Analysing the maps of children using bilateral cochlear implants

© 2016 Roghayeh AbdiFor more than a decade, bilateral cochlear implants (BICIs) have been considered as a standard treatment for children with significant hearing loss, with the aim of providing access to bilateral hearing advantages. During this time, a large number of studies have worked on different aspects such as spatial hearing, speech perception or the central auditory system development after receiving BICIs. One area which has had relatively minimal attention is the mapping-related outcomes among this group. The mapping of the cochlear implant (CI) is an important aspect of the post-operation management. With regard to BICIs, It is necessary to increase knowledge regarding the relationship of the mapping parameters [T- and C-levels, and dynamic ranges (DRs)] between the two implants. It is also important to know how this relationship changes over time due to the experience of hearing with two implants. This study will focus on the mapping-related outcomes of two implants for two groups of children with sequential and with simultaneous BICIs. This data will provide evidence- based information regarding the outcomes of current approaches for the mapping of BICIs among children. The main objective is to compare the difference between the mapping parameters of the two ears in the first 10 weeks, and at 2 years and 5 years after the bilateral operation in different electrode array regions and in two situations of different and similar device type at each ear. A secondary objective is to examine that how the degree of the difference between implants is related to the demographic factors of age at bilateral implantation and time between implants. This retrospective study involved 64 children with sequential BICIs and 29 children with simultaneous BICIs. All participants were bilaterally fitted with relatively recent versions of the Nucleus CI system [CI24M, CI24R, CI24RE (Freedom) or CI512]. T- and C-levels were extracted from maps obtained using the Custom Sound software in the first 10 weeks, and at 2 years and 5 years after bilateral implantation. More specifically, the T-and C-levels were extracted for three electrodes in each of three different electrode array regions: apical (electrodes 22, 20, 18), medial (electrodes 15, 13, 11) and basal (electrodes 3, 5, 7). The average of the three electrodes was used as the representative of each region. To represent the difference between the two implants, the T-ratio and C-ratio and DR-ratio were calculated by dividing the mean T- levels, C- levels and DRs of each electrode array region of the second implant (CI2) by those of the first implant (CI1) for the sequential group. For the simultaneous group, the left implant was considered as CI2 and the right implant was considered as CI1. A ratio of 1 would indicate that the levels being compared were the same for each implant. According to the statistical analyses, for the sequential group, the mean T-ratio of the two implants was found to be close to 1 at all three time points and three electrode array regions and two situations of different and similar device type at each ear. That is, the T-levels of the two implants were very similar over time and along the electrode array, even with different device type at each ear. By contrast, the mean C-ratio and DR-ratio of the two implants were found to be less than 1 at all three time points and three electrode array regions and two situations of different and similar device type at each ear. In other words, the C-levels and DRs for CI2 were lower than those for CI1 over time and, along the electrode array, even with similar device type at each ear. However, the degree of difference in the C-levels and DRs between the two implants significantly decreased from the initial to the 2-year post-operation time point. For the simultaneous group, the mean T-ratio, C-ratio and DR-ratio of the two implants were found to be close to 1 at all three time points and three electrode array regions. In other words, the mapping parameters of the two implants were very similar over time, and along the electrode array. For the sequential group, time between implants was a factor that affected the degree of difference in the C-levels and DRs between the two implants, with longer time between implants associated with a greater difference. For both the sequential and the simultaneous group, no significant relationship was found between age at bilateral implantation and the degree of difference in the mapping parameters between the two implants