 cases. Approximately 41% of the variability in interaural difference in pure tone average was explained by the difference in SGC counts. The findings would suggest that in a given individual, selection of the ear with better residual hearing for cochlear implantation is likely to result in accessing a higher number of residual SGCs. This, in turn, may result in better speech recognition with the implant.

Guinea pig auditory neurons are protected by glial cell line-derived growth factor from degeneration after noise trauma

For patients with profound hearing loss, cochlear implants have become the treatment of choice. These devices provide auditory information through direct electrical stimulation of the auditory nerve. Prosthesis function depends on survival and electrical excitability of the cochlear neurons. Degeneration of the auditory nerve occurs after lesions of its peripheral target field (organ of Corti), specifically, including loss of inner hair cells (IHCs). There is now evidence that local treatment of the cochlea with neurotrophins may enhance survival of auditory neurons after aminoglycoside-induced deafness. Glial cell line-derived neurotrophic factor (GDNF) has recently been shown to be an important survival factor in other regions of the nervous system. By in situ hybridization, we now show that IHCs of the neonatal and mature rat cochlea synthesize GDNF and that GDNF-receptor alpha, but not c-Ret, is expressed in the rat spiral ganglion. We also show that GDNF is a potent survival-promoting factor for rat cochlear neurons in vitro. Finally, we examined GDNF efficacy to enhance cochlear-nerve survival after IHC lesions in vivo. We found that chronic intracochlear infusion of GDNF greatly enhances survival of guinea pig cochlear neurons after noise-induced IHC lesions. Our results demonstrate that GDNF is likely to be an endogeneous survival factor in the normal mammalian cochlea and it could have application as a pharmacological treatment to prevent secondary auditory nerve degeneration following organ of Corti damage.

Effects of electrode configuration on psychophysical strength-duration functions for single biphasic electrical stimuli in cats

The interface between electrode and neural target tissue is thought to influence certain characteristics of neural and behavioral responses to electrical stimulation of the auditory system. At present, the biophysical properties of this interface are not well understood. Here the effects of biphasic phase duration and electrode configuration on psychophysical threshold in response to electrical stimulation in cats are described. Five cats were trained to respond to acoustic stimuli using food as a reward in an operant reinforcement paradigm. After training, the animals were unilaterally deafened and implanted with a multicontact intracochlear electrode array. Thresholds for single presentations of biphasic current pulses were measured as a function of phase duration and electrode arrangement. Statistical analyses of the data indicated that strength-duration function slopes between 200 and 1600 microseconds/phase were significantly different for the different electrode configurations and, overall, were unrelated to the absolute level of the strength-duration function (i.e., were independent of absolute threshold). For all subjects, the slope of this function for intermediate pulse durations was dependent on electrode configuration and most shallow for radial-bipolar configurations (-3.4 dB/doubling), was steepest for monopolar arrangements (-5.9 dB/doubling), and was intermediate for longitudinal-bipolar pairings. (-4.4 dB/doubling). Slopes for both shorter and longer phase duration stimuli were not significantly different. The underlying mechanisms for these effects may include, or be a combination of altered electrical field patterns, integrated activity across multiple fibers, and stochastic behavior of individual auditory neurons to electrical stimulation.

Neurotrophins can enhance spiral ganglion cell survival after inner hair cell loss

Following destruction of sensory cells of the organ of Corti, spiral ganglion cells (SGC) in the guinea pig degenerate. Chronic electrical stimulation via cochlear prostheses can enhance their survival, with the effect blocked by stopping the electrically elicited action potentials with tetrodotoxin. Blocking action potentials in the normal hearing ear with tetrodotoxin, however, does not cause degeneration. This suggests that in the pathological ear VIII N activity acts as a survival factor, while in the normal ear there are other survival factors that maintain SGCs. We examined neurotrophins, as survival factors in the deafened ear. Two weeks of treatment with BDNF (brain derived neurotrophic factor) administered chronically via a mini-osmotic pump into scala tympani at 50 ng/ml, provided a statistically significant enhanced SGC survival over untreated deafened ears or deafened ears treated with artificial perilymph. Neurotrophin 3 provided some enhanced survival, but this was not statistically significant over untreated deafened ears. These observations suggest there are survival factors in the inner ear, including those coupled to direct activation of the auditory nerve fibers, that may serve to maintain the auditory nerve. These factors may be applied following deafness to maintain and enhance neural populations and to increase benefits to the profoundly deaf receiving cochlear implants.

Psychometric functions and temporal integration in electric hearing

Temporal-integration functions and psychometric functions for detection were obtained in eight users of the Nucleus 22-electrode cochlear implant. Stimuli were 100-Hz, 200-microseconds/phase trains of biphasic pulses with durations ranging from 0.44 to 630.4 ms (1 to 64 pulses). Temporal-integration functions were measured for 21 electrodes. Slopes of these functions were considerably shallower than the 2.5 dB/doubling slopes typically observed in acoustic hearing. They varied widely across subjects and for different electrodes in a given subject, ranging from 0.06 to 1.94 dB/doubling of stimulus pulses, with a mean [standard deviation (s.d.)] value of 0.42 (0.38). Psychometric functions were measured for 11 of the same 21 electrodes. Slopes of psychometric functions also varied across subjects and electrodes, and were 2-20 times steeper than those reported by other investigators for normal-hearing and cochlear-impaired acoustic listeners. Slopes of individual psychometric functions for 1-, 2-, 4-, and 8-pulse stimuli ranged from 0.20 to 1.84 log d'/dB with a mean (s.d.) value of 0.77 (0.45). Psychometric-function slopes did not vary systematically with stimulus duration in most cases. A clear inverse relation between slopes of psychometric functions and slopes of temporal-integration functions was observed. This relation was reasonably well described by a hyperbolic function predicted by the multiple-looks model of temporal integration [Viemeister and Wakefield, J. Acoust. Soc. Am. 90, 858-865 (1991)]. Psychometric-function slopes tended to increase with absolute threshold and were inversely correlated with dynamic range, suggesting that observed differences in psychometric-function slopes across subjects and electrodes may reflect underlying differences in neural survival.

Effects of chronic high-rate electrical stimulation on the cochlea and eighth nerve in the deafened guinea pig

This study was undertaken to examine the effects of chronic high-rate stimulation on the eighth nerve and cochlea. Fifty-four male pigmented guinea pigs were deafened and implanted with single ball electrodes in scala tympani. Four groups of animals received chronic electrical stimulation at a level of 5 microCol/cm2/ph for 1000 h as follows: Group A: 1000 Hz, 100 microseconds/ph duration, 100 microA peak; Group B: 250 Hz, 100 microseconds/ph duration, 100 microA peak; Group C: 2750 Hz, 36 microseconds/ph duration, 250 microA peak; Group D: 250 Hz, 400 microseconds/ph duration, 25 microA peak. Also, two control groups received 20 min stimulation during weekly electrically evoked auditory brainstem response (eABR) measurement (Group E) and about 5 s stimulation (Group F) during a brief eABR 3 day postimplantation and at perfusion. On Day 50, animals were perfused, midmodiolar sections cut and a quantitative assessment of spiral ganglion cells (SGC) performed. All stimulated subjects showed a similar decrease in eABR thresholds and dynamic range over time. No stimulation conditions induced pathology. All stimulation conditions enhanced survival of SGCs compared to unimplanted ears and implanted non-stimulated ears (Group F). There were no statistically significant differences in SGC survival between any stimulated groups, including Group E stimulated once a week. In conclusion, high-rate stimulation, under the conditions of this study, provides no additional risks and the same benefits to SGC survival as low-rate stimulation.

Effect of chronic electrical stimulation on cochlear nucleus neuron size in normal hearing kittens

Very young cochlear-implant candidates may have undetected islands of residual hearing. Would the maturation of these functioning auditory neurons be affected by chronic cochlear stimulation? This was tested by examining neuron sizes in the cochlear nuclei of young, normal hearing kittens with and without chronic cochlear stimulation. Six animals received bilateral intra- or extracochlear implants and were electrically stimulated unilaterally for periods of 1,000-1,500 hours. After sacrifice, cross-sectional areas of approximately 11,000 neurons somata in the cochlear nuclei were measured with an image-analysis system. There were statistically significant differences between stimulated and unstimulated nuclei, especially the posteroventral cochlear nucleus (PVCN), in individual cats, but the directions of the differences were inconsistent. Overall, there was no significant effect of electrical stimulation on soma size. These results indicate that chronic electrical stimulation of the auditory nerve has no positive or negative trophic effects on otherwise innervated, maturing cochlear nucleus neurons.

Protective effect of electrical stimulation in the deafened guinea pig cochlea

The effect of chronic intrascalar electrical stimulation on the spiral ganglion cell survival of the ototoxically deafened guinea pig was investigated. Immediately after ototoxic drug administration, unilateral sinusoidal (1 kHz) charge-balanced electrical stimulation on a 50% duty cycle was administered for 2 hours per day, 5 days per week, at intensities from 0 (control) to 400 microAmp via an implanted scala tympani electrode. The relationship of electrically evoked middle latency response (EMLR) to stimulation protocol and cell survival was studied. At 9 weeks post-drug treatment, the animals were killed and temporal bones were prepared for morphometric analysis of spiral ganglion cell density. The subjects showed essentially complete elimination of outer hair sensory cells, with minimal remaining inner hair cells confined to apical turns. Variable loss of spiral ganglion cell populations was observed, which related to electrical stimulation. In animals that received daily unilaterally electrical stimulation, statistically significant increases in survival of spiral ganglion cells were observed in the stimulated ear, compared to the nonstimulated ear-particularly in basal cochlear regions near the electrode. Spiral ganglion cell density was a function of stimulation current intensity level. Moreover, the slope of the amplitude input/output (I/O) function of the EMLR was found to be dependent on stimulating current level. The effect of stimulation on induced survival may be dependent on a number of mechanisms, including metabolic effects of direct activation of "deafferented" spiral ganglion cells. These data support the suggestion that implantation may provide optimal benefits when performed shortly after deafness.

Intensity operating range measures as predictors of word-recognition ability in cochlear implant subjects

The purposes of the experiment were to examine the appropriateness of pre- and post-implant intensity measures (thresholds, most comfortable listening level, and loudness discomfort level) as predictors of post-implant phoneme-recognition ability and to study the relationship between pre- and post-implant intensity measures. Pre-implant intensity measures were obtained on 16 subjects who were eventually implanted with either a Nucleus device (n = 8) or a Symbion device (n = 8). Phoneme scores on a NU-6 word list were obtained on these 16 subjects at 1 month post-implant. Post-implant intensity measures were also made on the 8 Symbion subjects at 1 month post-implant. The results showed that none of the pre-implant intensity measures correlated significantly with post-implant phoneme scores. In addition, pre-implant intensity measures did not correlate with the same post-implant intensity measures. However, post-implant MCLs and LDLs correlated significantly with phoneme scores as reflected by correlation coefficients that were larger than 0.8. These preliminary results suggest that although intensity measures may relate to phoneme-recognition ability, their use as predictive measures (as in pre-implant measures) for post-implant ability is questionable.

Evaluation of eighth nerve integrity by the electrically evoked middle latency response

A reliable objective test for estimating the number and distribution of surviving eighth nerve fibers needs to be identified for selection of candidates for cochlear implantation. Kanamycin and ethacrynic acid administration in guinea pigs resulted in graded amounts of eighth nerve degeneration over time. The electrically-induced middle latency response (EMLR) was acutely recorded in these animals at specific post-drug times, followed by the immediate killing of the animals, histologic preparation, and spiral ganglion cell density determination. Significant progressive spiral ganglion cell loss was noted by 4 weeks that increased over time. While EMLR threshold remained stable over time, the slope of the EMLR input/output function decreased with increasing post-drug intervals in a manner directly correlated with reduction in spiral ganglion cell density.

Threshold functions for electrical stimulation of the human cochlear nucleus

Thresholds to sinusoidal and biphasic pulsatile electrical stimuli were measured in two patients with electrodes positioned on the cochlear nucleus. The threshold functions differ from those observed in patients with scala tympani electrodes, primarily at low sinusoidal frequencies and long pulse widths. This difference is probably due to differences in the biophysical properties of the stimulated neural tissues in the two regions.

Electrically stimulated increases in cochlear blood flow: I. Frequency and intensity effects

Charge-balanced, sinusoidal current was passed differentially between the apex and round window of the guinea pig cochlea. Cochlear blood flow was measured using a laser Doppler flow monitor. Systemic blood pressure was monitored from a cannula within the common carotid artery. Electrical stimulation increased cochlear blood flow, while systemic blood pressure was unaffected. A cochlear blood flow response parameter, normalized for transient changes in systemic blood pressure, was defined. The magnitude of the response parameter was found to be frequency selective and was also found to be an increasing function of current intensity, with maximum responses obtained with 500 Hz sinusoids. This cochlear blood flow response was not observed in dead animals; was present in preparations paralyzed with gallamine hydrochloride; and was correlated with an increase in cochlear red blood cell velocity, as directly observed by intravital microscopy. These observations imply that electrical stimulation induces a local vasodilation within the temporal bone. The fact that decreased cochlear blood flow was never observed with current injection implies that ischemia is not a likely mechanism of electrically induced tissue damage within the inner ear. The mechanism of this cochlear blood flow response is addressed in a companion report.

Pathology as it relates to ear surgery. VI. Cochlear implantation

The surgical anatomy and pathology of the cochlea have been reviewed in relation to cochlear implant surgery. Animal experimentation, as well as human temporal bone studies, have shown that the implant electrodes were well tolerated by the cochlea. The possible chemical and mechanical trauma induced by the electrodes can be avoided by better choice of shape, size, length and material of the implants. Long-term electrical stimulation did not seem to cause any deleterious effects on the neuronal population of the cochlea. In the present state of the art, cochlear implantation seems justified in well chosen cases.

Brainstem auditory pathway degeneration associated with chronic cochlear implants in the monkey

The form and pattern of first-order and transsynaptic degeneration in the central auditory pathway was studied in monkeys following inner ear stimulation by a cochlear implant. Multielectrode, scala tympani, and modiolar systems were implanted; in some cases, neomycin was perfused into the cochlea to destroy the organ of Corti at the time of implantation. The monkeys were maintained chronically for 5 to 120 weeks, then the cochleas and brainstems were examined histologically. The extent of spiral ganglion cell loss across animals showed variability, reflecting the different procedures and devices used. The degree and distribution of spiral ganglion cell loss was related to the degree and distribution of neural degeneration seen in the cochlear nucleus in all cases. Peripheral damage progressed toward the cochlear apex as survival time increased, and this progression was reflected in the cochlear nucleus by a ventrolateral shift in the locus of degeneration over time. In addition, evidence for transneuronal degeneration was seen at the superior olive, the lateral lemniscus and the inferior colliculus. Our findings indicate that several factors inherent in the use of a cochlear prosthesis, i.e., insertion trauma, host reaction, and/or electrical stimulation, may be associated with a long-term, continuing process of central degeneration visible at several levels of the auditory system.

Long-term intracochlear implantation in man

Whether long-term intracochlear implantation and direct electrical stimulation of the acoustic nerve will induce intracochlear bone growth or cause further degeneration of a severely compromised auditory system is an important clinical consideration. Thin-section CT evaluations of the cochleas of six subjects who have used their cochlear implant devices on a daily basis for 3 or more years demonstrated no evidence of osteoneogenesis of the cochlea in the vicinity of the active electrode. No corrosion of the electrode or insulation material was noted on electron microscopy of an explanted electrode system. Electrical threshold and dynamic range measurements have remained stable or even improved during the period of observation. Performance measures using a variety of audiologic tests and speech-tracking scores have demonstrated stability of performance.