Assessment of Depth Symmetricity of Bilateral Cochlear Implants using Plain X-ray Scans

Background: Pitch perception in cochlear implant patients is in association with the angular depth of activated electrodes. In this study, we evaluated the symmetricity of cochlear implant arrays in patients undergoing simultaneously bilateral implantation.

Material and Methods: Intraoperative anterior-posterior (AP) X-ray scans of 41 patients were evaluated retrospectively by two neurotologists. Evaluation was based on the standardized `C-View` method. All patients underwent bilateral simultaneous cochlear implantation.

We evaluated interobserver variability for each measurement . Subsequently we used the averages (both observer) to assess depth-of- insertion asymmetricity .

Results: 41 scans were evaluated (82 ears). Average depth pf insertion ranged between 423 to 434 degrees. Using a cut-off of 45 degrees for symmetricity, we found that 43% had asymmetrical insertion.

We found small interobserver variability for the degree of symmetricity. In only twenty percent of cases, both observer rated symmetricity within 25 degrees. It was found that allocation of modiulus had the greatest effect on the measurement, while the allocation of the round window had negligible effect. Comparison of AP scans and lateral scans demonstrated that head rotation can have a significant effect on measurements.

Conclusions: It is feasible to evaluate the depth of insertion using plain X-ray scans and the C-View method, with small interobserver variability. Asymmetrical insertion is very common, and might adversely affect the perceived pitch from both ears. Head rotation might influence this assessment, hence AP scans should be performed meticulously. Further studies should evaluate the effect of asymmetricity on the functional outcomes with bilateral cochlear implants.