Imaging of electrode position in relation to electrode functioning after cochlear implantation

This study assessed the electrode position in cochlear implant patients and evaluated the extent to which the electrode position is determinative in the electrophysiological functioning of the cochlear implant system. Five consecutively implanted adult patients received a multichannel cochlear implant. In all patients, the electrical impedance and the electrically evoked compound action potentials were recorded immediately after implantation. Multislice computer tomography was performed 6 weeks postoperatively before switch-on of the cochlear implant. The electrode position relative to the modiolus was assessed and correlated to the electrophysiological measurements. All electrodes were fully inserted; this was confirmed by computer tomography. The individual electrode distance toward the modiolus could be most precisely analyzed for the basal part of the electrode array. It was thus decided to study the data of electrodes one, four, and seven. No correlation was found between electrical impedance and electrode distance. A significant correlation was found between electrode distance and the electrically evoked compound action potentials, with a 96% probability using Kendall's rank correlation. We conclude that the electrode-modiolus distance is of importance to the stimulation of auditory nerve fibers. Future developments in imaging will further improve and refine our insight in the relation between electrode positioning.