Effects of long-term use of a cochlear implant on the electrically evoked compound action potential

Abstract

Background: 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.

Purpose: 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.

Research design: 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.

Study sample: 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.

Data collection and analysis: 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.

Results: 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.

Conclusion: 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.

Figure 1

ECAP waveforms recorded from two different CI users are shown. M33 is a postlingually deafened adult. ECAP growth functions were recorded at 3 and 96 mo posthookup. CM 12 is a child with congenital, progressive hearing loss implanted at 42 mo of age. ECAPs were recorded at 2 and 85 mo posthookup. Stimulation levels for both visits are shown. The response marked with the asterisk was identified as threshold. The panel on the right shows ECAP amplitude plotted as a function of the current level of the probe for both visits for these two study participants.

Figure 2

The two panels on the left show changes in ECAP threshold and slope over time for study participants who use the CI24M cochlear implant. The two panels on the right show changes in ECAP threshold and slope over time for study participants who use the CI24R cochlear implant. Data recorded from children are shown with open symbols and dashed lines. Data recorded from adults are shown with filled symbols and solid lines. The error bars indicate ± 1 SE around the mean. Stimulation level is specified in clinical programming units (CL).

Figure 3

Box plots showing distribution of the ECAP threshold and slope data for subjects tested at both the early and late intervals. The labels on the abscissa indicate the device type. Filled bars indicate results obtained from adult study recipients. Open bars indicate results obtained from pediatric CI users. Dots indicate the 5th and 95th percentiles. Whiskers indicate the 10th and 90th percentiles. The upper and lower boundaries of the box indicate the 25th and 75th percentiles. The mean and median are shown with the thick and thin lines, respectively, located near the center of the individual boxes. Stimulation level is specified in clinical programming units (CL).

Figure 4

These graphs show mean ECAP thresholds plotted as a function of the stimulating electrode. Data from children are shown with open symbols and dashed lines. Data from adults are shown with filled symbols and solid lines. The circles are used to indicate data obtained from study participants who used the Nucleus CI24M cochlear implant. Squares are used to show results obtained from Nucleus CI24R cochlear implant users. Error bars indicate ±1 SE around the mean. Stimulation level is specified in clinical programming units (CL).

Figure 5

These graphs show slope of the ECAP growth functions plotted as a function of the stimulating electrode. Data from children are shown with open symbols and dashed lines. Data from adults are shown with filled symbols and solid lines. Circles are used to indicate data obtained from study participants who used the Nucleus CI24M cochlear implant. Squares are used to show results obtained from Nucleus CI24R cochlear implant users. Error bars indicate ± 1 SE around the mean. Stimulation level is specified in clinical programming units (CL).