Evaluation of the Relationship between the NRT-Ratio, Cochlear Anatomy, and Insertions Depth of Perimodiolar Cochlear Implant Electrodes

The position of the cochlear implant electrode array within the scala tympani is essential for an optimal postoperative hearing benefit. If the electrode array changes in between the scalae intracochlearly (i.e., from scala tympani to scala vestibuli), a reduced auditory performance can be assumed. We established a neural response telemetry-ratio (NRT-ratio) which corresponds with the scalar position of the electrodes but shows within its limits a variability. The aim of this study was to determine if insertion depth angle or cochlea size influences the NRT-ratio. The intraoperative electrophysiological NRT data of 26 patients were evaluated. Using a flat panel tomography system, the position of the electrode array was evaluated radiologically. The insertion depth angle of the electrode, the cochlea size, and the NRT-ratio were calculated postoperatively. The radiological results were compared with the intraoperatively obtained electrophysiological data (NRT-ratio) and statistically evaluated. In all patients the NRT-ratio, the insertion depth angle, and the cochlea size could be determined. A significant correlation between insertional depth, cochlear size, and the NRT-ratio was not found. The NRT-ratio is a reliable electrophysiological tool to determine the scalar position of a perimodiolar electrode array. The NRT-ratio can be applied independent from insertion depth and cochlear size.</jats:p

Electrophysiological Detection of Scalar-Changing Perimodiolar Cochlear Electrode Arrays: A Six-Month Follow-Up Study

The position of the cochlear electrode array within the scala tympani is essential for optimal hearing benefit. An intraoperative neural response telemetry ratio (NRT ratio; a threshold ratio of pairs of apical and basal electrodes) has been established, which can provide information about the intracochlear electrode array position. Out of a previous collective of 85 patients, the 6-month follow-up electrophysiological NRT data of 37 patients have been included in this study. Comparing the intraoperatively estimated NRT ratio with the 6-month follow-up NRT ratio, it remained unchanged intraindividually in 92% of cases. Within this group the NRT ratio and the intracochlear position of the electrode array matched in all cases. There were two newly occurring mismatches and one new match was observed. After a period of 6 months the NRT ratio remained unchanged in most cases and showed a good correlation with the intracochlear position of the electrode array.</jats:p