Effect of Electrode-Modiolus-Distance on Electrophysiological and Psychophysical Parameters

Background: The electrode position within the cochlea is determined by the form of the electrode carrier. Straight electrode arrays (Nucleus™ 522 „SRA“) are pressed against the lateral wall of the cochlea, while precurved arrays (Nucleus™ 532 „SMA“ and 512 „Contour“) curl around the modiolus and ideally have no contact to the lateral wall. Recent improvements in postoperative CBCT (cone beam computational tomography) imaging resolution allow accurate determination of the space between each electrode contact and the medial wall of the cochlea and to observe the relationship between this distance and electrophysiological and psychophysical parameters.

Objective: To determine whether electrophysiological and psychophysical parameters are significantly different depending on the distance between the electrode contacts and the modiolus.

Methods: Postoperative CBCT-images of 30 patients with Cochlear® Nucleus™ 532, 522 and 512 implants were evaluated to determine the distance between the individual electrode contacts and the medial wall. During the first fitting ECAPs, T- and C-levels were measured. The neural spread of excitation (SOE) was determined using spatial masking (SM) with ECAPs and electric field imaging (EFI) was performed to measure the voltage spread.

Results: Novel tools can be used in the evaluation of high resolution CBCT-images to determine individual electrode-modiolus-distances after cochlear implantation. ECAP thresholds, T-Levels, C-Levels and SOE widths are larger with increased distance to the modiolus. These parameters are therefore overall lower and more uniform in perimodiolar arrays compared to straight arrays.

Conclusion: Our results show that the electrode position has a significant effect on both electrophysiological and psychophysical parameters.