The next generation of Nucleus(®) fitting: a multiplatform approach towards universal cochlear implant management.

OBJECTIVE: This article provides a detailed description and evaluation of the next Nucleus(®) cochlear implant fitting suite. A new fitting methodology is presented that, at its simplest level, requires a single volume adjustment, and at its advanced level, provides access to 22-channel fitting. It is implemented on multiple platforms, including a mobile platform (Remote Assistant Fitting) and an accessible PC application (Nucleus Fitting Software). Additional tools for home care and surgical care are also described. DESIGN: Two trials were conducted, comparing the fitting methodology with the existing Custom Sound™ methodology, as fitted by the recipient and by an experienced cochlear implant audiologist. STUDY SAMPLE: Thirty-seven subjects participated in the trials. RESULTS: No statistically significant differences were observed between the group mean scores, whether fitted by the recipient or by an experienced audiologist. The lower bounds of the 95% confidence intervals of the differences represented clinically insignificant differences. No statistically significant differences were found in the subjective program preferences of the subjects. CONCLUSIONS: Equivalent speech perception outcomes were demonstrated when compared to current best practice. As such, the new technology has the potential to expand the capacity of audiological care without compromising efficacy.

Neural response telemetry reconsidered: I. The relevance of ECAP threshold profiles and scaled profiles to cochlear implant fitting.

OBJECTIVE: 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. DESIGN: 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. RESULTS: 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. CONCLUSIONS: 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.

Neural response telemetry reconsidered: II. The influence of neural population on the ECAP recovery function and refractoriness.

OBJECTIVE: 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. DESIGN: 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). RESULTS: 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. CONCLUSIONS: : 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.

Clinical results of AutoNRT, a completely automatic ECAP recording system for cochlear implants.

OBJECTIVE: 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. DESIGN: 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. RESULTS: 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. CONCLUSIONS: 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.

AutoNR: an automated system that measures ECAP thresholds with the Nucleus Freedom cochlear implant via machine intelligence.

OBJECTIVE: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.