Test / Re-Test Variability of ECAP Thresholds: A Monte Carlo Simulation

Background

The reliability of an eCAP threshold depends on its precision and accuracy. The precision, or variability, of the eCAP threshold has received relatively little attention. We have determined the test / re-test variability of the eCAP threshold, reflecting the effect of the background noise in the recordings.

Objective

Evaluation of the precision of the eCAP threshold as a function of the number of waveform averages by using repeated measurements followed by a Monte Carlo simulation. The slope of the AGF, cochlear location, and eCAP threshold were taken into account.

Methods

eCAP thresholds were determined by constructing amplitude growth functions (AGFs) and linear extrapolation to zero-amplitude in subjects implanted with an Advanced Bionics cochlear implant. A total of 256 waveforms were obtained. The precision of the eCAP threshold was expressed as the standard deviation (SD) using a Monte Carlo simulation.

Results

The SD of the eCAP threshold was reduced by approximately the square root of two when the number of averages in the eCAP waveforms was doubled. The precision was significantly better in the cochlear base, and when AGFs had a steeper slope. Absolute eCAP threshold did not significantly affect precision. At the default number of averages in the Advanced Bionics system, we report a median SD of 3 – 5 μA, with a range of 2 – 18 μA across the cochlea.

Conclusions

The test / re-test variability in the eCAP threshold is small, and negligible compared to the total variability introduced by the linear extrapolation method.