Electrically Evoked Compound Action Potential Thresholds Research Articles

Electrical Compound Action Potentials Recorded With Automated Neural Response Telemetry: Threshold Changes as a Function of Time and Electrode Position

Since the introduction of neural response telemetry (NRT) for the Nucleus 24 cochlear implant (CI24), researchers and clinicians have investigated the feasibility of using the electrically evoked compound action potential (ECAP) threshold to objectively predict psychophysical measurements that are used in the programming of the speech processor. The ability to substitute objective for behavioral measurements, particularly measurements made at the time of surgery, would greatly facilitate programming the MAP for young children and other individuals who are not able to provide reliable behavioral data required for MAP programming. There have been a number of studies that have examined characteristics of the ECAP measured at the time of surgery and postoperatively; however, all the available published data are based on the CI24. With the introduction of the Nucleus Freedom device, an automated NRT (AutoNRT) program became available, which was capable of measuring ECAP thresholds at lower levels than was previously possible with NRT software associated with the CI24 device. It was hypothesized that the enhancements to the NRT program may improve the predictability of postoperative measurements from intraoperatively recorded ECAP thresholds. The purpose of this study was to track ECAP thresholds obtained using AutoNRT as a function of time and electrode position. ECAP thresholds were recorded from 71 children and adults implanted with the Nucleus Freedom device using the AutoNRT test protocol. ECAP thresholds were obtained at the time of surgery, at initial stimulation, and 3 mos poststimulation. Five electrodes located at basal, middle, and apical positions in the cochlea were tested at each time interval and thresholds were compared. Significant differences were found in ECAP thresholds measured with AutoNRT as a function of both time and electrode position. Basal electrodes had higher ECAP thresholds than apical electrodes and that relationship was consistent for each time period. Thresholds for all electrodes decreased between surgery and initial stimulation and remained relatively stable at 3 mos poststimulation. ECAP thresholds were consistently lower for children compared with adults at each time point. Mid-array electrodes (11 and 16) showed the least amount of change over time. AutoNRT thresholds demonstrated significant change over time, limiting the ability to use intraoperatively recorded ECAP thresholds to predict postoperative measurements. In this study, electrodes 11 and 16 showed the least amount of change in ECAP threshold over time and therefore would be the best choices for estimating postoperative ECAP thresholds. Although not an ideal solution, mid-array ECAP thresholds obtained intraoperatively may prove to be helpful in creating a first MAP when no other behavioral or electrophysiological data are available.

The Relationship between Thresholds of Electrically Evoked Compound Action Potential and Speech Perception in Children with Cochlear Implants

Background and ObjectivesZZNeuropathology around the cochlea could create variation from site to site in physiological thresholds of cochlear implant users. This variability would be detrimental to speech recognition with a cochlear implant for a variety of reasons, including distortion of the place code and variation in the number of neurons. The purpose of this study is to examine the relationship between thresholds of electrically evoked compound action potential (ECAP) and speech perception in children implanted with the Nucleus Freedom devices. Subjects and MethodZZFifty-seven children implanted with the Nucleus Freedom device participated in this study. ECAP thresholds were recorded using the automated neural response telemetry test protocol. We then calculated mean threshold and three metrics to assess acrosssite variation within subjects: 1) the variance of T levels for all tested sites, 2) the range of T levels (highest minus lowest) across all tested sites and 3) site-to-site variation. For each subject, these measures were compared with performance on tests of word recognition. ResultsZZThere was considerable across-site (within-subject) and across-subject variability in thresholds. However, we found no significant correlation between speech recognition and across-site variation of thresholds as well as mean threshold levels. ConclusionZZThese data suggest that the ECAP measures of thresholds may not be an accurate predictor of speech perception ability. � Korean J Otorhinolaryngol-Head Neck Surg 2011;54:688-92 Key WordsZZCochlear implant ㆍElectrically evoked compound action potential ㆍ Key WordsZZThreshold ㆍSpeech perception.

Neural Response Telemetry Reconsidered: II. The Influence of Neural Population on the ECAP Recovery Function and Refractoriness

The Neural Response Telemetry (NRT) recovery function measures the electrically evoked compound action potential (ECAP) in response to a second biphasic pulse (the probe) after masking by a first pulse (the masker). The masker-probe interval is varied and the ECAP amplitude is measured at each masker-probe interval, giving an inverse exponential recovery. The prevailing understanding of the recovery function has been that faster recovery indicates a more efficient response to the individual pulses within a pulse sequence. Psychophysical data in the past have not supported this view, and in fact, the opposite result has been observed. This study explores this phenomenon from theoretical and experimental viewpoints. Fundamentally, a distinction is made between the refractoriness of a single fiber and the refractoriness of the whole nerve. The hypothesis is that the size of the neural population heavily influences whole nerve refractoriness: large neural populations operate near threshold and are more susceptible to masking, leading to slower ECAP recovery; however, they maintain temporal responsiveness through greater numbers of nonrefractory neurons. In phase I, the hearing loss durations (indicators of neural survival) of 21 adult Nucleus Freedom implantees were compared with the corresponding median recovery function time-constants (calculated per implant array). The data were separated by implant (nine Contour, 12 Straight) and the means of these two groups were compared. The Straight array, delivering broader excitation, is expected to engage a larger neural population. In phase II, a computational model of the ECAP recovery function was constructed based on data from the cat auditory nerve. The model allows the neural population size to be manipulated; accordingly, recovery functions from different neural populations were compared. In phase III, ECAP thresholds (via AutoNRT), ECAP recovery functions, and T- and C-levels were obtained from a subset of 12 subjects. Psychophysical levels were measured using pulse train stimuli at six different stimulation rates, spanning 250 to 3500 Hz. At each electrode, the recovery function time-constant tau was compared with two measures of temporal responsiveness: (i) the gradient of the linear trend in psychophysical levels with stimulation rate; and (ii) the difference between ECAP threshold (a single pulse measure) and 900 Hz T-level (a pulse train measure). In phase I, a trend toward shorter recovery function time-constants with increasing hearing loss durations was observed. The mean recovery function time-constant of the Contour implant group (0.51 msec) was significantly shorter than that of the Straight implant group (0.90 msec). When, in phase II, the recovery functions from the computational model were compared at equal ECAP amplitude, the larger neural population was associated with slower ECAP recovery. In phase III, the recovery function time-constant was significantly correlated with both temporal responsiveness measures, with slower ECAP, recovery associated with greater temporal responsiveness, thus confirming the results of previous studies. : Slower ECAP recovery, at equal loudness, is associated with larger neural populations. The collective results suggest that this neural population view of the recovery function explains the observed association between slower ECAP recovery and greater temporal responsiveness.

Neural Response Telemetry Reconsidered: I. The Relevance of ECAP Threshold Profiles and Scaled Profiles to Cochlear Implant Fitting

For more than a decade, Neural Response Telemetry (NRT) has provided direct access to the electrically evoked compound action potential (ECAP) as elicited by the Nucleus cochlear implant. When used clinically during fitting, ECAP threshold profiles are applied by shifting the profile to the audible threshold and comfort level boundaries (the T- and C-level profiles, respectively). The resulting profiles, to date, have matched the curvature of the ECAP threshold profile exactly. When compared with psychophysical profiles, previous studies have shown that this approach incurs errors in program levels that are no better than flat or population mean profiles. However, C-level profiles are observed to be flatter than T-level profiles. Accordingly, ECAP threshold profiles are flattened in this study when applied at increasing stimulus levels, and the effectiveness of this approach is evaluated among other methods. In phase I, ECAP thresholds (via AutoNRT) and T- and C-levels were measured from 15 adult Nucleus Freedom implantees. Psychophysical levels were measured using pulse train stimuli at six different stimulation rates, spanning 80 to 3500 Hz. The different rates spread T- and C-levels across a range of stimulus levels. At each of these levels, a scaling factor of best fit was calculated such that the shifted ECAP threshold profile, when scaled (0 giving a flat profile, 1 giving an unmodified profile), gave the best fit to the corresponding psychophysical profile. From the 148 such T- and C-level profiles, a single profile scaling model was determined by a simple linear regression. In phase II, the model was tested on data using three separate stimulation rates (250, 900, and 2400 Hz) and 14 additional subjects. The root mean square psychophysical level mismatch of the ECAP threshold profile, the scaled ECAP threshold profile, a flat profile, and a mean population profile was calculated per subject and per stimulation rate, and the differences in the means of these calculations were compared. In phase III, 13 separate subjects evaluated the scaled ECAP-based program during a 2 wk trial, comparing the new program to a flat program and a conventional ECAP-based program with unmodified ECAP threshold profiles. A questionnaire captured their subjective preferences. In phase I, the profile scaling model constructed from the data prescribed a flattening of the ECAP threshold profile with increasing mean T- or C-level (in CL units): scale = 1.38 - 0.0043 PsychoMean. In phase II, the scaled ECAP-based profiles were found to fit the psychophysical profiles significantly better in all test configurations (typically of the order of 5% dynamic range) compared with all other profiles. In phase III, 62% of subjects preferred the scaled ECAP-based program, whereas 8% preferred the conventional ECAP-based program, 15% the flat program and 15% had no preference. Analyses of the questionnaires revealed significantly higher ratings for the scaled ECAP-based programs, whereas the conventional ECAP-based programs were not rated differently than the flat programs. The scaled ECAP threshold profile method provides a clinically significant enhancement to ECAP-based fitting methods, confirming the value of the ECAP threshold profile to cochlear implant fitting.

Using Evoked Compound Action Potentials to Assess Activation of Electrodes and Predict C-Levels in the Tempo+ Cochlear Implant Speech Processor

In this study, we analyze how electrically evoked compound action potential (ECAP) responses can be used to assess whether electrodes should be activated in the map and to estimate C levels in the Med-El Tempo+ Cochlear Implant Speech Processor. ECAP thresholds were measured using the ECAP Recording System of the Pulsar CI implant. Twenty-one postlingually and 28 prelingually deafened patients participated in this study. The relationship between ECAP responses and the activation of electrodes was analyzed. Because an error in the estimation of T levels (behavioral thresholds) has less effect on hearing quality than an error in the estimation of C levels in the Tempo+ cochlear implant speech processor (maximum comfort levels), correlation and regression analyses were performed between ECAP thresholds and C levels. The observation of an evoked potential generally implied that the electrode was activated because only 3.5% of electrodes that yielded measurable evoked responses were deactivated, because of collateral stimulations or an unpleasant hearing sensation. In contrast, the absence of an evoked potential did not imply that an electrode should be deactivated, because 20% of these electrodes provided a useful auditory sensation. ECAP responses did not predict the absolute behavioral comfort levels because of the excessive error between behavioral C levels and those derived from ECAP thresholds (the mean relative error is 43.78%). However, by applying a normalization procedure, ECAP measurements allowed the C-level profile to be predicted with a mean relative error of 6%; that is, they provided useful data to determine the C level of each electrode relative to the average C level of the patient. ECAP is a reliable and an useful objective measurement that can assist in the fitting of the Tempo+ cochlear implant speech processor. From results presented in this work, a protocol is proposed for fitting this cochlear implant system. This protocol facilitates appropriate cochlear implant fitting, particularly for children or uncooperative patients.

Packing of the Cochleostomy Site Affects Auditory Nerve Response Thresholds in Precurved Off-Stylet Cochlear Implants

To determine whether packing of the cochleostomy site influences electrically evoked compound action potential (ECAP) thresholds. Prospective nonrandomized control study. Tertiary referral pediatric hospital. Eleven consecutive children who received unilateral or bilateral cochlear implants (16 Nucleus 24RE Freedom precurved advanced off-stylet electrode arrays [Cochlear Corp., Lane Cove, New South Wales, Australia]) between June and September 2008. Electrically evoked compound action potential thresholds were evoked by stimulation of basal (electrode 3), mid (electrode 9), and apical (electrode 20) electrodes during cochlear implant surgery in the operating room. The first recording was completed immediately after insertion of the electrode array before cochleostomy packing, and the same measures were collected immediately after the packing. The time between recordings was 5 minutes. Electrically evoked compound action potential thresholds evoked by the apical electrode did not significantly change from pre- to post-cochleostomy packing, but ECAP thresholds increased significantly after packing when evoked by the mid-array electrode (mean, +10 clinical units; p < 0.001) and decreased significantly when evoked by the basal electrode (mean, -5.6 clinical units; p = 0.023). These changes were unrelated to passive changes occurring over the first 5 minutes after insertion of the electrode array. Packing of the cochleostomy causes changes in ECAP thresholds evoked by mid and basal areas of the implant electrode array. These findings suggest that packing shifts the electrode array in the cochlea, increasing the distance from the modiolus in the mid array and decreasing this distance at the basal end. This study is concerned with a single electrode array from a single manufacturer and a single insertion method (precurved off-stylet electrode). These conclusions may be too broad to apply to all precurved electrode arrays.

Comparison of Electrically Evoked Compound Action Potential Thresholds and Loudness Estimates for the Stimuli Used to Program the Advanced Bionics Cochlear Implant

In the mid-1990s, Cochlear Corporation introduced a cochlear implant (CI) to the market that was equipped with hardware that made it possible to record electrically evoked compound action potentials (ECAPs) from CI users of all ages. Over the course of the next decade, many studies were published that compared ECAP thresholds with levels used to program the speech processor of the Nucleus CI. In 2001 Advanced Bionics Corporation introduced the Clarion CII cochlear implant (the Clarion CII internal device is also known as the CII Bionic Ear). This cochlear implant was also equipped with a system that allowed measurement of the ECAP. While a great deal is known about how ECAP thresholds compare with the levels used to program the speech processor of the Nucleus CI, relatively few studies have reported comparisons between ECAP thresholds and the levels used to program the speech processor of the Advanced Bionics CI. To explore the relationship between ECAP thresholds and behavioral measures of perceptual dynamic range for the range of stimuli commonly used to program the speech processor of the Advanced Bionics CI. This prospective and experimental study uses correlational and descriptive statistics to define the relationship between ECAP thresholds and perceptual dynamic range measures. Twelve postlingually deafened adults participated in this study. All were experienced users of the Advanced Bionics CI system. ECAP thresholds were recorded using the commercially available SoundWave software. Perceptual measures of threshold (T-level), most comfortable level (M-level), and maximum comfortable level (C-level) were obtained using both "tone bursts" and "speech bursts." The relationship between these perceptual and electrophysiological variables was defined using paired t-tests as well as correlation and linear regression. ECAP thresholds were significantly correlated with the perceptual dynamic range measures studied; however, correlations were not strong. Analysis of the individual data revealed considerable discrepancy between the contour of ECAP threshold versus electrode function and the behavioral loudness estimates used for programming. ECAP thresholds recorded from Advanced Bionics cochlear implant users always indicated levels where the programming stimulus was audible for the listener. However, the correlation between ECAP thresholds and M-levels (the primary metric used to program the speech processor of the Advanced Bionics CI), while statistically significant, was quite modest. If programming levels are to be determined on the basis of ECAP thresholds, care should be taken to ensure that stimulation is not uncomfortably loud, particularly on the basal electrodes in the array.

Effects of Long-Term Use of a Cochlear Implant on the Electrically Evoked Compound Action Potential

Since the early 1990s, it has been possible to measure electrically evoked compound action potentials (ECAPs) from Nucleus cochlear implant users. Recording the ECAP does not require active participation by the subject, and the recordings are not adversely affected by attention or sleep, making this response an ideal tool for monitoring long-term changes. Previous research from our laboratory (Hughes et al, 2001) has shown that ECAP thresholds and slope of the ECAP growth functions are relatively stable over time. However, this conclusion was based on results obtained from a fairly limited number of study participants, each of whom used the Nucleus CI24M cochlear implant and were followed for less than two years. To evaluate the effect of long-term use of a cochlear implant on ECAP thresholds and slope of the ECAP input/output function for both pediatric and adult cochlear implant recipients. A longitudinal study that describes how ECAP thresholds and growth functions change over a period of 96 mo following initial activation. Changes over time in ECAP threshold and slope of the ECAP growth function were analyzed, and effects of the subject's age, type of CI (cochlear implant), and stimulating electrode are included in the analysis. 134 Nucleus CI users participated in this study. All were profoundly deaf. This subject pool included 84 individuals (40 adults and 44 children) who used the Nucleus CI24M cochlear implant and 50 individuals (21 adults and 29 children) who used the Nucleus CI24R cochlear implant. Electrodes 5, 10, 15, and 20 were stimulated, and ECAP growth functions were measured for each subject at regular intervals following the initial activation of the device. Small increases in mean ECAP thresholds were observed for both pediatric and adult CI users between an "early" visit that occurred within 3-6 mo following hookup and a "late" visit that occurred 4.8-6 yr later. For adults, the average increase in ECAP threshold was 3.94 CL (clinical programming units for Nucleus CIs). For children, the average increase was 4.16 CL. These differences, while small, were statistically significant. Slope of the ECAP growth functions measured over the same time interval did not change significantly. On average, pediatric CI users had ECAP thresholds that were 4-5 CL units higher than the adult CI recipients. The most striking outcome from this study, however, was the finding that when compared with postlingually deafened adults, pediatric CI users had ECAP growth functions that were substantially steeper. The differences between the results obtained from children and those obtained from adults were statistically significant and largely independent of device type or stimulating electrode. Results from this study show ECAP thresholds and growth functions to change very little over a 5-6 yr observation interval suggesting that long-term use of a CI is not likely to have a significant negative impact on the response of the peripheral auditory system. Pediatric CI users were shown to have, on average, higher ECAP thresholds and steeper ECAP growth functions than postlingually deafened adult CI users. This finding suggests potential differences between the two patient populations either in terms of the current fields within the cochlea or the effective distance between the stimulating electrode and the stimulable neural tissue.

Clinical use of a system for the automated recording and analysis of electrically evoked compound action potentials (ECAPs) in cochlear implant patients

Conclusion: The CustomSoundEP software can record ECAP traces in an automated manner and calculate the ECAP threshold from them. These values are in good agreement with those determined by audiologists. Manual optimization measurements, as needed when using the predecessor version 3 of the software, are done by the software itself in the newer system. Objectives: The programming of a cochlear implant speech processor is usually carried out in a subjective manner, influenced by the audiologist and the subject, and it takes considerable time. With increasing numbers of patients there is a growing need for a rapid and reliable technique to measure and analyse neural responses – one which could give suggestions for programming the speech processor and thus shorten the fitting procedure. Methods: In 30 patients, ECAP responses were recorded postoperatively in an automated procedure. ECAP thresholds were determined automatically by the AutoNRTTM (Automatic Neural Response Telemetry) software and visually by seven persons. Results: The time required to determine a TECAP threshold for a single electrode contact was found to be between 0.5 and 1 min.

Evolution of Cochlear Implant Arrays Result in Changes in Behavioral and Physiological Responses in Children

To determine whether a change in cochlear implant technology and electrode array design affects electrophysiological, behavioral, and functional measures of audition in pediatric cochlear implant users. Prospective nonrandomized control study over 6 months postimplantation. Tertiary referral pediatric hospital. A total of 115 children using unilateral cochlear implants were included in this study. Subjects were divided into 3 groups: 1) 38 using the Nucleus 24M straight array device, 2) 20 using the perimodiolar Nucleus 24RCS Contour array, and 3) 57 using the perimodiolar Nucleus 24RE array with advance off stylet insertion. The mean ages at implantation were 4.85 +/- 4 (24M), 3.88 +/- 3.4 (24RCS), and 5.41 +/- 4.36 years (24RE; not significant, p > 0.05). The electrically evoked compound action potential (ECAP) and the electrically evoked stapedius reflex and behavioral measures of stimulation threshold were evoked by stimulation of basal, mid, and apical electrodes. These measures were completed at regular intervals over 6 to 12 months of implantation. Age-appropriate speech perception skills were also assessed during this period. The 24RE array group had significantly lower ECAP and behavioral thresholds compared with the 24M and 24RCS array groups. The largest reductions of ECAP thresholds in the precurved array group were observed upon apical and basal electrode stimulation. Electrically evoked stapedius reflex thresholds were significantly higher in the 24RE group as compared with 24M and 24RCS groups. Comparing age-matched groups, open- and closed-set speech perception test scores were significantly higher in the 24RE array group. The precurved Freedom 24RE cochlear implant potentially provides a wider range of stimulation levels and better functional results than the straight electrode 24M and the precurved 24RCS devices in profoundly hearing impaired children.

Evaluation of a Novel, Noninvasive, Objective Test of Auditory Nerve Function in Cochlear Implant Candidates

To investigate whether the objective noninvasive recording of the electrically evoked amplitude modulation following response (EAMFR) can be applied for the assessment of auditory nerve function before cochlear implant surgery. Prospective clinical study from January 2005 to August 2008. Cochlear Implant Program at the Charité University Hospital (Berlin, Germany). One hundred twenty-one patients with severe to total bilateral sensorineural hearing impairment. Diagnostic. All EAMFR thresholds were correlated with the mean intraoperative electrically evoked compound action potential (ECAP) thresholds. In a subgroup of 16 patients, the objective thresholds of the EAMFR were compared with the subjective auditory thresholds for the electrical stimuli applied. The intraindividual left-right concordance of the EAMFR and ECAP thresholds was investigated in 22 patients with bilateral implants. The EAMFR-auditory nerve function test produced clear responses in all patients examined. The threshold of auditory perception for the applied stimuli correlated highly significantly with the EAMFR threshold (r = 0.89, p < 0.01). The correlation between the EAMFR and mean ECAP thresholds was again highly significant (r = 0.49, p < 0.01). In all patients with bilateral implants, the ear with the lower preoperative EAMFR threshold was also the one with the lower intraoperative mean ECAP threshold. These results indicate that the objective noninvasive recording of EAMFR seems to be a useful diagnostic tool for the evaluation of the auditory nerve function in cochlear implant candidates, especially in children or in adult patients who require objective examination.

Psychophysical and physiological measures of electrical-field interaction in cochlear implants

The primary purpose of this study was to determine whether the electrically evoked compound action potential (ECAP) can be used to predict psychophysical electrical-field interaction patterns obtained with simultaneous stimulation of intracochlear electrodes. The second goal was to determine whether ECAP patterns are affected by recording location because differences might influence the relation between ECAP and psychophysical measures. The third goal was to investigate whether symmetrical threshold shifts are produced with phase inversion of the interaction stimulus. Nine adults with Advanced Bionics cochlear implants participated. ECAP and psychophysical thresholds were obtained for basal, middle, and apical probe electrodes in the presence of a subthreshold interaction stimulus delivered simultaneously to each of seven to eight interaction electrodes per probe. The results showed highly significant correlations between ECAP and psychophysical threshold shifts for all nine subjects, which suggests that the ECAP can adequately predict psychophysical electrical-field interaction patterns for subthreshold stimuli. ECAP thresholds were significantly higher for recordings from the basal (versus apical) side of the probe, which suggests that recording location may affect relations between ECAP and psychophysical measures. Interaction stimulus phase inversion generally produced symmetrical threshold shifts for psychophysical measures but not for half of ECAP measures.

Adaptation of the Electrically Evoked Compound Action Potential (ECAP) Recorded from Nucleus CI24 Cochlear Implant Users

This study had three main goals. The first goal was to assess the extent to which neural adaptation varied across cochlear implant users. The second goal was to determine whether adaptation at the level of the auditory nerve was correlated with word recognition ability. The third goal was to determine whether peripheral neural adaptation had an impact on the relationship between the electrically evoked compound action potential (ECAP) thresholds and MAP levels. Neural response telemetry software was used to record the ECAP in 21 Nucleus cochlear implant users. A series of 110 ECAP recordings were made over a 5-min period at three different stimulation rates: 15, 80, and 300 Hz. The stimulation levels used to record this series of responses were held constant at or near the level the subject identified as his or her maximum comfort level (C-level) for the 300-Hz stimulation rate. Consistent decreases in ECAP amplitude as measured from the beginning to the end of the 5-min stimulation interval were interpreted as evidence of neural adaptation. Regression analysis procedures were then used to assess the relationship between neural adaptation and word recognition. Significant levels of adaptation were observed for all 21 subjects at stimulation rates of 80 and 300 Hz. Little or no adaptation was observed over the 5-min recording period when the 15-Hz rate was used. The amount of adaptation was greatest at the 300-Hz rate and varied substantially across cochlear implant users. No relationship between the amount of adaptation and word recognition was found. Neither was the degree of adaptation shown to influence the relationship between ECAP thresholds recorded at low rates and the levels used to program the speech processor. Cochlear implant users experienced varying degrees of long-term adaptation in response to continuous electrical stimulation. The effects of adaptation on the ECAP were apparent even at stimulation rates as low as 80 Hz. Although variations in the amount of adaptation are likely to reflect cross-subject differences in the status of the auditory nerve, no predictable relationship was found between these physiologic measures of peripheral neural function and either word recognition or the relationship between ECAP thresholds and MAP levels.

Clinical Results of AutoNRT,™ a Completely Automatic ECAP Recording System for Cochlear Implants

AutoNRT is the completely automatic electrically evoked compound action potential (ECAP) measuring algorithm in the recently released Nucleus Freedom cochlear implant system. AutoNRT allows clinicians to automatically record T-NRT profiles that in turn can be used as a guide for initial fitting. The algorithm consists of a pattern recognition part that judges if the traces contain an ECAP and an intelligent flow that optimizes the measurement parameters and finds the ECAP threshold (T-NRT). The objective of this study was to determine how accurate, reliable, and fast the automatic measurements are. Data on more than 400 electrodes were collected as part of the multicenter clinical trial of the Nucleus Freedom cochlear implant system. T-NRT values determined by the algorithm were compared with T-NRT determinations on the same data by different human observers. Also, the time the measurements took was analyzed. In 90% of the cases, the absolute difference between the AutoNRT and the human observer determined T-NRT was less than 9 CL; the median absolute difference was 3 CL. A second experiment, in which a group of human observers were asked to analyze NRT data, showed high variability in T-NRT; in some cases, two experienced clinicians disagreed by more than 30 current levels. Compared with the group, AutoNRT performed as well as the "average" clinician, with the advantage that the AutoNRT threshold determinations are objective. Analysis of the timing data showed an average intraoperative measurement time of less than 20 sec per electrode with a standard deviation of 5 sec, suggesting that the total array of 22 electrodes can be measured intraoperatively in about 7 minutes on average. AutoNRT provides comparable accuracy to an average clinician but with the added benefit of significant time savings over manual recordings. This makes it a valuable tool for clinical measurement of ECAP threshold in cochlear implant recipients.

Measuring the Refractoriness of the Electrically Stimulated Auditory Nerve

Intracochlear recordings in cochlear implant recipients provide access to the electrically evoked compound action potential (ECAP). ECAP thresholds are potential predictors of speech processor map’s threshold and comfortable loudness levels. The auditory nerve’s refractory properties can influence these levels due to high-rate stimulation with interpulse intervals within the relative refractory period. Recovery functions were investigated at 84 stimulation sites in 14 Nucleus CI24 recipients using neural response telemetry and a modified forward masking technique. This technique introduces a reference masker-probe interval (MPI). In our study, an appropriate value between 300 and 375 µs was determined for this reference MPI, and the use of a reference MPI of 300 µs is suggested for recovery and amplitude growth functions. A median absolute refractory period of about 390 µs and a median time constant of about 425 µs were obtained by fitting an exponential model to the data. Hence, the auditory nerve is usually in relative refractory state when standard neural response telemetry forward masking is selected because of its default MPI of 500 µs. This can bias the measurement of ECAP thresholds. Additionally, the shape of standard forward masking recovery functions was explained by the influence of latency shift of the neural response.

AutoNRT™: An automated system that measures ECAP thresholds with the Nucleus ® Freedom™ cochlear implant via machine intelligence

AutoNRT is an automated system that measures electrically evoked compound action potential (ECAP) thresholds from the auditory nerve with the Nucleus Freedom cochlear implant. ECAP thresholds along the electrode array are useful in objectively fitting cochlear implant systems for individual use. This paper provides the first detailed description of the AutoNRT algorithm and its expert systems, and reports the clinical success of AutoNRT to date. AutoNRT determines thresholds by visual detection, using two decision tree expert systems that automatically recognise ECAPs. The expert systems are guided by a dataset of 5393 neural response measurements. The algorithm approaches threshold from lower stimulus levels, ensuring recipient safety during postoperative measurements. Intraoperative measurements use the same algorithm but proceed faster by beginning at stimulus levels much closer to threshold. When searching for ECAPs, AutoNRT uses a highly specific expert system (specificity of 99% during training, 96% during testing; sensitivity of 91% during training, 89% during testing). Once ECAPs are established, AutoNRT uses an unbiased expert system to determine an accurate threshold. Throughout the execution of the algorithm, recording parameters (such as implant amplifier gain) are automatically optimised when needed. In a study that included 29 intraoperative and 29 postoperative subjects (a total of 418 electrodes), AutoNRT determined a threshold in 93% of cases where a human expert also determined a threshold. When compared to the median threshold of multiple human observers on 77 randomly selected electrodes, AutoNRT performed as accurately as the 'average' clinician. AutoNRT has demonstrated a high success rate and a level of performance that is comparable with human experts. It has been used in many clinics worldwide throughout the clinical trial and commercial launch of Nucleus Custom Sound Suite, significantly streamlining the clinical procedures associated with cochlear implant use.

Normative Findings of Electrically Evoked Compound Action Potential Measurements Using the Neural Response Telemetry of the Nucleus CI24M Cochlear Implant System

One hundred and forty-seven adult recipients of the Nucleus^® 24 cochlear implant system, from 13 different European countries, were tested using neural response telemetry to measure the electrically evoked compound action potential (ECAP), according to a standardised postoperative measurement procedure. Recordings were obtained in 96% of these subjects with this standardised procedure. The group results are presented in terms of peak amplitude and latency, slope of the amplitude growth function and ECAP threshold. The effects of aetiological factors and the duration of deafness on the ECAP were also studied. While large intersubject variability and intrasubject variability (across electrodes) were found, results fell within a consistent pattern and a normative range of peak amplitudes and latencies was established. The aetiological factors had little effect on the ECAP characteristics. However, age affected ECAP amplitude and slope of the amplitude growth function significantly; i.e., the amplitude is higher in the lowest age category (15–30 years). Principal component analysis of the ECAP thresholds shows that the thresholds across 5 electrodes can be described by two factors accounting for 92% of the total variance. The two factors represent the overall level of the threshold profiles (‘shift’) and their slopes across the electrode array (‘tilt’). Correlation between these two factors and the same factors describing the T- and C-levels appeared to be moderate, in the range of 0.5–0.6.

Electrically Evoked Compound Action Potential Amplitude Growth Functions and HiResolution Programming Levels in Pediatric CII Implant Subjects

To characterize the amplitude growth functions of the electrically evoked compound action potential (ECAP) in pediatric subjects implanted with the Clarion HiFocus electrode array with respect to electrode position and the presence or absence of a Silastic positioner. Electrophysiologic growth function data are compared with HiResolution (HiRes) psychophysical programming levels. ECAP growth functions were measured for all electrodes along the implant's array in 16 pediatric subjects. Nine of the patients were implanted with a Silastic positioner, whereas seven had no positioner. ECAP thresholds and growth function slopes were calculated. Fifteen of the 16 patients had psychophysical threshold and maximum comfort levels available. Programming levels and ECAP thresholds were compared within and among the subjects. ECAP thresholds showed variability among patients, ranging from 178 to 920 nA at 32 musec pulse width. ECAP thresholds did not depend on electrode position along the cochlea but were lower in the presence of the Silastic positioner (p < 0.001). Thresholds determined with the masker-probe versus the alternating polarity paradigms revealed moderate (r = 0.76) correlation. Growth function slopes also showed considerable variation among patients. Unlike thresholds, slopes decreased from apical to basal cochlear locations (p < 0.001) but showed no difference between the absence and presence of the positioner. Programming levels in HiRes were correlated with ECAP threshold levels. When ECAP thresholds were adjusted for each patient by the difference between M level and ECAP threshold at electrode 9, however, overall correlation between the two measurements was excellent (r = 0.98, N = 224). In pediatric subjects with the Clarion HiFocus electrode, ECAP growth function thresholds appear to decrease with the presence of the Silastic positioner but are unaffected by electrode position along the array. Growth function slope, however, depends on electrode position along the array but not on the presence of the positioner. ECAP programming levels can reliably predict stimulus intensities within the patients' dynamic ranges, but considerable variability is seen between ECAP thresholds and HiRes programming levels.

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 μA increase in psychophysical levels for every 1 μA 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 μA (95% confidence interval: 0.18–0.28) in T-level for every 1 μA 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.

Speech Perception in Nucleus CI24M Cochlear Implant Users with Processor Settings Based on Electrically Evoked Compound Action Potential Thresholds

Adjusting the speech processor of a cochlear implant, per electrode, to the individual’s response is a laborious task that may interfere with a user-friendly start of implant-mediated hearing, particularly in children. This research concerns the possibility of processor adjustment based on a profile derived from measurements of the electrically evoked compound action potential (ECAP) thresholds across the electrode array, followed by adjustment of the overall level of the profile to the hearing threshold and maximum comfortable loudness level using live voice. The results for CVC word lists show that speech perception is quite insensitive to the threshold setting of the speech processor. On average, the speech score does not decrease by more than 10% when, with the new method, the threshold setting comes out so much lower that the dynamic range has doubled. In contrast, the speech score appears to be sensitive to an increase of the maximum high-frequency stimulation settings for the basal electrodes, resulting in lower scores at these higher settings. The correlation between the overall ECAP thresholds and conventionally measured subjective thresholds is weak (r = 0.64). However, the correlation between the slopes of these threshold curves is satisfactory (r = 0.82). The correlation between the ECAP thresholds and the maximum stimulation levels is poor, both with respect to overall level and slope (r = 0.39 and 0.36, respectively). Applicability of the ECAP threshold in processor adjustment could not be demonstrated in this study. Prediction of the most critical factor in speech perception, the slope of the maximum stimulation curve, from the ECAP thresholds is poor. However, considering habituation to the initial processor setting of at least 6 months, the small decrease in the CVC scores with the new setting suggests that a more user-friendly adjustment procedure can be developed.