The effect of current amplitude and multi-electrode stimulation on eSRT for auditory fitting in cochlear implants...

Neurophysiologic measures like the electrically evoked Stapedius Reflex Threshold (eSRT) offer valuable insights into personalized responses to Cochlear Implant (CI) stimuli. This study evaluates the effects of stimulation current amplitude and the number of stimulating electrodes on eSRT and its correlation with the Most Comfortable Loudness level (MCL or C-level) in all-polar, pseudo-monophasic, pulse-width loudness coded stimulations.

The study was conducted on seventeen adult patients with Oticon Medical implants. In these implants, current amplitude remains constant while loudness is coded through pulse-width. Stimulation amplitudes were set to low (default clinical) and high values, using single or multi electrode groups of 1, 3, and 5 (G1, G3, and G5) across five cochlear regions, spanning from apical to basal. For each amplitude and group, the eSRT detection rate and correlation with MCL were analyzed regionally and overall.

Higher eSRT detection rates were observed in the contra-lateral ear, with G3 and G5 significantly enhancing detection over G1 for both low and high amplitudes. The current amplitude facilitated eSRT invocation, particularly for G1 and patients with higher MCLs. The correlation between eSRT and MCL was relatively high (r = 0.64-0.87) across all conditions, indicating accurate MCL estimation from eSRT at both low and high amplitudes.

Among all condition interactions (amplitude, side, electrodes), low-amplitude contra-lateral G5 and high-amplitude contra-lateral G3 showed optimal modes for estimating MCL from eSRT, with linear slopes near 1.0, and detection rates of 54% and 63%, respectively. High-amplitude contra-lateral G5 further increased detection to 68%, though with a slope of 0.8, requiring a correction factor. These findings provide insights into eSRT mechanisms and its application in pseudo-monophasic all-polar CI programming.

The eSRT detection rate can be improved in pulse-width modulation cochlear implants by using multi-electrode stimulation and/or increasing the stimulation amplitude beyond the default clinical value, without significantly affecting the correlation coefficient between eSRT and MCL.