Electrocochleography Latency: Correlation with Electrode Position during Cochlear Implantation

Objectives: Cochlear implant (CI) candidates increasingly exhibit some degree of residual hearing, which should be preserved despite the implantation. Today, cochlear health is monitored during CI surgery by tracking the cochlear microphonic (CM) amplitude from intracochlear electrocochleography (ECochG) measurements. However, recent studies indicate that the insertion depth of the measuring electrode must be considered to accurately interpret these signals. The acoustic path from the cochlear base to the apex induces excitation delays in deeper regions, which should be reflected in the CM measurements. In this study, we analyzed the potential of cochlear microphonic latency (CML) as an objective method for continuously tracking CI electrode position during cochlear implantation. In addition, we examined whether CML can be associated with residual hearing. Design: We recorded intraoperative pure-tone ECochG at maximum stimulation levels from 30 CI patients to derive CML. During CI electrode insertion, ECochG was continuously recorded at the 2 stimulation frequencies of 0.5 and 0.75 kHz. After complete insertion, ECochG was measured on all evenly numbered electrodes at frequencies of 0.25, 0.5, 0.75, and 1 kHz. The electrode locations (i.e., linear insertion depth) were identified by postoperative computed tomography (CT) scans. The location of the measuring electrode during the insertion period was then calculated backward, assuming a constant insertion speed. Finally, we used a linear regression model to relate CML to linear insertion depth. In addition, we evaluated the relationship between CML and preoperative residual hearing. Results: CML is significantly correlated to the linear insertion depth (p < 0.001) during and after electrode insertion (with restrictions on 0.25 kHz stimulus, presumably since the characteristic 0.25 kHz region is not within reach of the used CI electrode arrays). Despite high inter-individual variability, our results align with documented delays in the basilar membrane observed in other studies. However, we could not identify a significant association between CML and residual hearing. Conclusions: Our study demonstrates that objectively extracted CML encodes the intracochlear electrode location in CI patients but is not directly linked to residual hearing. Consequently, CML has the potential to enhance intraoperative ECochG analysis by providing real-time tracking of electrode position. To better understand the inter-individual variations in CML, future studies with larger patient cohorts are needed.