Hypothesis: The increased likelihood of facial nerve stimulation (FNS) with cochlear implantation in advanced cochlear otosclerosis is due to a lowering of the facial nerve excitation threshold with increasing bone demineralization.
Background: Facial nerve stimulation can complicate cochlear implant fitting, often necessitating the deactivation of certain electrode contacts.
Methods: High-resolution computed tomographic scans were used to estimate anatomic features of the cochlea and the facial nerve canal. These features were added to a detailed computational model of the implanted human cochlea to examine the consequences of increased conductivity of the bone of the otic capsule. The model took into account the electrode contact type (banded or otherwise) and position (perimodiolar or lateral wall) of the electrode array.
Results: Contrary to the hypothesis, facial nerve thresholds were found to be slightly elevated with increased conductivity of the surrounding bone. However, the threshold and most comfortable loudness levels of the auditory nerve increase more rapidly owing to the reduced current density in the scala tympani as current leaks more easily out of the cochlea. Lateral wall electrodes were predicted to result in an increased likelihood of FNS. A progressively reduced probability of FNS was indicated for the full-band, half-band, and plated electrode arrays, respectively.
Conclusion: The clinical observation of increased FNS in cases of cochlear otosclerosis has been demonstrated in a computational model. Rather than decreased FN threshold, it is the increased levels for cochlear stimulation that is the main factor. Particularly, perimodiolar designs with more shielding against lateral spread of current could reduce the likelihood of FNS.