Prediction of behavioral threshold and comfort values for Nucleus 22-channel implant patients from electrical auditory brain stem response test results

Clinical implications of intraoperative eABRs to the Evo®-CI electrode array recipients

INTRODUCTION

Electrically evoked auditory brainstem responses provide reliable clinical information to assist professionals in the auditory rehabilitation of cochlear implant users.

OBJECTIVE

This study aimed to investigate intraoperative evoked auditory brainstem response recordings in Evo®-cochlear implant electrode array recipients and its correlation with their behavioral levels and auditory performance.

METHODS

This is a retrospectivey study. Intraoperative evoked auditory brainstem responses were recorded in adult Evo®-cochlear implant electrode array recipients. Wave V latencies, amplitudes and interpeak III-V intervals were recorded in three different electrode locations and compared to the sentence recognition scores obtained from subjects after six months of device use. Evoked auditory brainstem responses thresholds were also recorded and compared to the behaviorally determined levels of the subjects in the sound processor activation.

RESULTS

Evoked auditory brainstem responses thresholds were significantly correlated with both, behavioral T- and C-levels and they were recorded at audible electrical stimulation levels in all subjects. There was a significant correlation between interpeak III-V interval recorded in the apical electrode and sentence recognition scores of the subjects.

CONCLUSIONS

Intraoperative evoked auditory brainstem responses can be used to establish audible levels for fitting the sound processor in Evo®-cochlear implant recipients and it could help professionals to plan further actions aiming to improve their auditory performance.

Extracochlear Stimulation of Electrically Evoked Auditory Brainstem Responses (eABRs) Remains the Preferred Pre-implant Auditory Nerve Function Test in an Assessor-blinded Comparison

OBJECTIVE

Electrically evoked auditory brainstem responses (eABRs) can be recorded before cochlear implant (CI) surgery to verify auditory nerve function, and is particularly helpful in to assess the function of the auditory nerve in cases of auditory nerve hypoplasia. This is the first study to compare three preimplant eABRs recording techniques: 1) standard extracochlear, 2) novel intracochlear, and 3) conventional intracochlear with the CI.

STUDY DESIGN

A within-participants design was used where eABRs were sequentially measured during CI surgery using three methods with stimulation from: 1) an extracochlear electrode placed at the round window niche, 2) two different electrodes on a recently developed Intracochlear Test Array (ITA), and 3) two different electrodes on a CI electrode array.

SETTING

New adults implantees (n = 16) were recruited through the Manchester Auditory Implant Centre and eABR measurements were made in theater at the time of CI surgery.

PATIENTS

All participants met the clinical criteria for cochlear implantation. Only participants with radiologically normal auditory nerves were recruited to the study. All participants were surgically listed for either a MED-EL Synchrony implant or a Cochlear Nucleus Profile implant, per standard practice in the implant centre.

OUTCOME MEASURES

Primary outcome measures were: 1) charge (μC) required to elicit a threshold response, and 2) latencies (ms) in the threshold waveforms. Secondary outcome measures were: 1) morphologies of responses at suprathreshold stimulation levels and 2) wave V growth patterns.

RESULTS

eABRs were successfully measured from 15 participants. In terms of primary outcome measures, the charge required to elicit a response using the extracochlear electrode (median = 0.075 μC) was approximately six times larger than all other electrodes and the latency of wave V was approximately 0.5 ms longer when using the extracochlear electrode (mean = 5.1 ms). In terms of secondary outcomes, there were some minor quantitative differences in responses between extracochlear and intracochlear stimulation; in particular, ITA responses were highly variable in quality. The ITA responses were rated poor quality in 33% of recordings and in two instances did not allow for data collection. When not disrupted by open circuits, the median ITA response contained one more waveform than the median extracochlear response.

CONCLUSIONS

In this first study comparing intracochlear and extracochlear stimulation, the results show that both can be used to produce an eABR that is representative of the one elicited by the CI. In the majority of cases, extracochlear stimulation was the preferred approach for preimplant auditory nerve function testing because of consistency, recordings that could be analyzed, and because extracochlear placement of the electrode does not require a cochleostomy to insert an electrode.

Electrically-evoked auditory steady-state responses as neural correlates of loudness growth in cochlear implant users

Loudness growth functions characterize how the loudness percept changes with current level between the threshold and most comfortable loudness level in cochlear implant users. Even though loudness growth functions are highly listener-dependent, currently default settings are used in clinical devices. This study investigated whether electrically-evoked auditory steady-state response amplitude growth functions correspond to behaviorally measured loudness growth functions. Seven cochlear implant listeners participated in two behavioral loudness growth tasks and an EEG recording session. The 40-Hz sinusoidally amplitude-modulated pulse trains were presented to CI channels stimulating at a more apical and basal region of the cochlea, and were presented at different current levels encompassing the listeners' dynamic ranges. Behaviorally, loudness growth was measured using an Absolute Magnitude Estimation and a Graphic Rating Scale with loudness categories. A good correspondence was found between the response amplitude functions and the behavioral loudness growth functions. The results are encouraging for future advances in individual, more automatic, and objective fitting of cochlear implants.

Population-based prediction of fitting levels for individual cochlear implant recipients

OBJECTIVES

This study analyzed the predictability of fitting levels for cochlear implant recipients based on a review of the clinical levels of the recipients.

DESIGN

Data containing threshold levels (T-levels) and maximum comfort levels (M-levels) for 151 adult subjects using a CII/HiRes 90K cochlear implant with a HiFocus 1/1 J electrode were used. The 10th, 25th, 50th, 75th and 90th percentiles of the T- and M-levels are reported. Speech perception of the subjects, using a HiRes speech coding strategy, was measured during routine clinical follow-up.

RESULTS

T-levels for most subjects were between 20 and 35% of their M-levels and were rarely (<1/50) below 10% of the M-levels. Furthermore, both T- and M-levels showed an increase over the first year of follow-up. Interestingly, levels expressed in linear charge units showed a clear increase in dynamic range (DR) over 1 year (29.8 CU; SD 73.0), whereas the DR expressed in decibels remained stable. T-level and DR were the only fitting parameters for which a significant correlation with speech perception (r = 0.34, p < 0.01, and r = 0.33, p < 0.01, respectively) could be demonstrated. Additionally, analysis showed that T- and M-level profiles expressed in decibels were independent of the subjects' across-site mean levels. Using mixed linear models, predictive models were obtained for the T- and M-levels of all separate electrode contacts.

CONCLUSIONS

On the basis of the data set from 151 subjects, clinically applicable predictive models for T- and M-levels have been obtained. Based on one psychophysical measurement and a population-based T- or M-level profile, individual recipients' T- and M-levels can be approximated with a closed-set formula. Additionally, the analyzed fitting level data can serve as a reference for future patients.

Accuracy of measurement in electrically evoked compound action potentials

BACKGROUND

Electrically evoked compound action potentials (ECAP) in cochlear implant (CI) patients are characterized by the amplitude of the N1P1 complex. The measurement of evoked potentials yields a combination of the measured signal with various noise components but for ECAP procedures performed in the clinical routine, only the averaged curve is accessible. To date no detailed analysis of error dimension has been published. The aim of this study was to determine the error of the N1P1 amplitude and to determine the factors that impact the outcome.

NEW METHODS

Measurements were performed on 32 CI patients with either CI24RE (CA) or CI512 implants using the Software Custom Sound EP (Cochlear). N1P1 error approximation of non-averaged raw data consisting of recorded single-sweeps was compared to methods of error approximation based on mean curves.

RESULTS

The error approximation of the N1P1 amplitude using averaged data showed comparable results to single-point error estimation. The error of the N1P1 amplitude depends on the number of averaging steps and amplification; in contrast, the error of the N1P1 amplitude is not dependent on the stimulus intensity.

COMPARISON WITH EXISTING METHOD(S)

Single-point error showed smaller N1P1 error and better coincidence with 1/√(N) function (N is the number of measured sweeps) compared to the known maximum-minimum criterion.

CONCLUSION

Evaluation of N1P1 amplitude should be accompanied by indication of its error. The retrospective approximation of this measurement error from the averaged data available in clinically used software is possible and best done utilizing the D-trace in forward masking artefact reduction mode (no stimulation applied and recording contains only the switch-on-artefact).

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.

Advantages of Cochlear Implantation in Prelingual Deaf Children before 2 Years of Age when Compared with Later Implantation

OBJECTIVES

To compare the auditory abilities and speech performance of children with a profound prelingual bilateral hearing-impairment when subjected to a cochlear implant (CI) before or after 2 years of age. To analyze the complications that arose during, or as a result of, the implantation process in these groups.

DESIGN

Prospective cohort single-subject, repeated-measures study of children with profound bilateral hearing impairment subjected to CI.

SETTING

Tertiary referral center with a program of pediatric CI from 1991.

PATIENTS

This study analyzed 130 children subjected to multichannel CI for profound prelingual bilateral hearing-impairment in two age groups: 0 to 2 (n = 36) and 2 to 6 years of age (n = 94).

INTERVENTIONS

The children were evaluated before, and each year after, the intervention (for up to 5 years) with both closed-set and open-set auditory and speech perception tests. Their speech ability was evaluated according to the Peabody Picture Vocabulary and Reynell general oral expression scales.

RESULTS

Auditory and speech perception tests improved significantly in all children after CI, regardless of the follow-up time. The infant's performance was better the earlier the implant was performed. Speech tests showed that the development of children treated before 2 years of age was similar to normal children, and no additional complications were observed when compared with CI in older children.

CONCLUSIONS

When performed before 2 years of age, CI offers a quicker and better improvement of performance without augmenting the complications associated with such an intervention.

Electrophysiologic Effects of Placing Cochlear Implant Electrodes in a Perimodiolar Position in Young Children

OBJECTIVE

The purpose of this study was to intraoperatively record the electrically evoked auditory brainstem response (EABR) before and after placement of the electrode positioning system (EPS) (CII Bionic Ear with HiFocus I cochlear implant electrode array) as well as before and after stylet removal (Nucleus Contour cochlear implant electrode array). It was hypothesized that physiologic changes would occur after perimodiolar positioning of the electrode array and these changes would be evident from the EABR recordings.

STUDY DESIGN

Consecutive young (11-36 month old) pediatric cochlear implant recipients (n = 17) had intraoperative EABRs recorded from three intracochlear electrodes that represented apical, medial, and basal locations. Wave V amplitudes and thresholds were studied relative to electrode location and pre- versus postperimodiolar positioning. These evoked potential measures were analyzed for statistical significance.

SETTING

Tertiary referral children's hospital/medical college.

RESULTS

Wave V thresholds of the EABR were lower, and amplitudes were larger after perimodiolar positioning, although the changes were dependent on electrode location and implant design. Statistically significant decreases in EABR wave V threshold and increases in suprathreshold wave V amplitude were found for the basal electrode for the CII Bionic Ear HiFocus I and for the apical electrode for the Nucleus Contour.

CONCLUSIONS

Placement of either the CII Bionic Ear HiFocus I or Nucleus Contour cochlear implant electrode array in the perimodiolar position in young children resulted in less electrical current necessary to stimulate the auditory system. Changes in electrophysiologic thresholds and amplitudes, measured with EABR, indicate that the electrode array is placed closer to the modiolus with both electrode designs.

Clinical uses of electrically evoked auditory nerve and brainstem responses

PURPOSE OF REVIEW

The purpose of this review is to summarize current thinking relative to clinical applications for the electrically evoked compound action potential and the electrically evoked auditory brainstem response with the focus on works published between 1998 and 2003.

RECENT FINDINGS

During the period of this review, a considerable body of research has been published describing how the electrically evoked compound action potential can be measured and how the electrically evoked compound action potential may be used in the clinical treatment of cochlear implant patients. During this same period, there has been a decline in the number of studies reporting potential clinical applications for the electrically evoked auditory brainstem response. Perhaps the strongest clinical application for the electrically evoked compound action potential today is as a tool to facilitate the process of fitting the speech processor of the cochlear implant. This is particularly important for pediatric patients who may not be able to be programmed using traditional behavioral techniques. The accuracy of these predictions is discussed in addition to the limitations of the studies reviewed.

SUMMARY

The introduction of cochlear implants with the capability of measuring the response of the auditory nerve to electrical stimulation from an intracochlear electrode has tremendous potential to impact clinical practice. Research into how best to use this information is ongoing.

Electrically evoked auditory brain stem responses for lateral and medial placement of the Clarion HiFocus electrode

OBJECTIVE

The purpose of this study was to compare the electrically evoked auditory brain stem response (EABR) for lateral and medial placement of the Clarion HiFocus cochlear implant electrode array via the electrode positioning system (EPS).

DESIGN

Twenty-five adult and pediatric cochlear implant recipients participated in the study. Intraoperatively recorded EABRs were evoked by stimuli via three intracochlear electrodes representing apical, medial, and basal locations, and responses were elicited before and after positioner insertion. Evoked potential measures of wave V amplitude and threshold were examined for statistical significance using ANOVA for repeated measures and Chi-Square methods.

RESULTS

For a given supra-threshold stimulus level, the increase in EABR wave V amplitude was significantly larger after EPS placement compared to before EPS placement for electrodes 1 (apical) and 13 (basal). Likewise, when the stimulus was decreased to obtain a minimal amplitude, the wave V threshold was significantly lower after EPS placement for electrodes 7 (medial) and 13. The number of measurements that showed decreased wave V threshold after EPS insertion was significantly dependent on intracochlear electrode location.

CONCLUSIONS

Placement of the Clarion Electrode Positioning System following HiFocus electrode insertion resulted in a reduction in the electrical current required to activate the auditory system. The effect of the EPS was greatest for the basal location, demonstrated by lower wave V thresholds and a larger percentage increase in wave V amplitude. The EABR reflected electrophysiologic changes relative to lateral-to-medial changes in intracochlear electrode position due to the EPS.

Neurophysiology of cochlear implant users I: effects of stimulus current level and electrode site on the electrical ABR, MLR, and N1-P2 response

OBJECTIVE

As the need for objective measures with cochlear implant users increases, it is critical to understand how electrical potentials behave when stimulus parameters are systematically varied. The purpose of this study was to record and evaluate the effects of implanted electrode site and stimulus current level on latency, amplitude, and threshold measures of electrically evoked auditory potentials, representing brainstem and cortical levels of the auditory system.

DESIGN

The electrical auditory brainstem response (EABR), electrical auditory middle latency response (EAMLR), and the electrical late auditory response (ELAR) were recorded from the same experimental subjects, 11 adult Clarion cochlear implant users. The Waves II, III, and V of the EABR, the Na-Pa complex of the EAMLR and the N1-P2 complex of the ELAR were investigated relative to electrode site (along the intra-cochlear electrode array) and stimulus current level. Evoked potential measures were examined for statistical significance using analysis of variance (ANOVA) for repeated measures.

RESULTS

For the EABR, Wave V latency was significantly longer for the basal electrode (7) compared with the mid (4) and apical (1) electrodes. For the EAMLR and ELAR, there were no significant differences in latency by electrode site. For all subjects and each of the evoked potentials, the apical electrodes tended to have the largest amplitude and the basal electrodes the smallest amplitude, although amplitude differences did not reach statistical significance. In general, decreases in stimulus current level resulted in statistically significant decreases in the amplitude of Wave V, Na-Pa and N1-P2. The evoked potential thresholds for Wave V, Na-Pa, and N1-P2 were significantly higher for the basal Electrode 7 than for Electrodes 4 and 1.

CONCLUSIONS

Electrophysiologic responses of Waves II, III, and V of the EABR, Na-Pa of the EAMLR, and N1-P2 of the ELAR were characterized as functions of current level and electrode site. Data from this study may serve as a normative reference for expected latency, amplitude and threshold values for the recording of electrically evoked auditory brainstem and cortical potentials. Responses recorded from cochlear implant users show many similar patterns, yet important distinctions, compared with auditory potentials elicited with acoustic signals.

Initial evaluation of the Clarion CII cochlear implant: speech perception and neural response imaging

OBJECTIVE

To evaluate the new Clarion CII cochlear implant with the perimodiolar HiFocus electrode array, including both speech perception outcomes and the device's capabilities of measuring the electrically evoked compound action potential (eCAP) of the auditory nerve (Neural Response Imaging, NRI).

DESIGN

The speech perception scores on CVC words without lip reading were monitored prospectively for the 10 postlingually deaf patients implanted with the Clarion CII device in the period July 2000 until May 2001 in the Leiden University Medical Center. Preoperative and postoperative NRI recordings were made, applying various combinations of monopolar stimulating and recording electrodes with the alternating polarity paradigm available in the test bench software.

RESULTS

Nine patients preferred the CIS, one the PPS strategy, none the SAS strategy. With their favorite strategy they acquired significant open set speech understanding within a few weeks, resulting in an average CVC phoneme score of 84% (word score 66%) at the end of the study (follow-up 3 to 11 mo). In speech-shaped noise, the average phoneme recognition threshold (PRT) was reached at a signal to noise ratio just below 0 dB. The NRI recordings had clear N1 and P1 peaks if there was at least one contact between the stimulating and recording electrodes, necessitating just 15 sweeps for a reliable recording. We observed considerable inter-patient and inter-electrode variability, but for a given situation NRI input/output curves were stable over time. More apical contacts generally elicited larger eCAPs. Response amplitudes tended to peak at recording sites around apical and basal stimulating electrodes, suggesting a limited spread of excitation. Preliminary recordings with the forward masking paradigm were consistent with the ones with the alternating polarity scheme.

CONCLUSIONS

The Clarion CII is a promising cochlear implant with which our first 10 patients have obtained excellent speech perception results. The NRI system yields high quality signals with a limited number of sweeps at a high sampling rate.

Neural response telemetry in 12- to 24-month-old children

The minimum age for cochlear implantation has been reduced to 12 months in an effort to provide auditory stimulation to children with hearing loss during early development. Because behavioral measures in such young children are limited, objective measures such as the electrically evoked compound action potential (EAP) from the auditory nerve are needed to facilitate measurement of stimulation level requirements. We assessed EAPs recorded by the Nucleus 24 neural response telemetry (NRT) system in children who underwent implantation between 12 and 24 months of age. We recorded EAPs in 37 such children (mean age at implantation, 18.1+/-3.6 months). The EAPs were of large amplitude, and thresholds fell between behavioral T and C levels. A correction factor applied to EAP thresholds provided useful predictions of T levels. The EAPs can be used to ensure that even very young children receive auditory stimulation with their cochlear implants upon device activation.

EAP recordings in ineraid patients--correlations with psychophysical measures and possible implications for patient fitting

OBJECTIVE

Objective measurements can be helpful for cochlear implant fitting of difficult populations, as for example very young children. One method, the recording of the electrically evoked compound action potential (EAP), measures the nerve recruitment in the cochlea in response to stimulation through the implant. For coding strategies implemented at a moderate stimulation rate of 250 pps per channel, useful correlations between EAP data and psychophysical data have been already found. With new systems running at higher rates, it is important to check these correlations again.

DESIGN

This study investigates the correlations between psychophysical data and EAP measures calculated from EAP amplitude growth functions. EAP data were recorded in 12 Ineraid subjects. Additionally, behavioral thresholds (THR) and maximum acceptable loudness levels (MAL) were determined for stimulation rates of 80 pps and 2,020 pps for each electrode.

RESULTS

Useful correlations between EAP data and psychophysical data were found at the low stimulation rate (80 pps). However, at the higher stimulation rate (2,020 pps) correlations were not significant. They were improved substantially, however, by introducing a factor that corrected for disparities due to temporal integration. Incorporation of this factor, which controls for the influence of the stimulation rate on the threshold, improved the correlations between EAP measures recorded at 80 pps and psychophysical MALs measured at 2,020 pps to better than r = 0.70.

CONCLUSIONS

EAP data as such can only be used to predict behavioral THRs or MCLs at low stimulation rates. To cope with temporal integration effects at higher stimulation rates, EAP data must be rate corrected. The introduction of a threshold-rate-factor is a promising way to achieve that goal. Further investigations need to be performed.

Cochlear implants in children

Cochlear implants are electronic prostheses that provide a high quality sense of hearing to severely and profoundly deaf children and adults. As improvements in surgical technique and device performance have occurred, indications for implantation have expanded.

Significance of auditory evoked responses (EABR and P300) in cochlear implant subjects

An electrically derived auditory brainstem response (EABR) was recorded intra-operatively, and the event-related potential (P300) postoperatively in post-lingually deaf adult cochlear implantees. EABR and P300 were recorded from two groups of 25 subjects each. All subjects had a cochlear mini-22 system implanted at our center between 1994 and 1998. Two parameters of EABR, threshold (T) and the gradient of the amplitude-growth (AG) curve, were used for correlation with postoperative speech reception scores. The consonant recognition score (CRS) measured 1 month postoperatively showed a significant correlation with the AG curve of EABR (Spearman rank order test, p = 0.004), but not at later points in time. No correlation was found between the T and CRS at any time postoperatively. As the AG curve reflects the number of remaining spiral ganglion cells in the inner ear, a large number of surviving neurons is thought to be advantageous only for initial speech learning. The latency of P300 was measured for correlation with the speech reception score. The division of the cochlear implant subjects into a fair and a good hearing group according to their CRS showed that the average latency of P300 was significantly longer for the fair hearing group than for the good hearing group. A follow-up study showed that P300 latency significantly correlated with CRS measured at 6 months, 1 year and at a later time. As the latency of P300 is thought to reflect the time for sound processing in the central auditory system, our results indicate that plasticity of the central auditory system is more important than that of the peripheral auditory system for speech learning in cochlear implant subjects.

Comparison of EAP thresholds with MAP levels in the nucleus 24 cochlear implant: data from children

OBJECTIVE

The purpose of this study is to examine the relationship between the electrically evoked compound action potential (EAP) thresholds and the MAP thresholds (T-levels) and maximum comfort levels (C-levels) in children implanted with the Nucleus 24 device.

DESIGN

EAP thresholds were measured using the Neural Response Telemetry system of the Nucleus 24 device. Twenty children implanted with the Nucleus 24 cochlear implant participated in this study. EAP thresholds were compared with the behavioral measures of T- and C-level used to construct the MAP these children used on a daily basis. For these subjects, both EAP and MAP T- and C-levels were obtained the same visit, which occurred at 3 to 5 mo postconnection.

RESULTS

EAP thresholds were shown to fall between MAP T- and C-level for 18 of 20 subjects tested; however, considerable variability across subjects was noted. On average, EAP thresholds fell at 53% of the MAP dynamic range. Correlations between EAP threshold and MAP T- and C-level improved substantially when combined with behavioral measures obtained from one electrode in the array.

CONCLUSIONS

Moderate correlations were found between EAP thresholds and MAP T- and C-levels for the children participating in this study. However, a technique is described for improving the accuracy of predictions of MAP T- and C-levels based on EAP data combined with a small amount of behavioral information.

The relationship between EAP and EABR thresholds and levels used to program the nucleus 24 speech processor: data from adults

OBJECTIVE

The objective of this study was to determine the relationship between electrically evoked whole nerve action potential (EAP) and electrical auditory brain stem response (EABR) thresholds and MAP threshold (T-level) and maximum comfort level (C-level) for subjects who use the Nucleus 24 cochlear implant system.

DESIGN

Forty-four adult Nucleus 24 cochlear implant users participated in this study. EAP thresholds were recorded using the Neural Response Telemetry System developed by Cochlear Corporation. EABR thresholds were measured for a subset of 14 subjects using standard evoked potential techniques. These physiologic thresholds were collected on a set of five electrodes spaced across the cochlea, and were then compared with behavioral measures of T-level and C-level used to program the speech processor.

RESULTS

EAP thresholds were correlated with MAP T- and C-levels; however, the correlation was not strong. A technique for improving the correlation by combining measures of T- and C-levels made on one electrode with the EAP thresholds was presented. Correlations between predicted and measured T- and C-levels using this technique were 0.83 and 0.77, respectively. Similar results were obtained using the EABR thresholds for a smaller set of subjects. In general, EABR thresholds were recorded at levels that were approximately 4.7 programming units lower than EAP thresholds.

CONCLUSIONS

Either EAP or EABR thresholds can be used in combination with a limited amount of behavioral information to predict MAP T- and C-levels with reasonable accuracy.

Electrically evoked potentials recorded in adult and pediatric CLARION implant users

The purpose of this study was to record electrical auditory brain stem responses (EABRs) and electrical middle latency responses (EMLRs) in the clinic from 3 adult CLARION Multi-Strategy Cochlear Implant subjects and to record EABRs in the operating room from 3 pediatric subjects. For 2 of the 3 adult subjects, EABR thresholds were within the subject's behavioral dynamic range, whereas 1 adult subject's EABR thresholds were either absent or, when present, exceeded the upper limit of the dynamic range. For this subject, EMLRs were absent or of poor morphology for the channels tested. Moreover, this subject was not able to understand speech in an open-set, auditory-only format. The EABR thresholds obtained with children were within the behavioral dynamic range for 2 of the 3 subjects, but exceeded comfortable loudness levels for 1 subject. Although the EABR thresholds were measured at stimulus levels that were audible for all subjects, the relationship of the EABR threshold levels to behavioral measures of loudness varied. Evoked potentials that originate more centrally, such as the EMLR, should be investigated further to determine the possible relationship to postimplant performance.

Relationship between EABR thresholds and levels used to program the CLARION speech processor

The purpose of this study was 1) to describe the relationship between electrically evoked auditory brain stem response (EABR) thresholds and the behavioral measures needed to program the speech processor of the CLARION Multi-Strategy Cochlear Implant, and 2) to determine whether the relationship between EABR threshold and the behavioral levels used to program the Clarion device was correlated with the temporal integration abilities of these subjects. The EABR thresholds, psychophysical thresholds, and the Clarion speech processor programming levels were recorded from 29 postlingually deafened adults. The mean EABR thresholds approximated most comfortable levels (M levels) for the programming stimulus. However, intrasubject variability was substantial. Significant intrasubject variability in temporal integration also was observed. Correlations were found between the ability of a cochlear implant user to perform temporal integration and the relationship between his or her EABR threshold and the threshold for the programming stimulus.

Summary of results using the nucleus CI24M implant to record the electrically evoked compound action potential

OBJECTIVE

This study outlines a series of experiments using the neural response telemetry (NRT) system of the Nucleus CI24M cochlear implant to measure the electrically evoked compound action potential (EAP). The goal of this investigation was to develop a protocol that allows successful recording of the EAP in a majority of CI24M cochlear implant users.

DESIGN

Twenty-six postlingually deafened adults participated in this study. A series of experiments were conducted that allowed us to examine how manipulation of stimulation and recording parameters may affect the morphology of the EAP recorded using the Nucleus NRT system.

RESULTS

Results of this study show consistent responses on at least some electrodes from all subjects. Cross-subject and cross electrode variations in both the growth of the response and the temporal refractory properties of the response were observed. The range of stimulus and recording parameters that can be used to record the EAP with the Nucleus NRT system is described.

CONCLUSIONS

Using the protocol outlined in this study, it is possible to reliably record EAP responses from most subjects and for most electrodes in Nucleus CI24M cochlear implant users. These responses are robust and recording these responses does not require that the subject sleep or remain still. Based on these results, a specific protocol is proposed for measurement of the EAP using the NRT system of the CI24M cochlear implant. Potential clinical implications of these results are discussed.

Correlation between electrical auditory brainstem response and perceptual thresholds in Digisonic cochlear implant users

OBJECTIVES

To examine the relationships between psychophysical perceptions and the electrically evoked auditory brainstem responses (EABRs) in multichannel cochlear implant (CI) users and to determine the effectiveness of EABRs in electrode failure.

DESIGN

A descriptive study reported the EABR characteristics while the different electrodes were activated. Characteristics of the EABR and of the perceptual measures served as compared variables in a correlational study.

SETTING

The study was carried out in the audiology clinic of an otolaryngology department at a university hospital.

PATIENTS

The subjects consisted of nine consecutively selected habitual Digisonic DX1OR multichannel CI users. Seven patients were postlinguistically deafened adult patients; two were congenitally deaf children.

MAIN OUTCOME MEASURES

Ipsilateral recordings were performed using a previously published method. Morphology, latency, and amplitude measures of the EABR recordings were described, computed, and compared with the literature data for EABRs obtained while activating other types of CI and for acoustically evoked ABRs. Correlations between EABRs and behavioral perception thresholds were analyzed using the parametric Pearson's correlation test.

RESULTS

EABRs allowed the authors to detect failure of no. 10 electrode integrity in one child. Perceptual threshold measures were found to be highly significantly related to the EABR threshold across subjects and electrode position (n = 31, r = 0.98; P < 0.001; linear regression equation: perceptual threshold = 1.06 EABR threshold + 0.76). The latencies and amplitudes were found to be similar to those described in the literature.

CONCLUSIONS

EABRs may be used to estimate settings for the Digisonic DX10 CI even in a pediatric population, although they cannot entirely replace behavioral measurements, especially in children. The EABR can be employed for electrode dysfunction diagnosis. Further studies are needed to determine whether recordings of EABR quality could contribute to the evaluation of functional prognosis during the rehabilitation.

The effect of loudness imbalance between electrodes in cochlear implant users

OBJECTIVE

The aim was to determine the effect of loudness imbalance between electrodes in patients using the 22-electrode cochlear implant (Cochlear Pty Ltd). It was hypothesized that speech perception scores would be greater when the loudness of electrodes was balanced at the comfort (C) levels than when the C levels were unbalanced.

DESIGN

Ten adult patients received a monosyllabic word test (CNC) words) in quiet and a sentence test (CUNY sentences) in noise under two conditions: with C levels balanced for equal loudness and with unbalanced C levels.

RESULTS

When the C levels across electrodes were pseudo-randomly unbalanced by 0 to +/- 20% of the electrodes' dynamic ranges (20% unbalancing), 6 of the 10 subjects showed a significant drop in sentence perception scores. Of these patients, none had a significant decrease in perception when the degree of unbalancing was halved. Of the four patients who showed no change with 20% unbalancing, three revealed a significant decline in sentence perception when the degree of unbalancing was doubled. There also were significant group effects for phonemes on the word test as well as for sentences in noise for the 20% unbalancing.

CONCLUSIONS

The implications for clinical practice are that it is important to balance the C levels and that clinicians should be encouraged to refine methods for setting C levels in very young children, who may be using unbalanced MAPs. Nevertheless, although most patients revealed a statistically significant drop in sentence perception with 20% imbalance of the C levels, the changes in percentage scores often were only small.

Cochlear implants in children

Cochlear implants allow the rehabilitation of children with severe to profound hearing loss. They are beneficial for not only postlingual children with hearing loss but also for children with congenital or prelingual hearing loss. Issues regarding cochlear implant candidacy and surgery are discussed. The results of cochlear implants in children and the complications related to cochlear implant surgery in children are reviewed.

Electrically evoked whole nerve action potentials in Ineraid cochlear implant users: responses to different stimulating electrode configurations and comparison to psychophysical responses

Electrically evoked whole nerve action potentials (EAP) have been recorded from 20 ineraid cochlear implant users in response to bipolar and/or monopolar electrical stimulation of the cochlea. EAP growth functions and refractory recovery functions were obtained for a variety of different stimulating conditions. Where possible, parallel psychophysical experiments were conducted that measured the just detectable increment in stimulus current level (JND), forward-masking functions, and detection thresholds for a range of different stimuli. Variations in EAP threshold, slope of the EAP growth function, and the rate of recovery of the EAP from the refractory state were observed both across subjects for a given place and by mode of stimulation (bipolar versus monopolar), as well as across electrodes within a subject. A poor correlation between slope of the EAP growth functions and current JNDs was obtained. However, good correlations were observed between EAP threshold and psychophysical detection threshold and between the EAP refractory recovery functions and the psychophysical forward-masking functions. Our interpretation of these findings is that these particular physiological measures are related to the excitability of the auditory nerve to electrical stimulation and further that these measures are related to performance of cochlear implant patients on listening tasks. Consequently, such information may prove to be useful in adjusting the stimulation parameters of the cochlear implant speech processor in order to maximize an individual's performance with the device.

Functional responses from guinea pigs with cochlear implants. I. Electrophysiological and psychophysical measures

We examined electrophysiological and psychophysical measures of the electrically stimulated auditory system of guinea pigs implanted with chronic intracochlear electrodes. Guinea pigs were trained to detect low-level acoustic stimuli and then unilaterally deafened and implanted with one extracochlear and two intracochlear electrodes. Electrically evoked auditory brainstem responses (EABRs) and psychophysical detection thresholds were obtained from the same animals using pulsatile stimuli. Supplementary EABR data were obtained from additional, untrained, animals. Thresholds were obtained as a function of stimulus phase duration and monopolar and longitudinal-bipolar electrode configurations. The slopes of the EABR and psychophysical functions for bipolar stimulation, averaged across subjects within 1 month after implantation, were -5.25 and -6.18 dB per doubling of pulse duration, respectively. These slopes were obtained with pulse durations ranging from 20 to 400 microseconds/phase; slope was reduced at longer pulse durations. Strength-duration slope also varied as a function of electrode configuration: monopolar stimulation produced steeper functions than did bipolar stimulation. Differences between EABR and psychophysical strength-duration measures suggest the existence of central mechanisms of stimulus integration in addition to that occurring at the level of the auditory nerve. Differences observed with variation of stimulus parameters (e.g., monopolar vs. bipolar stimulation modes) suggest that the specific mode of intracochlear electrical stimulation can influence stimulus integration. Such observations may be useful in the design of prosthetic devices and furthering our understanding of electrical excitation of the auditory system.

Behavioral and electrophysiological responses to electrical stimulation in the cat. I. Absolute thresholds

Estimates of electrical auditory brainstem response (EABR) thresholds are compared with behavioral thresholds for electrical stimulation in the same subject using identical stimuli and electrode configurations. Four cats were behaviorally trained to measure acoustic auditory thresholds using food as a reward in an operant reinforcement paradigm. One of the animals was then implanted, in an otherwise normal ear, with a scaled-UCSF multi-contact electrode array containing four intracochlear electrodes. Three animals were implanted with an electrode array containing eight intracochlear contacts and one extracochlear contact under the temporalis muscle following unilateral cochlear perfusion with 10% neomycin solution. Stimuli for the behavioral studies were single presentations of 200 us/phase biphasic current pulses. For the EABR studies, the same stimulus was presented at a rate of 32/s. In general, for the animal with the four-contact array and two of the three subjects with the eight-contact implant, changes in electrode configuration produced well-differentiated changes in threshold. For these three subjects, comparisons of behavioral and EABR thresholds for the majority of monopolar and bipolar electrode configurations tested showed excellent agreement (r2 = 0.88). Correlations between behavioral and EABR measures in these animals were comparable for bipolar and monopolar arrangements (r2 = 0.88 for bipolar and 0.87 for monopolar). For one subject with the eight-contact electrode, who showed similar monopolar and bipolar electrode behavioral thresholds for all tested electrode spacings or configurations, most EABR thresholds were substantially higher than, and poorly correlated with, behavioral thresholds (r2 = 0.15; r2 = 0.28 for monopolar arrangements, and r2 = 0.12 for bipolar arrangements).