The Effect of Advanced Age on the Electrode-Neuron Interface in Cochlear Implant Users

Abstract

Objectives: This study aimed to determine the effect of advanced age on how effectively a cochlear implant (CI) electrode stimulates the targeted cochlear nerve fibers (i.e., the electrode-neuron interface [ENI]) in postlingually deafened adult CI users. The study tested the hypothesis that the quality of the ENI declined with advanced age. It also tested the hypothesis that the effect of advanced age on the quality of the ENI would be greater in basal regions of the cochlea compared to apical regions.

Design: Study participants included 40 postlingually deafened adult CI users. The participants were separated into two age groups based on age at testing in accordance with age classification terms used by the World Health Organization and the Medical Literature Analysis and Retrieval System Online bibliographic database. The middle-aged group included 16 participants between the ages of 45 and 64 years and the elderly group included 24 participants older than 65 years. Results were included from one ear for each participant. All participants used Cochlear Nucleus CIs in their test ears. For each participant, electrophysiological measures of the electrically evoked compound action potential (eCAP) were used to measure refractory recovery functions and amplitude growth functions (AGFs) at three to seven electrode sites across the electrode array. The eCAP parameters used in this study included the refractory recovery time estimated based on the eCAP refractory recovery function, the eCAP threshold, the slope of the eCAP AGF, and the negative-peak (i.e., N1) latency. The electrode-specific ENI was evaluated using an optimized combination of the eCAP parameters that represented the responsiveness of cochlear nerve fibers to electrical stimulation delivered by individual electrodes along the electrode array. The quality of the electrode-specific ENI was quantified by the local ENI index, a value between 0 and 100 where 0 and 100 represented the lowest- and the highest-quality ENI across all participants and electrodes in the study dataset, respectively.

Results: There were no significant age group differences in refractory times, eCAP thresholds, N1 latencies or local ENI indices. Slopes of the eCAP AGF were significantly larger in the middle-aged group compared to the elderly group. There was a significant effect of electrode location on each eCAP parameter, except for N1 latency. In addition, the local ENI index was significantly larger (i.e., better ENI) in the apical region than in the basal and middle regions of the cochlea for both age groups.

Conclusions: The model developed in this study can be used to estimate the quality of the ENI at individual electrode locations in CI users. The quality of the ENI is affected by the location of the electrode along the length of the cochlea. The method for estimating the quality of the ENI developed in this study holds promise for identifying electrodes with poor ENIs that could be deactivated from the clinical programming map. The ENI is not strongly affected by advanced age in middle-aged and elderly CI users.

Figure 1.

Upper panels: electrically-evoked compound action potential (eCAP) waveforms measured at different masker-probe intervals (MPIs) for stimulating electrode 3 in one middle-aged adult (A27) and one elderly adult (A25). Waveforms are arranged based on MPI duration (in ms), with responses evoked by shorter MPIs and longer MPIs displayed toward the top and bottom, respectively. Lower panels: refractory recovery functions (round symbols) obtained from the waveforms in the upper panels. The fitted exponential decay function for each refractory recovery function (black line), the goodness of fit (i.e., R²) and the resulting estimation for the refractory time (t₀) are also provided. The participant and electrode number are included in the upper left corner of the top panels.

Figure 2.

Upper panels: electrically-evoked compound action potential (eCAP) waveforms measured at different stimulation levels for electrode 3 in one middle-aged adult (A37) and one elderly adult (A38). Waveforms are arranged based on stimulation level (in current levels [CLs]), with responses evoked by the smallest stimulation level (i.e., eCAP threshold) displayed at the top. The largest stimulation level presented was the maximum comfortable level (i.e., C level) and is displayed at the bottom. Lower panels: eCAP amplitude growth functions (AGFs, round symbols) obtained from the waveforms in the upper panels after converting stimulation levels to logarithmic units (dB re 1 nanocoulomb [nC]). The slope is provided for each eCAP AGF, along with a representation of the slope estimated with sliding window linear regression (solid gray line). The participant and electrode number are included in the upper left corner of the top panels.

Figure 3.

Results of electrically-evoked compound action potential (eCAP) parameters (mean and standard deviation) measured for children with cochlear nerve deficiency (CND) and children with normal-sized cochlear nerves (NSCNs) used for model training. Ordered from left to right are the estimated absolute refractory recovery times (i.e., t₀), eCAP thresholds, slopes of eCAP amplitude growth functions, and N1 peak latencies. Significant group differences are indicated with asterisks.

Figure 4.

Local electrode-neuron interface indices for children with normal-sized cochlear nerves and sensorineural hearing loss (S) and children with cochlear nerve deficiency (CND) included in the validation dataset.

Figure 5.

The means and standard deviations of estimated absolute refractory recovery times (i.e., t₀) for two age groups at seven electrode locations.

Figure 6.

Results of electrically evoked compound action potential (eCAP) thresholds (means and standard deviations) measured for two age groups at seven electrode locations.

Figure 7.

The means and standard deviations of slopes of electrically evoked compound action potential (eCAP) amplitude growth functions (AGFs) for two age groups at seven electrode locations.

Figure 8.

Results of N1 peak latencies (means and standard deviations) measured for two age groups at seven electrode locations.

Figure 9.

The means and standard deviations of local electrode-neuron interface indices for two age groups at seven electrode locations.