The facial nerve canal: An important cochlear conduction path revealed by clarion electrical field imaging

Hypothesis: Electrical properties of the implanted scala tympani could be accurately modeled by means of a simple resis-tive ladder network model. The subject-specific model parameters can be obtained from electrical field imaging (EFI) recordings. It is a powerful tool for analysis of the Cochlear current spread. Background: In EFI mode, the telemetry systems of contemporary cochlear implants can measure the intracochlear potential distribution. At present, the clinical use of EFI is typically limited to checking the implant's proper functioning. Methods: Accurate EFI measurements and estimation algorithms have been developed to fit a small, yet physically relevant electrical model of the conductivity of the intracochlear structures. Results: The model can attain up to 95% agreement with in vivo EFI data. A first discovery is that in a majority of the tested subjects, a substantial fraction of the monopolar current leaves the scala along the facial nerve canal. The role of the facial nerve canal has been confirmed by a temporal bone study and a high-resolution computed tomography (HRCT) scan in two of the implanted subjects. Conclusions: The clinical use of EFI is not limited to checking the implant's status. For the Clarion II implant, a purely resistive model is able to match in vivo EFI recordings. The model indicates that the facial nerve canal is an important conduction path to the reference electrode. EFI can provide clinically relevant information, especially in problematic cases of cochlear malformations, postoperative fibrosis/ossification, implanted otosclerotic cochleae, postoperative facial nerve stimulation, increased stimulation thresholds, and so on.