Effect of Interaural Angular Insertion Depth Mismatch on Sound Localization in Adolescent Bilateral Cochlear Implant Users.

OBJECTIVES: To examine whether interaural angular insertion depth (AID) mismatch affects horizontal sound localization accuracy (SLA) in adolescents with bilateral cochlear implants (BiCI) since early childhood. STUDY DESIGN: Cross-sectional study. SETTING: University hospital. PATIENTS: Twenty subjects with mean age of 16.9 years (range: 12.8 to 21.6 y) and 14.7 years (9.6 to 19.6 y) of BiCI experience. METHODS/MAIN OUTCOME MEASURES: On the basis of three-dimensional modeling of photon-counting detector-computed tomography images of the inner ears, interaural AID mismatch of the most apical electrodes was estimated. SLA was objectively measured using 4 stimuli providing various spatial cues, quantified by an Error Index (EI). The effect of interaural AID mismatch on the EI was studied using linear regression analyses, and by comparing the EI between a matched and a mismatched group stratified on a mismatch criterion of 75 degrees. RESULTS: The median interaural AID absolute difference was 45.5 degrees (range: 4 to 256 degrees). Across sounds, no linear relationship between interaural AID mismatch and EI existed. No effect of group (matched vs. mismatched) on the EI existed. A significant effect of stimulus (P<0.001) and an interaction between stimulus and group (P<0.02) was found, with performance differences between stimuli for the mismatched but not the matched group. CONCLUSIONS: Individuals who have been listening through BiCI since early childhood show large variability in AID mismatch between the ears. Horizontal SLA seems unrelated to this mismatch. For large mismatches, the spatial cues present in a sound seem to affect localization accuracy.