Proefschrift_vd_Beek

The progression from single-channel to multichannel electrode arrays enabled the use of the tonotopic organization of the neural fibers in the cochlea. This technique proved to be a crucial improvement that made speech perception with cochlear implants possible [Mudry and Mills, 2013]. Although all current cochlear implant systems provide a higher number of channels, speech perception does not improve with the use of more than seven channels [Friesen et al., 2001]. Not all electrode contacts provide independent spectral information. The spread of currents through the highly conductive fluid in the cochlea prevents neuronal excitation in a restricted area. Various electrode arrays have been used to improve spectral resolution. Electrode contacts medially positioned in the cochlea near the neural elements facilitate excitation [Shepherd et al., 1993]. Hence, different cochlear implant manufacturers have developed medially positioned electrode arrays. These so-called perimodiolar electrodes offer improved speech perception [van der Beek et al., 2005a;Holden et al., 2013]. Furthermore, with the increased emphasis on preserving residual hearing, cochlear implants’ electrode arrays are designed to induce as little trauma as possible [Lenarz et al., 2013;Tavora-Vieira and Rodrigues, 2013]. The result is short, thin and flexible electrodes that are less likely to damage vulnerable cochlear microstructures. Moreover, when residual hearing is preserved, the combination of electric and acoustic stimulation is feasible. Finally, even an optimized electrode-neural interface should be adapted to the individual patient and to specific circumstances at different locations in the individual cochlea. This individualized tuning is performed during the fitting process, and numerous parameters can be set; however, the core parameters involve defining the threshold and maximum levels along the array. Research data concerning the stimulation levels that are useful in clinical practice primarily focus on speeding up the fitting process [Plant et al., 2005;Smoorenburg, 2007;Pfingst and Xu, 2004], and only a few studies report fitting improvements that would provide better speech perception [Gani et al., 2007;Zhou and Pfingst, 2014;Noble et al., 2014]. Outline of the present thesis In this thesis, the parameters that influence the performance of cochlear implant users are analyzed. Specifically, we analyze the signal-to-noise ratio at the input of the processor, the intracochlear position of the electrode design, the spread of excitation (SOE) and settings of the clinically used levels. In Chapter 2, the effect of background noise on speech perception is assessed in a trial studying the improvement of speech perception in noise using directional microphones versus an omnidirectional microphone. To mimic real-life situations, speech-in-noise was presented in a specially designed set-up with a diffuse noise field. In Chapter 3, the effect of electrode design and intracochlear position is analyzed by comparing the speech perception scores of 25 patients with cochlear implants that were forced into a perimodiolar position with a silastic positioner and the speech perception scores of 20 patients in whom no positioner was used. The 20 no-positioner patients were further subdivided into superficially and deeply implanted subgroups, both of which included 10 patients. The intrascalar position of the individual electrode contacts was analyzed using HDCT scans, and stimulation thresholds, maximum comfort levels, and dynamic ranges were obtained.

14 | Chapter 1