Longitudinal Analysis of Intracochlear Electrocochleographic Amplitude Patterns in Cochlear Implant Recipients.

objectivesIntracochlear electrocochleography (ECochG) in cochlear implant (CI) recipients is a potential tool for monitoring cochlear function during and after electrode array (EA) insertion. However, mechanisms underlying ECochG amplitude variations along the cochlear duct, and their significance for hearing preservation (HP), remain unclear. Therefore, a longitudinal study was conducted to monitor maximum ECochG amplitude and its tonotopic location from EA insertion to 1 yr postimplantation. It was hypothesized that changes in maximum amplitude (>30%) and/or shifts in its location (>1 octave) across timepoints reflect intracochlear alterations associated with residual hearing changes.

designECochG recordings were obtained in 80 adult CI recipients with measurable residual hearing. For Contour Advance (CI612) and Slim Straight (CI622) arrays (Cochlear Ltd.), recordings were taken from every second intracochlear electrode. For HiFocus SlimJ and MidScala arrays (Advanced Bionics LLC), recordings were obtained from all electrodes. Measurements were conducted at four timepoints: (1) intraoperatively, during EA insertion (Intraop1), (2) intraoperatively, immediately after full insertion (Intraop2), (3) approximately 7 wk after surgery (Postop1), and (4) approximately 1 yr after surgery (Postop2). 500 Hz tone bursts were used for acoustic stimulation and the magnitude of the difference between responses to alternating-polarity stimuli was analyzed. Tonotopic electrode locations were determined from postoperative cone beam computed tomography scans. Pure-tone audiograms were obtained preoperatively and at approximately 7 wk and 1 yr postoperatively. HP was determined using the HEARRING group formula.

resultsMaximum ECochG amplitudes remained largely stable intraoperatively, with no significant difference between Intraop1 and Intraop2 in complete-case analysis (n = 44). In contrast, a significant decrease in maximum amplitude was observed between Intraop2 and Postop1 ( p < 0.001). Participants with >30% amplitude reduction between the 2 intraoperative recordings (Intraop1 versus Intraop2) did not differ significantly in HP from those with stable amplitudes. However, those showing a >30% reduction in the early postoperative period (Intraop2 versus Postop1) showed significantly lower HP ( p = 0.028). Nonapical peak location during Intraop1 occurred in 41% of the cases, although tonotopic location of the maximum peak during insertion monitoring (Intraop1) did not show a relationship with HP. Tonotopic location shifts of the maximum amplitude (>1 octave) were observed in a small subset of cases between consecutive recordings up to Postop2. However, peak location changes (apical, basal, stable) were not associated with significant differences in HP.

conclusionsOur results suggest that nonapical peak patterns are not necessarily markers of insertion trauma and may instead reflect variability in cochlear integrity (e.g., dead regions). Peak location during insertion monitoring was not associated with postoperative HP, and both maximum amplitude and tonotopic peak location remained stable intraoperatively. In contrast, early postoperative reductions in ECochG amplitude were common and associated with HP, highlighting the need to investigate strategies to minimize early intracochlear reactions. Overall, the study demonstrates the value of ECochG for monitoring intracochlear processes over time.