Otolaryngology - Theses
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ItemTrauma and residual hearing loss after cochlear implantation surgery( 2019)For the last 30 years, cochlear implantation (CI) surgery has been successfully used for the treatment of severe to profound deafness. With improvements in cochlear implant technology, the surgical criteria have expanded to include patients with residual low frequency hearing. These patients can benefit from simultaneous use of the implant and a hearing aid in the same ear, termed electro-acoustic stimulation (EAS). EAS has been shown to be particularly beneficial for perceptual tasks relying on pitch resolution, such as music appreciation and speech perception in background noise (Gantz et al., 2005, Santa Maria et al., 2013). Unfortunately, residual hearing loss, termed post-implantation hearing loss, occurs in half of all patients, which degrades many of the benefits of EAS (Carlson et al., 2011). Post-implantation hearing loss may arise from direct surgical trauma and indirect damage to the cochlea (Eshraghi and Van de Water, 2006). The causes of direct trauma include surgical access to the cochlea and electrode insertion trauma. This results in several types of tissue injury, which may cause further hearing loss through a biological response, also termed indirect damage. However, the precise contributions of direct and indirect damage to post-implantation hearing loss remain indeterminate, in part, because of the lack of a standardised animal implantation model. Animal experimentations have been partly hampered by the difficult access to the mammalian inner ear and the absence of a consistent implantation technique (Rowe et al., 2016, Smeds et al., 2015, James et al., 2008) that has led to marked differences in the level of trauma and hearing loss (O'Leary et al., 2013, Farhadi et al., 2013, Honeder et al., 2015). The principal aim of this thesis was to examine the different types of cochlear trauma, how they relate to hearing loss and how hearing loss can be prevented. The issues that have been addressed here include cochlear anatomy and imaging techniques, pathology, pharmacology, electrophysiology and force recordings. This thesis begins by establishing a reproducible animal model of cochlear implantation surgery, followed by systematically examination the relevant contributors to surgical trauma and post-implantation hearing loss. From these findings, pharmacological therapy targeting the indirect damage and recording techniques to reduce post-implantation hearing loss have been investigated. An anatomical study of the guinea pig cochlea was initially performed using novel imaging and processing techniques. This study quantitatively described the soft and bony tissue relationships in the complex hook region of the unoperated guinea pig cochlea. A reproducible technique for implanting in an animal was established in this study for the remaining in vivo studies, including the use of a cochleostomy in an anteroinferior plane (highest predictability and smallest risk of trauma) and closer to the round window (lowest force profile). The first in vivo experiment of this thesis investigated the relative influence of implant insertion depth on trauma and hearing loss. This study also explored the use of glucocorticosteroids (‘steroids’) to preserve hearing and target the biological response to the implant. Key findings were that implants deeply inserted or in the presence of an osseous spiral lamina fracture caused the greatest low-frequency hearing loss. Steroids reduced the biological response in the most apical regions but had no effect on hearing thresholds. A second in vivo experiment was performed to further investigate the efficacy of steroids and to explore if trauma and hearing loss could be predicted at the time of surgery through force or electrocochleography recordings. Preserving the compound action potential of the electrocochleography recording was found to be predictive of an atraumatic insertion and reduced post-implantation hearing loss. Conversely, force was correlated with trauma but not hearing loss. Pre-operative steroids were effective in reversing the loss of hearing amplitudes evoked by lower frequency tones during implantation with hard electrodes. The results presented in this thesis will help inform the cochlear implant community of potential techniques to improve hearing preservation surgery. Specifically, these results provide a validated experimental model of hearing preservation surgery. These results also suggest a role for steroids in reducing tissue response and synaptopathy, and for electrocochleography as an intraoperative recording paradigm to reduce trauma and improve post-implantation hearing loss.
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ItemA study of electrical stimulation levels over 10 years for adults using Nucleus cochlear implants( 2017)Cochlear implants have improved sound perception for thousands of people with severe and profound sensorineural hearing loss. To ensure a good quality sound signal, the implant must be individually programmed throughout the user’s lifetime. Programming determines the electrical stimulation level requirements for each electrode of the implant. The frequency with which programming occurs has so far been based primarily on clinical experience and resources available to the clinic for programming purposes. To develop an evidence-based schedule for the frequency of programming, a comprehensive investigation of the change in electrical stimulation level requirements over time for a large group of adults is necessary. The overall purpose of this retrospective study was therefore to investigate the change in electrical stimulation levels up to 10 years postimplantation for a large group of adults using Nucleus cochlear implants. In addition, this study also aimed to investigate whether demographic characteristics and electrode array segments were predictive of the change in electrical stimulation levels. The Cochlear Implant Clinic of the Royal Victorian Eye and Ear Hospital, Victoria, Australia, has been providing programming services for cochlear implant users for over 25 years. The electrical stimulation data obtained at the programming sessions were extracted for 680 participants who used a Nucleus® cochlear implant over the course of 10 years. For each implant user, programming data for the following time points were extracted from Cochlear Limited’s TM Custom Sound 3.2® fitting software: 2, 3, 6, 9 months postimplantation and biennial time points from 1 year up to 10 years postimplantation. For each time point, the mean T level, C level, and dynamic range (DR) were calculated separately for four electrode array segments: apical (mean of electrodes 22, 20, 18), medial (16, 14, and 10) upper-basal (7, 6), and lower-basal (4, 3). The degree of change (DC) in levels between consecutive time points was also calculated. Long-term changes in electrical stimulation levels were also investigated for 128 participants from the same group who had adequate programming data up to 10 years postimplantation. The trends and amount of change in programming levels were investigated separately for the four electrode array segments using regression analyses. The effects of several demographic characteristics (e.g., aetiology and onset of hearing loss) were also investigated. For the 680 participants, the average T levels and C levels were consistent from 3 months and 6 months postimplantation, respectively. In terms of the degree of change between time points, the majority of participants showed an average of ≤ 20% change in levels as a function of DR after the 3- to 6-month time point comparison postimplantation. Long-term patterns of change for 128 individual participants showed that, for 42% of these participants, the electrical stimulation levels remained relatively stable without any significant trends for either the T levels or C levels, from 6 months up to 10 years postimplantation. Some significant changes in levels were evident up to 10 years postimplantation; however, changes were small, equating to less than 6% per year as a function of DR for 75% of the participants and a maximum of 10% change in levels for the remaining 25% of participants. Additional noteworthy findings include the following: 1. For the participants who showed more change in levels in the first 3 to 6 months postimplantation, approximately 70% of these participants showed a non-significant long-term trend in levels up to 10 years postimplantation, 2. The mean T levels and C levels were found to differ significantly for the four electrode array segments in the first 2 years postimplantation, but were relatively consistent in the long-term, and 3. The mean degree of change in levels as a function of DR was significantly greater for the Otosclerosis pathological group (n = 72) compared to all other groups, and for the prelingual onset of deafness group compared to the postlingual group. This thesis provided strong evidence that electrical stimulation levels change in the first 6 months but expected to remain relatively stable up to 10 years postimplantation for the majority of adults using Nucleus cochlear implants. Based on this evidence, a programming protocol for 10 years postimplantation has been proposed. The proposed protocol recommends more frequent programming sessions in the first 6 months compared to the number of sessions thereafter. The reduced number of sessions over an implant user’s lifetime will assist in managing the increasing caseload with the amount of resources currently available for programming. Given that the levels differed across the segments in the first 2 years postimplantation, but not in the long-term, the proposed protocol recommends measuring levels separately for the four segments in the early postimplantation period. This can then be limited to one or two segments beyond 2 years postimplantation. The protocol also suggests that more programming sessions may be required in the first two years for implant users with Otosclerosis and those with a prelingual onset of hearing loss.
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ItemBilateral advantage and spatial unmasking for children with normal hearing, bimodal devices and bilateral implants( 2012)This thesis describes bilateral advantage and spatial unmasking for children using bimodal devices and for children using bilateral implants. It also describes the development of spatial unmasking with age in children with normal hearing. Spatial unmasking was measured with a speech detection in noise task, and bilateral advantage was measured with both speech detection and speech recognition tasks in background noise. The relationships between aided thresholds and bilateral advantage for children with bimodal devices were examined. The frequencies of the phonemes that contributed to bilateral advantage in children with bimodal devices and children with bilateral implants were investigated. Comparisons were made between children with bimodal devices and children with bilateral implants on bilateral advantage, head-shadow effect, and types of phonemes contributing to bilateral advantage. Results showed that spatial unmasking was demonstrated in children with normal hearing across three different age groups: infants, preschoolers, and school children. Spatial unmasking was also demonstrated in children with a hearing loss, both in children using bimodal devices and in children using bilateral implants. For children with normal hearing, spatial unmasking changed with age, with infants and preschoolers showing a greater amount of spatial unmasking than school children and/or adults. This change in spatial unmasking with age could imply that the development of the binaural processing of non-spatial cues is slower than that of spatial cues. Differences in attention and motivation between the age groups may also have contributed to the results. These results suggest that sources of noise and target speech should be spatially separated wherever possible for children with bimodal devices and bilateral implants in educational and home settings. For children with bimodal devices or bilateral implants, both speech detection and speech recognition in noise were better when listening with two hearing devices rather than one. This reinforces the importance of fitting a contralateral hearing aid or a second implant to children who use a single implant. Results showed that aided thresholds affect bilateral advantage in children with bimodal devices. Children with better aided thresholds at 250 and 500 Hz, and/or poorer aided thresholds at 4 kHz demonstrated greater bilateral advantage. This finding is possibly due to the fact that a hearing aid provides finer low-frequency spectral information than a cochlear implant, so the low-frequency information from a hearing aid could be complementary to the signal received from an implant in the opposite ear. There may also be a mismatch of high-frequency information provided by the hearing aid and the cochlear implant, resulting in the negative impact of aided thresholds at 4 kHz on bilateral advantage. Results of information transmission analyses for children with bimodal devices and bilateral implants showed that bilateral advantage in speech recognition was spread over a wide range of frequencies of phonemes for both groups. This research identified two main differences between children with bimodal devices and children with bilateral implants that were consistent across the speech detection and speech recognition experiments. Firstly, children with bilateral implants demonstrated a greater head-shadow effect than children with bimodal devices. Secondly, children with bimodal devices demonstrated a greater bilateral advantage than children with bilateral implants when signal and noise are both presented from the front. These results imply that bimodal devices and bilateral implants may each have their own advantages and disadvantages. Careful pre-operative counseling is therefore important for children with bimodal devices who are considering a second implant. The speech detection method for measuring spatial unmasking and bilateral advantage described in this thesis can be used for evaluating the outcome of bimodal devices and bilateral implants for young children with minimal language ability for speech recognition tasks.