The Precision of eCAP Thresholds Derived From Amplitude Growth Functions

An amplitude growth function (AGF) shows the amplitude of an electrically evoked compound action potential (eCAP) as a function of the stimulation current. AGFs can be used to derive the eCAP threshold, which represents the minimum amount of current needed to elicit a measurable eCAP. eCAP thresholds have been widely used clinically to, for example, assist with sound processor programming. However, no eCAP precision has been included to date. The aim of this study was to investigate the precision of eCAP thresholds and determine whether they are precise enough for clinical use.

The study is retrospective, and the data comprised 826 AGFs, intraoperatively measured in 111 patients implanted with a HiRes90K cochlear implant (Advanced Bionics). For each AGF, the eCAP threshold was determined using two commonly used methods: linear extrapolation (LE) toward the x axis and detection of the last visible (LV) eCAP. Subsequently, the threshold confidence interval (TCI) of each eCAP threshold was calculated to serve as a metric for precision, whereby a larger TCI means a lower precision or reliability. Additionally, the eCAP thresholds results were compared with most recent behavioral fitting thresholds (T profile) to put the eCAP threshold analysis in clinical context. Thereby, the association between eCAP and behavioral thresholds was calculated, both for all subjects together (group analysis) and, in contrast to previous studies, within individual subjects.

Our data show that the TCIs were larger with the LE method than with the LV method. The eCAP thresholds estimated by the LE method were systematically smaller than those estimated by the LV method, while the LE thresholds with the smallest TCIs correlated best with the LV thresholds. Correlation analysis between eCAP and behavioral thresholds revealed correlation coefficients of r = 0.44 and r = 0.54 for the group analysis of LE and LV thresholds, respectively. Within individual subjects, however, the correlation coefficients varied from approximately -1 to +1 for both LE and LV thresholds. Further analysis showed that across subjects, the behavioral thresholds fell within the TCIs of the eCAP threshold profiles.

This study shows that eCAP thresholds have an uncertainty that can be estimated using TCIs. The size of the TCI depends on several factors, for example, the threshold estimation method and measurement conditions, but it is often larger than one would expect when just looking at the threshold values. Given these large TCIs, future research on eCAP thresholds should be accompanied by a measure of precision to correctly apply eCAP thresholds in clinical practice. Comparing our eCAP threshold results with T profiles indicates that the eCAP thresholds are possibly not precise enough to predict T profiles.