Helix electrode pull back: electrophysiology and surgical results

Abstract

electrodes. Recent reports showed the effect of an increased focusing of the intracochlear electrical field by a surgical modification of the commonly performed insertion (Todt et al., 2005; 2008) called ‘pull-back’. Focusing the intracochlear field is assumed to further increase the number of virtual channels. It has been shown that this further increase of the perimodiolar position led to a better frequency discrimination with the Nucleus Advance array (Basta et al., 2010). The aim of the present study was to investigate the effect of the modified insertion technique in the temporal bone (TB) to be able to perform the technique with the helix electrode. In a second step, we looked at the intraoperative electrophysiological changes due to the electrode pull-back. Method Ten TB with a view on the scala tympani were investigated. The intracochlear movements of the helix electrode positioning were recorded. After assessing the video-based recordings, the absolute change of three different distances (tip change, modiolus approximation distance, and pull back distance) was calculated. To define the pixel-based frame distances, the known contact-to-contact distances of the electrode provided by the manufacturer were used. Video analysis was performed with Metra software. Additionally, we investigated the electrophysiological effect of the electrode pull-back by pre- and post-pull-back measurements of the spread of excitation (SOE) and the neural response imaging (NRI) threshold in 10 implanted patients. A modified round window approach was performed. The pullback distance from the surgeons perspective was subjectively limited to 1.5 mm. Therefore, the first and the second cochleostomy markers were used (distance 3 mm). Measurements of the SOE were performed with the RSPOM software (Advanced Bionics software platform, Valencia, CA, USA) using the variable recording algorithm. Measurement contacts were electrodes 4, 8, and 12. Pre- and post-pullback current levels were intraindividually equally chosen, but varied interindividually. For the calculation of the pull-back effect, the electrode width of the SOE at 60% was used. RSPOM and equally current level were used for the measurements of the NRI thresholds.