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

Recording EEG in Cochlear Implant Users: Guidelines for Experimental Design and Data Analysis for Optimizing Signal Quality and Minimizing Artifacts

Cochlear implants (CI) are neural prostheses that can restore hearing in individuals with severe to profound hearing loss. Although CIs significantly improve quality of life, clinical outcomes are still highly variable. An important part of this variability is explained by the brain reorganization following cochlear implantation. Therefore, clinicians and researchers are seeking objective measurements to investigate post-implantation brain plasticity. Electroencephalography (EEG) is a promising technique because it is objective, non-invasive, and implant-compatible, but is nonetheless susceptible to massive artifacts generated by the prosthesis's electrical activity. CI artifacts can blur and distort brain responses; thus, it is crucial to develop reliable techniques to remove them from EEG recordings. Despite numerous artifact removal techniques used in previous studies, there is a paucity of documentation and consensus on the optimal EEG procedures to reduce these artifacts. Herein, and through a comprehensive review process, we provide a guideline for designing an EEG-CI experiment minimizing the effect of the artifact. We provide some technical guidance for recording an accurate neural response from CI users and discuss the current challenges in detecting and removing CI-induced artifacts from a recorded signal. The aim of this paper is also to provide recommendations to better appraise and report EEG-CI findings.

The Difficulties Encountered by Pediatric Cochlear Implant Patients and Their Parents during the COVID-19 Pandemic

INTRODUCTION

The daily escalation in incidence and mortality caused by Coronavirus disease (COVID-19) has mandated forced curfew in our country (same as many other countries) to limit the spread of infection. This is predicted to have a more negative impact on cochlear implant (CI) patients since this group of patients needs a unique type of psychological, medical, and technical care in addition to a daily rehabilitation program.

METHODS

A cross-sectional study based on Arabic questionnaire that looked into the collateral consequences of COVID-19 on the pediatric CI patients. The questionnaire was designed to highlight different problems such as exposure to head trauma or ear infection, difficulties in device maintenance and getting spare parts, impacts of the delay of programing or switch on appointments, and the impacts of missing rehabilitation sessions. Different ways of management of these problems are presented and discussed.

RESULTS

A total of 174 parents responded to the questionnaire. The main problem met by the patients was missing their device programing and rehabilitation sessions. Many children had device maintenance and spare parts problems. Virtual clinics were helpful in solving different problems. Additionally, children who needed device programing were scheduled for remote programing sessions.

CONCLUSION

Although the inevitable consequences of the COVID-19 pandemic are catastrophic, they are forcing the medical field to explore new opportunities by sitting up an infrastructure for future usage of telemedicine. Telemedicine is cost-effective and more convenient and enables health-care providers to be immune to future circumstances.

eABR THR Estimation Using High-Rate Multi-Pulse Stimulation in Cochlear Implant Users

We estimated the electrically-evoked auditory brainstem response thresholds (eABR THRs) in response to multi-pulses with high burst rate of 10,000 pulses-per-second (pps). Growth functions of wave eV amplitudes, root mean square (RMS) values, peak of phase-locking value (PLV), and the lowest valid data point (LVDP) were calculated in 1-, 2-, 4-, 8-, and 16-pulses conditions. The growth functions were then fitted and extrapolated with linear and exponential functions to find eABR THRs. The estimated THRs were compared to psychophysical THRs determined for multi-pulse conditions as well as to the clinical THRs measured behaviorally at the rate of 1,000 pps. The growth functions of features showed shallower growth slopes when the number of pulses increased. eABR THRs estimated in 4-, 8-, and 16-pulses conditions were closer to the clinical THRs, when compared to 1- and 2-pulses conditions. However, the smallest difference between estimated eABR THRs and clinical THRs was not always achieved from the same number of pulses. The smallest absolute difference of 30.3 μA was found for the linear fittings on growth functions of eABR RMS values in 4-pulses condition. Pearson's correlation coefficients (PCCs) between eABR THRs and psychophysical THRs were significant and relatively large in all but 16-pulses conditions. The PCCs between eABR THRs and clinical THRs, however, were smaller and in less cases significant. Results of this study showed that eABRs to multi-pulse stimulation could, to some extent, represent clinical stimulation paradigms, and thus in comparison to single pulses, could estimate clinical THRs with smaller errors.

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.

EEG-based diagnostics of the auditory system using cochlear implant electrodes as sensors

The cochlear implant is one of the most successful medical prostheses, allowing deaf and severely hearing-impaired persons to hear again by electrically stimulating the auditory nerve. A trained audiologist adjusts the stimulation settings for good speech understanding, known as "fitting" the implant. This process is based on subjective feedback from the user, making it time-consuming and challenging, especially in paediatric or communication-impaired populations. Furthermore, fittings only happen during infrequent sessions at a clinic, and therefore cannot take into account variable factors that affect the user's hearing, such as physiological changes and different listening environments. Objective audiometry, in which brain responses evoked by auditory stimulation are collected and analysed, removes the need for active patient participation. However, recording of brain responses still requires expensive equipment that is cumbersome to use. An elegant solution is to record the neural signals using the implant itself. We demonstrate for the first time the recording of continuous electroencephalographic (EEG) signals from the implanted intracochlear electrode array in human subjects, using auditory evoked potentials originating from different brain regions. This was done using a temporary recording set-up with a percutaneous connector used for research purposes. Furthermore, we show that the response morphologies and amplitudes depend crucially on the recording electrode configuration. The integration of an EEG system into cochlear implants paves the way towards chronic neuro-monitoring of hearing-impaired patients in their everyday environment, and neuro-steered hearing prostheses, which can autonomously adjust their output based on neural feedback.

Investigation of Electrically Evoked Auditory Brainstem Responses to Multi-Pulse Stimulation of High Frequency in Cochlear Implant Users

We investigated the effects of electric multi-pulse stimulation on electrically evoked auditory brainstem responses (eABRs). Multi-pulses with a high burst rate of 10,000 pps were assembled from pulses of 45-μs phase duration. Conditions of 1, 2, 4, 8, and 16 pulses were investigated. Psychophysical thresholds (THRs) and most comfortable levels (MCLs) in multi-pulse conditions were measured. Psychophysical temporal integration functions (slopes of THRs/MCLs as a function of number of pulses) were -1.30 and -0.93 dB/doubling of the number of pulses, which correspond to the doubling of pulse duration. A total of 15 eABR conditions with different numbers of pulses and amplitudes were measured. The morphology of eABRs to multi-pulse stimuli did not differ from those to conventional single pulses. eABR wave eV amplitudes and latencies were analyzed extensively. At a fixed stimulation amplitude, an increasing number of pulses caused increasing wave eV amplitudes up to a certain, subject-dependent number of pulses. Then, amplitudes either saturated or even decreased. This contradicted the conventional amplitude growth functions and also contradicted psychophysical results. We showed that destructive interference could be a possible reason for such a finding, where peaks and troughs of responses to the first pulses were suppressed by those of successive pulses in the train. This study provides data on psychophysical THRs and MCLs and corresponding eABR responses for stimulation with single-pulse and multi-pulse stimuli with increasing duration. Therefore, it provides insights how pulse trains integrate at the level of the brainstem.

Electrical dynamic range is only weakly associated with auditory performance and speech recognition in long-term users of cochlear implants

OBJECTIVE

The electrical dynamic range (EDR) has been suggested to be related to auditory performance in cochlear implant (CI) users. However, few reports have evaluated postlingual CI users who have used CIs for long periods in comparison with prelingual CI users. Here, we evaluated auditory perception and speech performance in terms of the EDR in long-term CI users. The EDR, and auditory and speech performances, were compared between pre- and post-lingual CI users.

METHODS

We enrolled all patients who received CIs from April 2000 to December 2010 at Seoul National University Hospital, and who had ≥5 years of experience with CIs. The EDRs affording subjective responses at the threshold level (T-level) and comfortable level (C-level) were analyzed in terms of their relationships with pure tone audiometry levels, speech evaluation scores, including those on the Phonetically Balanced (PB) Word List test, vowel and consonant tests, a sentence test, and the Korean version of the Central Institute for the Deaf (K-CID) test; we also calculated Category in Auditory Performance (CAP) scores.

RESULTS

We found no significant difference in the average EDR, CAP, K-CID, PB word, consonant, or vowel scores between pre- and post-lingual CI users. The EDR was weakly associated with the PB word (P = 0.003, r = 0.462) and consonant scores (P = 0.005, r = 0.438). Other speech evaluations, such as the CAP, K-CID, and vowel scores, were not significantly associated with the EDR T-level. We found no association between pure tone thresholds at 0.5, 1, or 2 kHz, and the speech evaluation scores or EDRs of low-, middle-, or high-frequency channels.

CONCLUSIONS

The EDR was only weakly associated with speech performance, such as scores on consonant and PB word tests in long-term CI users, irrespective of pre- or post-lingual deafness status.

Auditory steady-state response in cochlear implant patients

INTRODUCTION AND OBJECTIVE

Auditory steady state responses to continuous amplitude modulated tones at rates between 70 and 110Hz, have been proposed as a feasible alternative to objective frequency specific audiometry in cochlear implant subjects. The aim of the present study is to obtain physiological thresholds by means of auditory steady-state response in cochlear implant patients (Clarion HiRes 90K), with acoustic stimulation, on free field conditions and to verify its biological origin.

METHODS

11 subjects comprised the sample. Four amplitude modulated tones of 500, 1000, 2000 and 4000Hz were used as stimuli, using the multiple frequency technique. The recording of auditory steady-state response was also recorded at 0dB HL of intensity, non-specific stimulus and using a masking technique.

RESULTS

The study enabled the electrophysiological thresholds to be obtained for each subject of the explored sample. There were no auditory steady-state responses at either 0dB or non-specific stimulus recordings. It was possible to obtain the masking thresholds. A difference was identified between behavioral and electrophysiological thresholds of -6±16, -2±13, 0±22 and -8±18dB at frequencies of 500, 1000, 2000 and 4000Hz respectively.

CONCLUSIONS

The auditory steady state response seems to be a suitable technique to evaluate the hearing threshold in cochlear implant subjects.

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.

Auditory steady state responses and cochlear implants: Modeling the artifact-response mixture in the perspective of denoising

Auditory steady state responses (ASSRs) in cochlear implant (CI) patients are contaminated by the spread of a continuous CI electrical stimulation artifact. The aim of this work was to model the electrophysiological mixture of the CI artifact and the corresponding evoked potentials on scalp electrodes in order to evaluate the performance of denoising algorithms in eliminating the CI artifact in a controlled environment. The basis of the proposed computational framework is a neural mass model representing the nodes of the auditory pathways. Six main contributors to auditory evoked potentials from the cochlear level and up to the auditory cortex were taken into consideration. The simulated dynamics were then projected into a 3-layer realistic head model. 32-channel scalp recordings of the CI artifact-response were then generated by solving the electromagnetic forward problem. As an application, the framework’s simulated 32-channel datasets were used to compare the performance of 4 commonly used Independent Component Analysis (ICA) algorithms: infomax, extended infomax, jade and fastICA in eliminating the CI artifact. As expected, two major components were detectable in the simulated datasets, a low frequency component at the modulation frequency and a pulsatile high frequency component related to the stimulation frequency. The first can be attributed to the phase-locked ASSR and the second to the stimulation artifact. Among the ICA algorithms tested, simulations showed that infomax was the most efficient and reliable in denoising the CI artifact-response mixture. Denoising algorithms can induce undesirable deformation of the signal of interest in real CI patient recordings. The proposed framework is a valuable tool for evaluating these algorithms in a controllable environment ahead of experimental or clinical applications.

The Relationship Between EABR and Auditory Performance and Speech Intelligibility Outcomes in Pediatric Cochlear Implant Recipients

PURPOSE

The primary purpose of the present study was to investigate the relationship between postimplant electrically evoked auditory brainstem response (EABR) measures (wave V threshold, wave V latency, input-output functions) and auditory performance and speech intelligibility outcomes measured using parental rating scales, such as Categories of Auditory Performance (CAP; Archbold, Lutman, & Marshall, 1995) and the Speech Intelligibility Rating scale (SIR; Allen, Nikolopoulos, Dyar, & O'Donoghue, 2001), respectively. The secondary purpose was to evaluate the relationship between age at implantation and ratings on the CAP and SIR.

METHOD

Forty children with congenital sensorineural hearing loss participated. Preimplant parental ratings on the CAP and SIR were obtained, and all the children underwent cochlear implantation. Intracochlear EABRs were recorded postimplantation. Postimplant parental ratings on the CAP and SIR were obtained. The relationships between EABR parameters and auditory performance and speech intelligibility outcomes were studied. The effect of age at implantation on auditory performance and speech intelligibility outcomes was also investigated.

RESULTS

A significant negative correlation was found between EABR wave V thresholds and SIR growth (r = -.415, p = .016). Children with better CAP growths tended to have lower wave V thresholds than those with poorer CAP growths. Age at implantation had an effect on the auditory performance as measured using the CAP.

CONCLUSIONS

The present study provides evidence for the relationship between wave V thresholds of the intracochlear EABR and auditory performance and speech intelligibility outcomes measured using parental rating scales in pediatric cochlear implant recipients. Data also indicate that early intervention has a positive impact on auditory performance outcomes.

Impact of modulating phase duration on electrically evoked auditory brainstem responses obtained during cochlear implantation

Objective To investigate the effect of increasing phase duration (pulse width, T-pulse) using a biphasic pulse composed of an initial anodic active phase followed by a balancing cathodic phase on the electrically evoked auditory brainstem responses (eABRs) recorded at the time of cochlear implantation. Design eABRs recorded during 188 surgeries for cochlear implantation from 1999 to 2006 in a single center were retrospectively reviewed by two independent observers. All patients were fitted with a NEURELEC cochlear implant (CI) device, initially DIGISONIC(®) then DIGISONIC SP(®) (2004-2006). Result Immediately following cochlear implantation, stimulation by the CI resulted in reliable wave III and V eABR waveforms (mean wave III latency 2.23 ± 0.38 ms SD and wave V latency 4.28 ± 0.42 ms SD). Latencies followed an apical to basal gradient (0.32 ms increase in mean eV latency and 0.12 ms for eIII latency). With increasing phase duration, wave III and wave V latencies significantly decreased in association with a shortening of the eIII-eV interwave gap, while amplitudes of both waves increased. Conclusion The impact of increasing phase duration on latency and amplitude of brainstem responses in a large set of patients implanted with NEURELEC CIs was reported.

The relationship between electrical auditory brainstem responses and perceptual thresholds in Digisonic® SP cochlear implant users

Determining the electrical stimulation levels is often a difficult and time-consuming task because they are normally determined behaviorally - a particular challenge when dealing with pediatric patients. The evoked stapedius reflex threshold and the evoked compound action potential have already been shown to provide reasonable estimates of the C- and T-levels, although these estimates tend to overestimate the C- and T-levels. The aim of this study was to investigate whether the evoked auditory brainstem response (eABR) can also be used to reliably estimate a patient's C- and T-levels. The correlation between eABR detection thresholds and behaviorally measured perceptual thresholds was statistically significant (r = 0.71; P < 0.001). In addition, eABR Wave-V amplitude increased with increasing stimulation level for the three loudness levels tested. These results show that the eABR detection threshold can be used to estimate a patient's T-levels. In addition, Wave-V amplitude could provide a method for estimating C-levels in the future. The eABR objective measure may provide a useful cochlear implant fitting method - particularly for pediatric patients.

Evaluation of the Digisonic® SP cochlear implant: patient outcomes and fixation system with titanium screws

Cochlear implants have revolutionized the way patients affected by severe hearing loss experience the world. Neurelec developed a fixation system with two titanium screws that requires no skull bone drilling.

Objective

To describe the outcomes and procedure-related details of a series of patients implanted with the Digisonic® SP cochlear implant.

Method

This retrospective study analyzed patients submitted to cochlear implant placement within a period of 18 months. All patients had postlingual hearing impairment. Data was collected from patient charts and standard questionnaires answered by the surgeons in charge of carrying out the procedures.

Results

The six patients offered the Digisonic® SP cochlear implants were operated by experienced surgeons. The procedures took 95 to 203 minutes (mean = 135') to be completed, which is less time than what has been described for other fixation approaches. No complications were recorded and hearing improvement was satisfactory.

Conclusion

The Digisonic® SP cochlear implant developed by Neurelec offered good audiological results for adult patients, shorter surgery time, and no surgical or postoperative complications.

The prediction of hearing thresholds with auditory steady-state responses for cochlear implanted children

OBJECTIVES

The objectives of this study were to investigate the efficacy of auditory steady-state response (ASSR) in predicting cochlear implanted children's actual behavioural thresholds at various frequencies.

METHODS

Twenty-two children with cochlear implants (Nucleus 24 devices) participated in this study. Behavioural thresholds and/or ASSR in cochlear implanted children were obtained to determine the predictive accuracy rates and errors by comparing the estimated range of thresholds with the actual behavioural thresholds.

RESULTS

Stable ASSR threshold results were obtained in 22 cochlear implanted children. The predictive accuracy rates were the highest and prediction errors were the smallest at 4000 Hz, among the four tested frequencies, using either the Rance 1995 or the Rance 2002 linear regression formulae. The Rance 2002 regression formulae had a higher predictive accuracy and fewer errors than the Rance 1995 regression formulae in predicting the hearing threshold of cochlear implanted patients at all tested frequencies.

CONCLUSIONS

Both behavioural thresholds and ASSR thresholds revealed significant improvements after cochlear implantation. ASSR may be used as a tool in predicting hearing thresholds after cochlear implantation in children. The predictive accuracy is best at 4000 Hz with the Rance 2002 linear regression formulae.

Effects of auditory pathway anatomy and deafness characteristics? (1): On electrically evoked auditory brainstem responses

The purpose of this study was to distinguish the effects of different parameters on latencies of wave IIIe, wave Ve, and interpeak interval IIIe-Ve of electrical auditory brainstem responses (EABRs). EABRs were recorded from all the intra-cochlear electrodes in eight adult HiRes90K((R)) cochlear implant users. The relationship between latencies and stimulation sites in the cochlea was characterized to assess activity along the auditory pathway. Audiograms before implantation, psychophysics at first fitting and duration of deafness were used to describe the influence of deafness on latencies. A decreasing baso-apical latency gradient was found for waves IIIe and Ve, while the interpeak interval IIIe-Ve remained the same along the electrode array. Electrical stimulation enabling to stimulate various parts of the cochlea at the same time, this could indicate an anatomical way of compensating for the delay the acoustic wave takes to reach the cochlea apex in a non-implanted ear. However, psychophysical levels were also found to increase at the cochlear base showing that the latency gradient could result from an increasing gradient of neural degeneration toward the base. Correlations of EABR latencies with psychophysics, audiometric data and duration of deafness show that factors linked to deafness have indeed an influence on EABR latencies. The possible explanations for the latency shift observed, whether they are anatomical and/or pathological, are exposed.

A longitudinal study of electrical stimulation levels and electrode impedance in children using the Clarion cochlear implant

CONCLUSIONS

Electrical stimulation levels and electrode impedance values (EIVs) in children using the Clarion cochlear implant (CI) programmed with CIS strategy stabilized after 3 months of implant use. The data presented here may be useful as a general guideline for the programming of infants and young children and may further be of help for the identification of patients who fall outside the "average" range.

OBJECTIVES

The purpose of the present study was to evaluate changes in electrical stimulation levels, i.e. threshold (T) levels, comfortable (M) levels, dynamic range (DR), and EIVs during the first 18 months of implant use, in children using the Clarion CI.

MATERIALS AND METHODS

The maps of 18 pre-lingual children (mean age at implantation 4.2 years; range 1-8), using the Enhanced Bipolar 1.2 or Bipolar standard electrode with the S-Series speech processor programmed with CIS strategy, were examined at five time points: connection, and 3, 6, 12, and 18 months post-initial stimulation. T levels, M levels, DR and EIVs were analyzed according to four cochlear segments: apical, apical-medial, medial-basal, and basal.

RESULTS

During the first 3 months of implant use T levels increased to some extent, whereas M levels and DR increased significantly. From 3 months and through the entire follow-up, T and M levels as well as DR were stable. EIVs of current carrying electrodes decreased significantly from connection to the 3-month visit; thereafter a stabilization of values was evident. Electrical stimulation levels and EIVs did not differ among the cochlear segments during the entire follow-up.

Electrical stimulation levels and electrode impedance values in children using the Med-El Combi 40+ cochlear implant: a one year follow-up

The present study was designed to follow changes in electrical stimulation levels and electrode impedance values (EIV) in children using the Med-El Combi 40+ cochlear implant (CI) during the first 12 months of implant use. The maps of 24 prelingually deaf children implanted at a mean age of 5.9 years (range 1-15.9 years) using the TEMPO+ speech processor programmed with CIS+ strategy were examined at five time points: initial stimulation, and 1, 3, 6, and 12 months post-initial stimulation. Most comfortable levels (M) and electrode impedance values (EIV) were analyzed according to three cochlear segments: apical, medial, and basal. Results indicated a significant increase in M levels until the 3-month time point, thereafter stabilization was evident. Furthermore, M levels in the apical segment were lower than those in the medial and basal segments. EIV decreased from initial stimulation to the 3-month time point and was then stable through the study follow up. Interestingly, the finding of higher EIV in the apical segment may be attributed to the physical characteristics of the Med-El electrode. In conclusion, the pattern of stabilization of M levels found in the present study is similar to that reported for children using other devices. The data presented here may be useful as a guideline for programming M levels and monitoring EIV in infants and young children. They may further help clinicians to identify those children that fall outside the 'typical' range.

An evoked potential study of the developmental time course of the auditory nerve and brainstem in children using cochlear implants

Central auditory responses to electrical stimulation from a cochlear implant were studied in 75 pre-lingually deafened children and 11 adults. Electrically evoked auditory brainstem response (EABR) latencies significantly decreased with duration of cochlear implant use and were not significantly affected by the age at implant activation. Significant decreases in early latency waves and interwaves occurred within the first 1-2 months of implant use, whereas longer term changes (6-12 months) were found for eV and eIII-eV, which measure activity in the more rostral brainstem. Comparisons to acoustically evoked auditory brainstem response (ABR) in children with normal hearing suggested shorter interwave EABR latencies, reflecting either distinct neural generators or increased neural synchrony, but similar rates of change in the later latency eV and eIII-eV with time in sound. In sum, normal-like development of the rostral auditory brainstem is promoted by cochlear implant use in children of a wide range of ages.

Toward a Battery of Behavioral and Objective Measures to Achieve Optimal Cochlear Implant Stimulation Levels in Children

OBJECTIVES

Children require audible and comfortable stimulation from their cochlear implants immediately after device activation. To accomplish this, a battery of objective measures may be needed that could include the electrically evoked stapedius reflex (ESR), compound action potential from the auditory nerve (ECAP), and/or auditory brain stem response (EABR). In the present study, the following specific research questions were asked: In children using cochlear implants, 1) Can the ECAP, EABR, and ESR be recorded at the time of cochlear implantation? 2) What is the feasibility of measuring the ECAP, EABR, and the ESR repeatedly without the use of sedation over the first year of implant use? 3) Do ECAP, EABR, and ESR thresholds or behavioral measures change over time? 4) What is the relation between ECAP, EABR, and ESR thresholds and behavioral measures of threshold and comfortably loud levels?

DESIGN

In 68 children, ECAP, EABR, and ESR responses as well as behavioral measures of stimulation threshold and maximum stimulation were recorded at regular intervals over the first year of implant use. In each child, responses were recorded to electrical pulses provided by three different electrodes along the implanted array. Visual inspections of the stapedius reflex (V-ESR) evoked by activation of the same three electrodes at the time of surgery were performed in an additional 20 children.

RESULTS

ECAP and EABR measures were obtained in more than 84% of electrodes tested and 89% of children tested both in the operating room at the time of implant surgery (OR) and after surgery in nonsedated children. ESRs were recorded by using immittance measures in more than 65% of electrodes tested and 67% of children tested by 3 mo of implant use, but this technique was less successful in the OR and during early stages of device use. V-ESRs and ECAP thresholds were higher in the OR than ESRs and ECAPs at postoperative recording times. EABR and ECAP thresholds did not significantly change over the first 6 and 12 mo of implant use, respectively, whereas ESR thresholds increased. Behavioral measures of threshold decreased over time, whereas maximum stimulation levels rose over time. Behavioral measures of threshold and loudness were highly correlated at all test times. ECAP, EABR, and behavioral measures were lower when evoked by an electrode at the apical end of the implanted array than by more basal electrodes. Behavioral thresholds could be predicted mainly by ECAP thresholds, whereas maximum stimulation levels could best be predicted by ESR thresholds; both were significantly affected by the age at implantation.

CONCLUSIONS

A combination of nonbehavioral measures can aid in the determination of useful cochlear implant stimulation levels, particularly in young children and infants with limited auditory experience. These measures can be made in the operating room and can be repeated after surgery when needed. Correction factors to predict threshold stimulation levels should be based on ECAP thresholds or EABR thresholds if necessary. Correction factors should be made for at least one apical and mid-array electrode, should take into account the age of the child, and may have to be revised during the first year of implant use. Maximum stimulation levels may be best determined by using the ESR.

Activity-dependent developmental plasticity of the auditory brain stem in children who use cochlear implants

OBJECTIVES

1) To determine if a period of early auditory deprivation influences neural activity patterns as revealed by human auditory brain stem potentials evoked by electrical stimulation from a cochlear implant. 2) To examine the potential for plasticity in the human auditory brain stem. Specifically, we asked if electrically evoked auditory potentials from the auditory nerve and brain stem in children show evidence of development as a result of implant use. 3) To assess whether a sensitive or critical period exists in auditory brain stem development. Specifically, is there an age of implantation after which there are no longer developmental changes in auditory brain stem activity as revealed by electrically evoked potentials?

DESIGN

The electrically evoked compound potential of the auditory nerve (ECAP) and the electrically evoked auditory brain stem response (EABR) were recorded repeatedly during the first year of implant use in each of 50 children. The children all had pre- or peri-lingual onset of severe to profound sensorineural hearing loss and received their implants at ages ranging from 12 mo to 17 yr. All children received Nucleus cochlear implant devices. All children were in therapy and in school programs that emphasized listening and required the children to wear their implants consistently.

RESULTS

Initial stimulation from the cochlear implant evoked clear responses from the auditory nerve and auditory brain stem in most children. There was no correlation between minimum latency, maximum amplitude, or slope of amplitude growth of initial responses with age at implantation for ECAP eN1, EABR eIII and eV components (p > 0.05). During the first year of implant use, minimum latency of these waves significantly decreased (p < 0.01, p < 0.0001, p < 0.0001, respectively). Neural conduction time, measured using the interwave latency of ECAP eN1-EABR eIII for lower brain stem and EABR eIII-eV for upper brain stem, decreased during the period of 6 to 12 mo of cochlear implant use (p < 0.01 (lower), p < 0.0001(upper)). The ECAP wave eN1 and the EABR wave eV showed significant increases in amplitude during time of implant use (p < 0.05 and p < 0.01, respectively). There were no correlations between the rate of interwave latency decrease and the rate of amplitude increases and the age at which children underwent implantation (p < 0.05).

CONCLUSIONS

Activity in the auditory pathways to the level of the midbrain can be evoked by acute stimulation from a cochlear implant. EABR measures are not influenced by any period of auditory deprivation. Auditory development proceeds once the implant is activated and involves improvements in neural conduction velocity and neural synchrony. Underlying mechanisms likely include improvements in synaptic efficacy and possibly increased myelination. The developmental plasticity that we have shown in the human auditory brain stem does not appear from EABR data to be limited by a critical period during childhood.

NRT-based versus behavioral-based MAP: a comparison of parameters and speech perception in young children

The present study was designed to evaluate the effect of neural response telemetry (NRT)-based cochlear implant (CI) programming versus behavioral-based programming on electrical stimulation parameters (MAP) threshold (T) and comfortable (C) levels and speech perception abilities in young children, during the first year of implant use. Ten congenitally deaf children at the age of 12-39 months (mean age: 25.2 months) implanted with the Nucleus 24R(CS) CI participated in the study. The group was randomly divided into two: (1) NRT-based MAP group (n = 5) consisted of children who were programmed using intra-operative NRT measurements; (2) behavioral-based MAP group (n = 5) consisted of children who were programmed using the behavioral responses of the patients. MAP parameters as well as sound-field aided thresholds and speech perception abilities were compared between the two groups at consecutive programming sessions: 1, 3, 6, and 12 months post initial stimulation. Results indicated no significant differences among NRT-based MAPs and behavioral-based MAPs. Although MAP profiles at initial stimulation differed in the apical region, these differences decreased with time. In addition, a gradual increase of T and C levels of NRT-based MAPs as well as those of behavioral-based MAPs was evident until the 1-month time point, thereafter stabilization occurred. Sound-field aided thresholds improved with time for both groups; however, they were found to be significantly better for the NRT-based MAP group. Despite these differences, speech perception abilities were comparable among groups at 12 months post initial stimulation. NRT-based programming was found to be significantly shorter than behavioral-based programming. In conclusion, for this small group of children, our findings support the use of NRT for programming of young children during the initial period after implantation.

Auditory late cortical response and speech recognition in Digisonic cochlear implant users

OBJECTIVE

The purpose of the study was to test for differences in late electrically evoked auditory potentials between subjects exhibiting "good" versus "poor" speech recognition performances with their cochlear implants.

METHODS

Late auditory evoked responses were measured in 30 subjects equipped with the Digisonic (MXM, Antibes, France) cochlear implant, 15 of whom had "good" speech recognition scores (i.e., more than 89% correct phoneme identification without lip reading). The 15 other subjects had poorer speech recognition scores (i.e., less than 85%).

RESULTS

Differences in N1P2 amplitude, as well as P1, N1, and P2 latencies, and N1-P1 and N1-P2 latency intervals were tested. Wave P2 latency was found to be significantly different between the two groups (P =.016), being shorter in "good" than in "poor" performers. The strength of the statistical relationship between electrophysiological and speech recognition variables (r2 = 17%) was substantially smaller than that observed using electrically evoked auditory brainstem response (EABR) for the same implanted device (r2 = 49% for the EABR wave III-V latency interval).

CONCLUSIONS

Some characteristics of late electrically evoked auditory potentials differ significantly among cochlear implant users depending on their speech recognition performance. However, the relationship between electrophysiological and speech recognition variables is more pronounced when early (brainstem) rather than late (cortical) evoked responses are considered.

Electrophysiological findings in two bilateral cochlear implant cases: does the duration of deafness affect electrically evoked auditory brain stem responses?

Bilateral cochlear implantation provides an interesting model for in vivo study of the effect of long-term profound deafness on neural transmission. We present electrophysiological observations on 2 patients implanted with the MXM Binaural Digisonic Convex system. This uncommon design consists of 2 electrode arrays placed bilaterally into the scala tympani and controlled by a single speech processor. In both patients, the duration of deafness before cochlear implantation clearly differed from one ear to the other. Electrically evoked auditory brain stem responses (EABRs) were measured and the EABRs from the ear with the longer deafness duration showed a lengthening of wave V latency. In I patient, recordings from this ear also showed a lack of reproducibility of wave III. The data suggest that neural responsiveness in the peripheral and intermediate auditory pathways is adversely affected by deafness duration. Poor EABRs on one ear possibly result from the total duration of deafness in this ear and/or compensation by the other ear.

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.

Factors contributing to limited open-set speech perception in children who use a cochlear implant

Cochlear implants have enabled many children with severe to profound sensorineural hearing loss to develop speech perception skills. However, some children experience few gains while others develop high levels of speech perception. We identified potential factors contributing to poor performance with an implant by studying implanted children who do not develop functional speech perception. Five children were identified as developing no open-set word recognition skills after at least 2 years of implant use. This study group was compared to a randomly selected control group (n=10) and an age-matched control group (n=5). Pre-implant factors were examined using a Graded Profile Analysis and post-implant factors were assessed in a retrospective chart review. A greater number of pre-implant concerns were raised in the study group than in randomized controls (P<0.01). Chronological age and duration of deafness were pre-implant concerns in all study group subjects. A greater number of post-implant concerns were found in the study group than in randomly selected controls (P<0.005). We conclude that while appropriate selection of candidates for cochlear implantation is important in predicting speech perception outcomes, post-implant follow-up is also essential and must include regular monitoring of equipment, monitoring of stimulation levels with use of objective measures of stimulation levels if necessary, and consistent habilitation.