Otolaryngology - Theses
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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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ItemTemporal processing in the deafened auditory cortex( 2012)The cochlear implant (CI) has proven highly successful in treating sensorineural hearing loss, however we remain unable to explain much of the variability in recipient outcomes. Studies of language impairment in normal hearing, in addition to more limited studies in CI recipients, suggest that the temporal processing capacity of the central auditory pathway may be a limiting factor in some CI users. In particular, the detection of amplitude modulation (AM) appears to be a good predictor of speech perception ability. The central aim of this thesis is to better understand the neural correlates of AM ICES detection, and identify what changes occur following deafness and chronic ICES experience. We recorded multiunit neural responses in the primary auditory cortex (AI) of anaesthetised rats that were either acutely (n=4) or long-term deaf (n=4) from adulthood. While previous reports show that temporal processing declines following neonatal long-term deafness, we found that the representation of AM intracochlear electrical stimulation (ICES) is maintained, or even improves, following adult-onset long-term deafness. We trialed a novel fully-implantable stimulator in seven animals, which was capable of chronically delivering AM ICES. While the stimulator itself generally proved reliable, functioning for up to five months in vivo, the intracochlear electrode array failed in all but one animal. The effects of chronic ICES in that subject were similar to that of LT deafness, although there was some evidence that phase locking was enhanced at high modulation frequencies, which justifies further study. Using a spike train classifier, we explored the temporal resolution necessary to detect AM ICES in the acutely-deafened auditory cortex. Modulated stimuli could be readily detected using both spike timing and rate. These results suggest that AM detection remains robust following deafness, and is supported by multiple redundant neural codes, reinforcing the notion that envelope information is privileged by the auditory system in order to support speech perception.
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ItemEffects of chronic electrical stimulation and exogenous neurotrophins in the deafened cochlea( 2011)The loss of cochlear hair cells is a common cause of deafness. Auditory perception can be restored to some deaf patients using a cochlear implant, which electrically stimulates the spiral ganglion neurons (SGN) of the auditory nerve. In the normal cochlea, hair cells produce peptides called neurotrophins which provide a pro-survival signal to the SGNs. The loss of hair cells can therefore result in the degeneration of SGNs, which could potentially affect implant performance. The application of exogenous neurotrophins to the cochlear fluid can prevent SGN degeneration and may induce the formation of new neurons (neurogenesis), as suggested by the results of some studies. Neurotrophin treatment may also promote the ectopic regrowth of SGN peripheral processes. The cochlear implant relies in part upon the orderly spatial organisation of SGNs to impart pitch information. Therefore, the disorganised process regrowth seen following neurotrophin treatment could degrade implant performance. Furthermore, the morphology of SGN peripheral processes has not been studied following the combined application of neurotrophins and chronic intracochlear electrical stimulation (ICES). This is an important treatment combination to investigate for the clinical advancement of intracochlear neurotrophin use. This study aimed to describe the effects of neurotrophin and/or ICES treatment on several aspects of SGN biology, with the primary emphasis on the potential relationship between SGN process disorganisation and the spatial selectivity of ICES. Ototoxically deafened guinea pigs were treated with neurotrophins and/or chronic ICES for the following studies: • The ectopic growth of peripheral processes was quantified using single SGN peripheral process tracings. It was found that SGN process disorganisation was significantly increased following neurotrophin treatment, and that ICES had no effect. • The spatial selectivity of ICES was measured by recording the spread of activation in the auditory midbrain. Neurotrophin treatment did not affect the spread of midbrain activation, whereas chronic ICES produced increased spread, as well as decreased the depth of electrode tuning – another indication of degraded spatial selectivity. However, neurotrophins did reduce the activation threshold to ICES.• The potential induction of SGN neurogenesis was investigated using a marker for DNA synthesis. However, no evidence of neurogenesis was seen.• Finally, SGN peripheral processes were examined using electron microscopy to determine if ultrastructural changes were caused by the treatments. Some key observations were that peripheral process degeneration tended to precede myelin degeneration, neurotrophins reduced the extent of peripheral process shrinkage caused by deafness, and chronic ICES caused further process shrinkage. In summary, this study has demonstrated that neurotrophin treatment causes functional and anatomical changes that may be beneficial to cochlear implant patients. The potential problem of increased peripheral process disorganisation seems to be of little functional consequence with regards to spatial selectivity, likely due to the small scale of process disorganisation compared to the electrical current pathways produced by ICES.