Assessing Array-Type Differences in Cochlear Implant Users Using the Panoramic ECAP Method

Cochlear implant companies manufacture devices with different electrode array types. Some arrays have a straight geometry designed for minimal neuronal trauma, while others are precurved and designed to position the electrodes closer to the cochlear neurons. Due to their differing geometries, it is possible that the arrays are not only positioned differently inside the cochlea but also produce different patterns of the spread of current and of neural excitation. The panoramic electrically evoked compound action potential method (PECAP) provides detailed estimates of peripheral neural responsiveness and current spread for individual patients along the length of the cochlea. These estimates were assessed as a function of electrode position and array type, providing a normative dataset useful for identifying unusual patterns in individual patients.

ECAPs were collected from cochlear implant users using the forward-masking artifact-reduction technique for every combination of masker and probe electrode at the most comfortable level. Data were available for 91 ears using Cochlear devices, and 53 ears using Advanced Bionics devices. The Cochlear users had straight arrays (Slim Straight, CI-22 series, n = 35), or 1 of 2 precurved arrays (Contour Advance, CI-12 series, n = 43, or Slim Modiolar, CI-32 series, n = 13). Computed tomography scans were also available for 41 of them, and electrode-modiolus distances were calculated. The Advanced Bionics users had 1 of 2 straight arrays (1J, n = 9 or SlimJ, n = 20), or precurved arrays (Helix, n = 4 or Mid-Scala, n = 20). The ECAPs were submitted to the PECAP algorithm to estimate current spread and neural responsiveness along the length of the electrode array for each user. A linear mixed-effects model was used to determine whether there were statistically significant differences between different array types and/or for different electrodes, both for the PECAP estimates of current spread and neural responsiveness, as well as for the available electrode-modiolus distances. Correlations were also conducted between PECAP's estimate of current spread and the electrode-modiolus distances.

For Cochlear users, significant effects of array type ( p = 0.001) and of electrode ( p < 0.001) were found on the PECAP's current-spread estimate, as well as a significant interaction ( p = 0.006). Slim Straight arrays had a wider overall current spread than both the precurved arrays (Contour Advance and Slim Modiolar). The interaction revealed the strongest effect at the apex. A significant correlation between PECAP's current-spread estimate and the electrode-modiolus distances was also found across subjects ( r = 0.516, p < 0.001). No effect of array type was found on PECAP's estimate of current spread for the Advanced Bionics users ( p = 0.979).

These results suggest that for users of the Cochlear device, precurved electrode arrays show narrower current spread within the cochlea than those with lateral-wall electrode arrays, with the strongest effect present at the apex. No corresponding effects of array type were found in the Advanced Bionics device. This could have implications for device selection in clinical settings, although the authors underscore that this is a post-hoc analysis and does not demonstrate a causal link wherein device selection can be expected to give rise to specific neural excitation patterns.