Using Neural Response Telemetry to Monitor Physiological Responses to Acoustic Stimulation in Hybrid Cochlear Implant Users

Abstract

Objective This report describes results of a series of experiments where we use the neural response telemetry (NRT) system of the Nucleus cochlear implant (CI) to measure the response of the peripheral auditory system to acoustic stimulation in Nucleus Hybrid CI users. Our objectives were to determine whether we could separate responses from hair cells and neurons, and to evaluate the stability of these measures over time. Design Forty-four CI users participated. They all had residual acoustic hearing and used a Nucleus Hybrid S8, S12, or L24 CI or the standard lateral wall CI422 implant. The NRT system of the CI was used to trigger an acoustic stimulus (500 Hz tone burst or click), which was presented at a low stimulation rate (10, 15 or 50 per second) to the implanted ear via an insert earphone and to record the cochlear microphonic (CM), the auditory nerve neurophonic (ANN) and the compound action potential (CAP) from an apical intracochlear electrode. To record acoustically evoked responses, a longer time window than is available with the commercial NRT software is required. This limitation was circumvented by making multiple recordings for each stimulus using different time delays between the onset of stimulation and the onset of averaging. These recordings were then concatenated offline. Matched recordings elicited using positive and negative polarity stimuli were added off line to emphasize neural potentials (SUM) and subtracted off line to emphasize potentials primarily generated by cochlear hair cells (DIF). These assumptions regarding the origin of the SUM and DIF components were tested by comparing the magnitude of these derived responses recorded using various stimulation rates. Magnitudes of the SUM and DIF components were compared to each other and to behavioral thresholds. Results SUM and DIF components were identified for most subjects, consistent with both hair cell and neural responses to acoustic stimulation. For a subset of the study participants, the DIF components grew as stimulus level was increased, but little or no SUM components were identified. Latency of the CAPs in response to click stimuli was long relative to reports in the literature of recordings obtained using extracochlear electrodes. This difference in response latency and general morphology of the CAPs recorded was likely due to differences across subjects in hearing loss configuration. Use of high stimulation rates tended to decrease SUM and CAP components more than DIF components. We suggest this effect reflects neural adaptation. In some individuals, repeated measures were made over intervals as long as 9 months. Changes over time in DIF, SUM and CAP thresholds mirrored changes in audiometric threshold for the subjects who experienced loss of acoustic hearing in the implanted ear. Conclusions The Nucleus NRT software can be used to record peripheral responses to acoustic stimulation at threshold and supra-threshold levels, providing a window into the status of the auditory hair cells and the primary afferent nerve f...

Auditory synaptopathy, auditory neuropathy, and cochlear implantation

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“…ECAP measurements are used intraoperatively to confirm auditory nerve, device integrity, and electrode functionality and postoperatively by audiologists for speech processor programming. ECAP measurements have been correlated with both detection thresholds (T-levels) and maximum comfortable loudness (C-levels), with a greater correlation found with T-levels (Brown et al, 1996;1998;Hughes et al, 2000;Franck and Norton, 2001;Abbas et al, 2017). The ECAP maximum amplitude has been linked to speech perception scores after CI (Kim et al, 2010;Schvartz-Leyzac and Pfingst, 2018).…”

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