Bilateral cochlear implants in children : clinical and methodological studies
Abstract: A cochlear implant (CI) restores functional hearing in individuals with bilateral severe- to-profound sensorineural hearing loss. Despite hearing loss in both ears, CIs are usually provided unilaterally, excluding the alleged benefits associated with bilateral auditory stimulation. Recently, however, bilateral cochlear implantation is increasingly common, with the main objectives of enhancing sound localization abilities and speech recognition, particularly in the presence of background noise. Here, using a within-subject longitudinal design in a large clinical study sample of children using bilateral cochlear implants (BiCI), a large, sustained, and significant bilateral benefit in horizontal sound localization accuracy (SLA) was demonstrated. A significant bilateral benefit also existed in speech recognition in noise spatially separated from the target signal. Speech recognition in quiet, however, was comparable under BiCI and unilateral CI listening conditions. Parental reports corroborated behavioral findings. Yet, the bilateral benefit was not uniform across subjects, large intersubject variability existed both with BiCI and unilateral CI, and neither SLA, nor speech recognition performance, was restored to that found in children with normal hearing (NH). Clinically important, a significant improvement of horizontal SLA with increasing BiCI experience (21 percentage points per year) was demonstrated from onset of bilateral stimulation until about 3 years post bilateral implantation (r = -0.51, p < 0.0001, n = 66), with a very similar developmental rate observed intraindividually (mean of the individual slopes = 19 percentage points per year of BiCI experience, n = 21), suggesting an experience-driven maturation of SLA. Of further clinical importance, no relationship between SLA and age or age at implantations was found, albeit, improvements in SLA as a function of BiCI experience were faster, and the bilateral SLA benefit was larger, when bilateral implantation occurred before 4 years of age. In an attempt to accommodate measurements of spatial hearing to clinical requirements, a SLA method using objectively recorded gaze was developed. Pupil positions toward spatially distributed auditory and visual events were recorded using corneal reflection eye tracking technique. The spatial resolution of the methodology allowed detailed objective analyses of gaze patterns in NH listeners from 6 months of age. SLA was rapidly measured in children (age range = 29 – 157 weeks; mean = 168 seconds, n = 12) and adults (mean = 162 seconds, n = 8). Data showed immature SLA in children, with increasing performance as a function of age (r = -0.68, p = 0.015). Highly reliable results existed in adults, who revealed high SLA across the entire spatial range tested. The findings in this thesis have important methodological implications for the clinical management of children with CI, and provide valuable data which may be used in counseling prior to bilateral cochlear implantation. Moreover, the objective and rapid SLA methodology may aid clinicians at an early stage of the process of early intervention with cochlear implants and/or hearing aids in children with hearing loss.
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