Otolaryngology - Theses
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ItemExtending the application of virtual reality simulation in temporal bone anatomy and advanced surgical training( 2021)Cochlear implant surgery has a strong foundation in the treatment of profound hearing loss. In the last decade, there have been marked developments in technology which have enabled the expansion of eligibility criteria. Cochlear implantation is now offered to patients with residual low-frequency hearing or those with unilateral or asymmetrical hearing loss. It is evidenced that to optimise hearing outcomes for patients with residual hearing, it is necessary to reduce the trauma during the insertion of the cochlear implant, in particular by precise surgical technique. Current temporal bone surgery training, including cochlear implant surgery, is based on an apprenticeship model, where registrars observe and practice with consultant supervision. Prior to performing surgery on a patient, it is common practice to perform cadaveric temporal bone dissections. In addition to concern regarding decreasing availability of cadaveric temporal bones are financial constraints and regulations that reduce teaching time available in surgery. The generally low caseload, specifically relevant to cochlear implant surgery, minimises the opportunities for apprenticeship training. As such, this traditional model of training is not maintainable. Similar pressures face the training of anatomy of the temporal bone to medical students and junior doctors. While otologic presentations make a sizable proportion of presentations to emergency departments and general practice, due to the reduction in medical school training time, education in otology is in decline. Virtual reality (VR) surgical training is an attractive adjunct to the current training pathway as it provides a cost effective platform where risk-free, repetitive practice is readily available. VR also has several unique benefits. By presenting automated feedback, VR training allows for self-directed learning. In addition, automated assessment tools have been validated to objectively measure performance. While the effectiveness of VR simulation for mastoidectomy training has previously been well established, to the author’s knowledge there have been no VR simulators adapted to teach more complex temporal bone surgery such as cochlear implant surgery, or clinically oriented temporal bone anatomy. The aims of this thesis were: 1) to determine the viability of expanding the role of VR simulation in otology, including anatomy education and advanced temporal bone surgery and 2) to explore patient factors that relate to surgical technique in cochlear implant surgery. To these ends, several investigations were performed. Firstly, a randomised control trial was conducted to determine whether a clinically oriented VR temporal bone simulator module improved anatomy knowledge of medical students and junior doctors. Participants were randomly allocated to three groups of differing display modality: stereoscopic 3D, monoscopic 3D and 2D presentations. The participants completed a pre-tutorial questionnaire before working through the self-guided tutorial. The module was followed by a post-tutorial questionnaire and a retention questionnaire at 6 weeks. The questionnaires assessed factual anatomic knowledge, spatial relationships and clinically oriented knowledge as well as student’s perception of the display modality. It was observed that the module was effective in imparting factual knowledge in all modalities. The students exposed to the 3D technologies performed better in the spatial relationship and clinically oriented questions. The Stereoscopic 3D modality showed particular benefit for ease of use. Secondly, a training module specifically designed for the surgical approach to cochlear implant surgery was assessed with a prospective pre- and post-study of ENT registrars. All participants were exposed to the same training module that included concurrent and terminal feedback on temporal bones with a range of common anatomical variations. The participants’ performances were compared before and after the training. The assessment temporal bones used at the end of training consisted of a mirror image of the one used prior to training and one novel temporal bone. It was found that there was a significant improvement with a large effect size after training for both the previously encountered temporal bone and the novel temporal bone. Thirdly, a conceptual anatomical study was performed using the University of Melbourne temporal bone simulator data. The study explored patient factors that affect the surgical technique used in approaching cochlear implantation. In particular, the relationship of surgical preparation of the facial recess to the acceptable electrode trajectories into the cochlea. It was found that acceptable trajectories through a round window membrane approach most likely originated superiorly in the facial recess, adjacent to the facial nerve. Conversely, acceptable trajectories through a cochleostomy approach most likely originated inferiorly in the facial recess, adjacent to the junction of the facial nerve and chorda tympani. Furthermore, the skeletonisation of the facial recess was found to be critical in the preparation of the temporal bone for a cochleostomy approach. The results presented throughout the thesis will help guide medical educators in the areas of otology and cochlear implant surgery. These results suggest the viability of an expanded application of virtual reality temporal bone simulations to use in anatomy education and advanced temporal bone surgery training.
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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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ItemLocal delivery of Dexamethasone for the preservation of residual hearing in an animal cochlear implant model( 2015)This thesis uses an experimental model of cochlear implantation to examine the hearing protection afforded by the application of dexamethasone to the round window prior to surgery.
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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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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.
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ItemSystemic steroid protects residual hearing in a guinea pig model of cochlear implantation( 2011)Background: The protection of residual hearing is an important goal of cochlear implant surgery. Local application of steroids to the round window membrane (RWM) of the guinea pig prior to cochleostomy has been shown to protect hearing, but requires a period of pre-treatment for at least one hour. To determine whether this waiting period can be avoided, the efficacy of administering systemic steroids prior to cochlear implantation is investigated. Methods: Seventeen normal hearing guinea pigs were randomly assigned to receive a single preoperative intravenous injection of: 1) normal saline, 2) dexamethasone 0.2 mg/kg or 3) dexamethasone 2 mg/kg, 60 minutes before cochlear implantation. Implantation was completed with a silastic/platinum dummy electrode. Prior to surgery pure tone auditory brainstem response (ABR) thresholds were estimated from both ears separately in response to tone-pips from 2-32 kHz. This was again completed at 1 and 4 weeks postoperatively. The primary outcome measure was threshold shift at 1 and 4 weeks. Histology was examined for evidence of insertion trauma and foreign body reaction. Results: Preoperative injection of 2 mg/kg dexamethasone prevented an elevation in ABR thresholds at all frequencies compared with the control group (8 - 32 kHz) at 1 and 4 weeks post implantation. This protection was not seen with a lower dose (0.2mg/kg) of dexamethasone. There was a foreign body reaction observed in control and low-dose treated animals, however this was suppressed in all but one of the high-dose dexamethasone-treated animals. Conclusions: Intravenous high dose dexamethasone protects hearing during cochlear implantation and prevents the development of an inflammatory histological response. The prolonged intra-operative delay required for local delivery is avoided in this model. Furthermore, it may provide better protection of low frequency hearing than locally administered steroid.
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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.
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ItemCochlear implant sound coding with across-frequency delays( 2009)The experiments described in this thesis investigate the temporal relationship between frequency bands in a cochlear implant sound processor. Initial studies were of cochlea-based traveling wave delays for cochlear implant sound processing strategies. These were later broadened into studies of an ensemble of across-frequency delays. Before incorporating cochlear delays into a cochlear implant processor, a set of suitable delays was determined with a psychoacoustic calibration to pitch perception, since normal cochlear delays are a function of frequency. The first experiment assessed the perception of pitch evoked by electrical stimuli from cochlear implant electrodes. Six cochlear implant users with acoustic hearing in their non-implanted ears were recruited for this, since they were able to compare electric stimuli to acoustic tones. Traveling wave delays were then computed for each subject using the frequencies matched to their electrodes. These were similar across subjects, ranging over 0-6 milliseconds along the electrode array. The next experiment applied the calibrated delays to the ACE strategy filter outputs before maxima selection. The effects upon speech perception in noise were assessed with cochlear implant users, and a small but significant improvement was observed. A subsequent sensitivity analysis indicated that accurate calibration of the delays might not be necessary after all; instead, a range of across-frequency delays might be similarly beneficial. A computational investigation was performed next, where a corpus of recorded speech was passed through the ACE cochlear implant sound processing strategy in order to determine how across-frequency delays altered the patterns of stimulation. A range of delay vectors were used in combination with a number of processing parameter sets and noise levels. The results showed that additional stimuli from broadband sounds (such as the glottal pulses of vowels) are selected when frequency bands are desynchronized with across-frequency delays. Background noise contains fewer dominant impulses than a single talker and so is not enhanced in this way. In the following experiment, speech perception with an ensemble of across-frequency delays was assessed with eight cochlear implant users. Reverse cochlear delays (high frequency delays) were equivalent to conventional cochlear delays. Benefit was diminished for larger delays. Speech recognition scores were at baseline with random delay assignments. An information transmission analysis of speech in quiet indicated that the discrimination of voiced cues was most improved with across-frequency delays. For some subjects, this was seen as improved vowel discrimination based on formant locations and improved transmission of the place of articulation of consonants. A final study indicated that benefits to speech perception with across-frequency delays are diminished when the number of maxima selected per frame is increased above 8-out-of-22 frequency bands.