Electrocochleographic Patterns Predicting Increased Impedances and Hearing Loss after Cochlear Implantation

Abstract

Objectives: Different patterns of electrocochleographic responses along the electrode array after insertion of the cochlear implant electrode array have been described. However, the implications of these patterns remain unclear. Therefore, the aim of the study was to correlate different peri- and postoperative electrocochleographic patterns with four-point impedance measurements and preservation of residual hearing. Design: Thirty-nine subjects with residual low-frequency hearing which were implanted with a slim-straight electrode array could prospectively be included. Intracochlear electrocochleographic recordings and four-point impedance measurements along the 22 electrodes of the array (EL, most apical EL22) were conducted immediately after complete insertion and 3 months after surgery. Hearing preservation was assessed after 3 months. Results: In perioperative electrocochleographic recordings, 22 subjects (56%) showed the largest amplitude around the tip of the electrode array (apical-peak, AP, EL20 or EL22), whereas 17 subjects (44%) exhibited a maximum amplitude in more basal regions (mid-peak, MP, EL18 or lower). At 3 months, in six subjects with an AP pattern perioperatively, the location of the largest electrocochleographic response had shifted basally (apical-to-mid-peak, AP-MP). Latency was analyzed along the electrode array when this could be discerned. This was the case in 68 peri- and postoperative recordings (87% of all recordings, n = 78). The latency increased with increasing insertion depth in AP recordings (n = 38, median of EL with maximum latency shift = EL21). In MP recordings (n = 30), the maximum latency shift was detectable more basally (median EL12, p < 0.001). Four-point impedance measurements were available at both time points in 90% (n = 35) of all subjects. At the 3-month time point, recordings revealed lower impedances in the AP group (n = 15, mean = 222 Ω, SD = 63) than in the MP (n = 14, mean = 295 Ω, SD= 7 6) and AP-MP groups (n = 6, mean = 234 Ω, SD = 129; AP versus MP p = 0.026, AP versus AP-MP p = 0.023, MP versus AP-MP p > 0.999). The amplitudes of perioperative AP recordings showed a correlation with preoperative hearing thresholds (r2=0.351, p = 0.004). No such correlation was detectable in MP recordings (r2 = 0.033, p = 0.484). Audiograms were available at both time points in 97% (n = 38) of all subjects. The mean postoperative hearing loss in the AP group was 13 dB (n = 16, SD = 9). A significantly larger hearing loss was detectable in the MP and AP-MP groups with 28 (n = 17, SD = 10) and 35 dB (n = 6, SD = 13), respectively (AP versus MP p = 0.002, AP versus AP-MP p = 0.002, MP versus AP-MP p = 0.926). Conclusion: MP and AP-MP response patterns of the electrocochleographic responses along the electrode array after cochlear implantation are correlated with higher four-point impedances and poorer postoperative hearing compared to AP response patterns. The higher impedances suggest that MP and AP-MP patterns are associated with increased intracochlear fibrosis.

Four-Point Impedance: A Potential Biomarker for Residual Hearing After Cochlear Implantation

“…A recent human trial reported lower 4PI in participants receiving a dexamethasone eluting array than controls at 6, 12, and 24 months (11). When considered in light of our observations here and recently (9) that higher 4PI predicts poorer residual hearing, this opens up the exciting prospect that dexamethasone-eluting cochlear implants may be effective in preserving residual hearing, probably by reducing fibrosis.…”

“…For the Slim Straight implant electrode used in this study, there is a high chance that intracochlear fibrosis will involve the basilar membrane and dampen transmission of the traveling wave in the basal-half of the cochlea, negatively affecting residual hearing (28). We have recently published evidence in support of this (9). In that clinical study, the pattern of ECochG recorded across the electrode array transitioned from that expected from normal cochlear mechanics (immediately after surgery) to that expected from fixation of the basilar membrane (3 mo later) in association with a significant rise in 4PI.…”

“…Real-time electrocochleography (rt-ECochG) has shown significant promise in this area; poorer residual hearing is associated with a drop in cochlear microphonic (CM) amplitude during implantation (3)(4)(5)(6)(7), and surgical intervention in response to an ECochG drop improves residual hearing (8). Bester et al (8,9) proposed that the likely cause of CM amplitude drop was contact between the electrode and the basilar membrane, resulting in impaired residual hearing.…”

Uncovering Vulnerable Phases in Cochlear Implant Electrode Array Insertion: Insights from an In Vitro Model

“…Amplitude drops have been reported to occur at any time during the insertion process, and this behavior may be related to the present findings (32). Intracochlear electrode movement caused by regripping can alter the electrode's contact with the basilar membrane, affecting the mechanics of acoustic stimulation and, consequently, ECochG measurements (33). In addition, exposure to loud noises and associated pressure peaks can cause temporary hearing threshold shifts by disrupting synaptic connections, impairing mechanoelectrical transduction channels of the hair cells, and inducing morphological changes in the tectorial membrane (34).…”

Classification of Acoustic Hearing Preservation After Cochlear Implantation Using Electrocochleography

“…Finally, two types of electrode arrays were used in this study. Lateral wall electrodes, such as the SlimJ electrode array, are prone to ride up and touch the basilar membrane, causing hearing loss through disruption of cochlear mechanics ( Bester et al, 2022 ). The mechanisms of hearing loss with Mid-Scala electrode arrays likely differ, and this heterogeneity in the data may have impacted the study findings.…”