Proefschrift_vd_Beek

basal current drain [van der Beek et al., 2005]. In addition to the higher levels at the base, Boyd [2010] mentioned nonuse of the most basal electrodes in a substantial number of cases. This nonuse of basal electrodes was not present in our study population. In spite of the fact that T-levels are higher for basal electrodes, figure 1c does not show a decrease in DR towards the base. This is a consequence of the emphasis on the higher frequencies in the preset M-level profile, which, in turn, might be beneficial for speech understanding, as discussed in relation to figure 4. Propst et al. [2006] showed that the variation in eCAP amplitudes along the array was etiology dependent (GJB2 vs. non-GJB2) and argued that this was due to differences in neural survival. This explanation is in line with the general finding of steeper eABR growth functions in the apex than at the base and the fact that eCAP growth curves predict speech perception in individuals with significant residual hearing [Gordon et al., 2007; Kim et al., 2010]. In line with this, the large intersubject variability in fitting levels is commonly attributed to differences in neural survival. The present study, however, demonstrated that the increase in the levels towards the base was independent of the levels themselves (fig. 5). Therefore, it is less likely that this increase was caused by differences in neural survival along the cochlea. CONCLUSIONS A practical aid to the fitting procedure has been introduced, enabling fast fitting in cochlear implant recipients. Based on one measurement and a population-based Tor M-level profile, individual recipients’ Tand M-levels can be predicted with a closed-set formula. Although fitting levels increased consistently over time, the electrical DR (in decibels) appeared to be constant, with T-levels between 20 and 35% of M-levels. In recipients lacking reliable behavioral feedback, the percentile plots of levels for our population can serve as a reference to avoid underand overstimulation.

110 | Chapter 5