gms | German Medical Science

Introduction: ECAPs can be used to estimate cochlear health [1]. For this purpose, responses to stimuli with 30 µs IPG and 2.1 µs IPG are typically compared. Earlier studies have demonstrated a steeper ECAP slope for stimulations with 30 µs IPG compared to 2.1 µs [1]. Furthermore, loudness perception is increased when measuring ECAPs with 30µs IPG vs. 2.1µs with otherwise identical stimulation intensities [2]. Knowingly, post-op ECAP function and loudness perception change over time [3]. Both are evaluated in this study.

Methods: The stimulation comprised continuous, biphasic rectangular pulses (phase duration 40 µs) with alternating phases and steadily increasing amplitude (0–750 µA in 20 seconds). During each session, the IPG was varied: 30 µs/2.1 µs. The respective measurement of the paradigms was aborted at a stimulation intensity of 30 nC if the parameterization was not successful beforehand. Loudness perception was recorded in parallel using a continuous scaling procedure [scale 0–1]. Measurement time points were: 0 months (intra-op), 1 and 3 months post-op.

Results: The pooled median ECAP thresholds (eTHR) was decreasing intra-op vs. 3 months post-op for IPG 30 µs and for 2.1 µs. The median ECAP slope (eSLOPE) was increasing over time for 30 µs IPG and stable for 2.1 µs. For all cochlear regions, steeper median eSLOPEs were found for 30 µs IPG after 3 months compared to intra-op whereas median eSLOPEs were stable for 2.1 µs IPG.

Differences in median perception thresholds (pTHR) were found between 30 µs IPG vs. 2.1 µs at 1 and 3 months. For all regions and over time the median pTHR were stable. Maximum loudness was higher and maximum stimulation intensity was lower for 30 µs IPG vs 2.1 µs at 1 and 3 months. Whereas the maximum stimulation intensity was similar over all regions, loudness percept was highest in the apex > base > middle, for both IPG and keeping this order over time.

Discussion: Our results show the expected change over time for eTHR. Furthermore, an increase of eSLOPE for IPG 30 µs was observed. The louder percept for 30 µs vs. 2.1 µs is partly hidden by a reduced stimulation intensity necessary for ECAP parametrization. In the apex, loudness percept was found to be highest compared to other regions at same stimulation intensities. This is in good agreement with the steepest slopes indicating a higher number of recruited nerve fibres.