Electrically evoked compound action potentials in adult and pediatric cochlear implant users: I. Cochlear nerve responsiveness and electrode-neuron interface index.

Adult and pediatric cochlear implant (CI) users differ in age, duration of deafness, and etiology, yet cochlear nerve (CN) response properties have not been systematically compared between these populations. To address this gap, this study examined electrically evoked compound action potentials (eCAPs) in pediatric and postlingually deafened adult CI users. Seven eCAP-derived indices were evaluated, including threshold, amplitude at the maximum comfortable level, overall and maximal slopes of the amplitude growth function (AGF), absolute and relative refractory periods (t0 and τ), and the local electrode-neuron interface (ENI) index. Sixty-four adult and 56 pediatric users of Cochlear™ Nucleus® CI devices were tested. eCAP AGFs and refractory recovery functions (RRFs) were measured at three to seven electrodes along the array. AGF slopes were quantified using linear regression and the window method, while t0 and τ were derived by fitting RRFs with exponential decay functions. Local ENI index was calculated using a machine-learning model incorporating several eCAP measurements. Linear mixed-effects models revealed both shared and distinct patterns of CN responsiveness between pediatric and adult CI users. Pediatric users demonstrated lower eCAP thresholds than adults when pooled across electrode locations. Adults exhibited shallower AGF slopes at basal electrodes than middle and apical electrodes. RRF-derived indices revealed shorter refractory periods in adults, likely influenced by group differences in maximum comfortable stimulation levels. Across both groups, several eCAP measures showed poorer responses at middle electrode locations relative to apical and/or basal sites. Future studies incorporating additional eCAP metrics may further clarify group differences in CN health and function.