Improving the clinical applicability of electrophysiological assessments and cochlear imaging in adult cochlear implant patients

Cochlear implants (CI) are recognized as the most successful neural prosthesis to date. With the technology and surgical techniques for implantation continually improving, the potential for improved speech perception abilities in individuals with severe-to-profound hearing loss, also increases. While different factors which influence these outcomes have been identified, no consensus exists regarding the degree to which each factor contributes (see Holden et al., (2013)). A major challenge of using speech perception scores to evaluate cochlear implantation outcomes is that understanding degraded (such as vocoded speech from a cochlear implant) speech uses both linguistic and cognitive resources (working memory and attention). Therefore, it is not a sensitive outcome measure for assessing minor technological or surgical modifications for the implant. On the other hand, electrophysiological measures of auditory function, such as electrically evoked auditory brainstem responses (eABRs) could provide a more sensitive measure of the effects of electrode positioning, surgical outcomes, or cochlear neural integrity. However, a robust understanding of how these measures could be used to inform functional outcomes of implantation has not yet been reached. The research reported in this thesis is based on an online questionnaire data from clinical audiologists working at the Sydney Cochlear Implant Centre (SCIC) in NSW and retrospective datasets collected from the centre, which has a long history of measuring objective responses intra-operatively during cochlear implantation surgery. This thesis firstly explores the current clinical use of these objective measures by the clinical audiologists, then investigates the potential clinical utility of imaging and intraoperative electrophysiological measures in CI to increase test battery efficiency and applicability. Further, this thesis describes a clinically viable tool of measuring cochlear length using Cone beam Computed Tomography (CBCT) with the implant array in situ, providing a more accurate method of measuring cochlear lengths and thereby provides insights into a variable which influences final electrode placement. results of these investigations demonstrate that considerably greater clinical use of the intra-operative measures could be made to increase the efficiency of cochlear implant programming (or mapping) during the switch-on appointment. In particular, the results showed that electrically-evoked auditory brainstem response (eABR) measures are a sensitive tool for predicting CI mapping parameters particularly at cochlear implant “switch-on”, although this is negatively affected by poor scalar placement. As such, this thesis demonstrates that CBCT can be used to identify scalar placement within the basal turn of the cochlea, thereby providing greater accuracy of predicting mapping outcomes. Finally, this thesis demonstrates the considerable inter-individual cochlear anatomical variation which highlights the need for a more individualized approach to CI for more consistent electrode placement.