Electrophysiological detection of scalar changing perimodiolar cochlear electrode arrays: a long term follow-up study

The position of the cochlear electrode array within the scala tympani is essential for an optimal hearing benefit. An intraoperative NRT-ratio was established, which can provide information about the intraoperative intracochlear electrode array position for perimodiolar electrodes. The aims of this study were to verify the longterm reliability for the NRT-ratio in perimodiolar electrodes. In a retrospective controlled study in a Tertiary Referral Center the electrophysiological data sets of 123 patients with implanted Nucleus Contour Advance electrodes were enclosed. Intraoperative and up to 1 year follow-up Auto-NRTs were evaluated. A NRT-ratio was calculated by dividing the average Auto-NRT data from electrode 16 to 18 with the average from electrode 5 to 7. Using a flat panel tomography system, the position of the electrode array was certified radiological. 31 patients with perimodiolar electrodes with 1 year follow-up data were included in the study. Eleven patients showed regular follow-up NRT-ratio with a correlated and radiologically confirmed electrode position. 20 patients showed mismatches between the NRT-ratio and the radiological position. These patients were highly variable in terms of duration of deafness and neural spectrum disorders. The NRT-ratio can be used to determine the intracochlear position of the electrode array for perimodiolar electrodes. Intraoperatively the NRT-ratio predicts the array position within the cochlea highly reliable for perimodiolar electrodes. We showed that after 6 months and a year, the NRT-ratio remains unchanged in most of the cases and shows a good correlation to the radiological determined position of the array. Nevertheless, the condition of the neural structures is highly important for reproducible responses. Limited validity is given in patients with degenerative and structural neural disorders.