The Failure Index as marker of cochlear health in MED-EL CI users: Anatomy, demographics, and speech correlations

Cochlear implants (CIs) enable hearing with the deafened ear, via direct, electrical stimulation of the spiral ganglion neurons (SGN). Thus, the outcome depends on the number and excitability of the SGNs. We recently established the electrically-evoked compound action potential (eCAP)-derived Failure Index (FI) as cochlear-health marker in the animal model. The FI informs about the presence, site, and size of a SGN lesion. Here, we translated the FI to clinical recordings of MED-EL CI users. For the retrospective study, we selected patient data from the database of the German Hearing Center Hannover recorded 2017 to 2024. We included 199 post-lingually and 79 pre-lingually deafened ears. Averaged FIs over all contacts of a CI were stable within the analysis period (3rd month to 1st year postoperatively). The FI increased with age and was elevated for etiologies associated with higher SGN loss. Utilizing 3D information from cone beam-computed tomography scans, we confirmed that the FI was independent of distance (0.1-2.5 mm) to the modiolus. The FI showed individual patterns along the array with maxima usually at basal contacts, corresponding to elevated SGN loss at high frequencies. In a selected group of post-lingually deaf ears, we confirmed the correlation of the FI with speech perception in quiet and in noise (n=28, r2=0.12-0.55). Thus, we propose the FI as promising clinical tool to identify CI-implanted ears with reduced neural health and contacts close to areas of SGN loss. Thereby, it can serve to guide speech-processor fitting to optimize CI outcomes.