Otolaryngology - Theses
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ItemFour-point impedance measurements in cochlear implantation, and correlations with clinical outcomes( 2023-08)With the expansion of inclusion criteria for cochlear implantation, there has been an increased focus on the preservation of residual hearing and reduction of post-operative dizziness in cochlear implant recipients. Preservation of residual hearing allows for electroacoustic stimulation, which results in improved music appreciation, noise localisation and comprehension in noisy environments. Understanding and the prevention of post-operative dizziness is important especially given the increasing frequency of cochlear implant surgery. Investigations into peri-operative monitoring (including intra-operative and post-operative monitoring) has yielded promising results, and this has provided new insights into the physiology of the cochlea, the pathophysiology of cochlear implantation, as well as factors that impact clinical outcomes such as loss of residual hearing and the development of post-operative dizziness. One such peri-operative measurement, four-point impedance, has been theorised to detect intra-cochlear bleeding, inflammation or fibrosis. This thesis explores the use of four-point impedance measurements during and after cochlear implantation and correlates these measurements with clinical outcomes in cochlear implant recipients. The first section investigates the use of four-point impedance alongside a known predictor of residual hearing (real-time intra-cochlear electrocochleography) in predicting loss of residual hearing. We see that four-point impedance significantly improves our ability to predict loss of residual hearing compared to real-time electrocochleography alone, and potentially fluctuates with the robustness of the early acute inflammatory response and the magnitude of fibrosis. The second section expands on the theories proposed in the first section, investigating the use of four-point impedance in predicting post-operative dizziness, alongside a test for utricular function (Subjective Visual Vertical). We find that four-point impedance is also closely associated with the development of dizziness, and this aligns with the inflammation theory as well. These findings suggest four-point impedance may be a useful biomarker for predicting clinical outcomes in future cochlear implantation surgery, and at the very least warrants further investigation.
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ItemPrevalence of ear disease and its associated hearing loss among primary school children in Tonga: urban versus rural( 2020)Background. Ear disease and hearing loss among children are significant world- wide health problems that potentially cause speech and language development delay, social isolation, and academic failure. The prevalence is highest in low- and middle- income countries including the Pacific island countries where epidemiological data and resources are limited. National programs to raise awareness and detection rarely existed in most Pacific island countries. The purpose of this case study was to determine the prevalence of ear disease and associated hearing loss in primary school children. The data will be used as informed evidence to develop a propose strategies to address ear and hearing problems among primary school children in Tonga. Methods. This was a cross sectional population- based study which used otoscopy, tympanometry and pure tone audiometry to determine the prevalence of ear disease and hearing loss in 10 urban and 12 rural primary schools in Tongatapu. Results were collected on a digitised form from 489 students (88% of total selected population) class 1 to 6 (5 to 14 years of age) and data were analysed using quantitative statistical and mixed model method. Results. Wax was the most common ear problem found in 54.6% (267 students) of the primary school children both partial and occluded wax and more common in urban children (31%) as compare to rural (24%). This was statistically significant for age and school location but not gender. Followed with middle ear diseases in 12% (60 students, rural 8%, urban 4%) including children with suppurative otitis media (1.4%, perforation with ear discharge), suspected cholesteatoma (0.6%) and possible otitis media with effusion (10%, immobile, dull, retracted, atelectatic ear drum and type B tympanogram). Seven percent (37 students) of the students failed hearing test (defined as > 25 dB of best hearing ear) with middle ear disease (15 students, 41%) the most common cause followed by occluded wax (9 students, 24%), wax plus OME (7 students, 19%) and 6 students (16%) under unknown cause. The mean threshold difference between occluded and non-occluded ears among primary school children were 3 dB. The improvement in mean threshold among children with occluded wax after removal was 7 dB. Of the 15% of children with bilateral hearing loss due to occluded wax, there was 5% reduction in number of students post wax removal. Wax removal benefitted 30% and 35% of students with unilateral and bilateral hearing respectively. Dental caries affected 28% of the primary school children. Conclusion. Ear disease causing hearing loss was commonly found among primary school children in Tonga. A national plan for ear and hearing care for primary school children including early detection and management through school screening are very important to assist children to achieve better education.
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ItemStochastic resonance in a neuron model with application to the auditory pathway( 2001-03)In this thesis, the transmission of spike trains in a neuron model is studied in order to obtain a better understanding of the role played by stochastic activity, i.e. uncorrelated spikes, in the auditory pathway. Fluctuations of the neuron membrane potential are given by a first-order stochastic differential equation, using a leaky integrate-and-fire model. In contrast to most previous studies the model has a finite number of synapses, and the usual diffusion approximation does not hold. The input signal is modeled by spike trains with spiking times described by inhomogenous Poisson processes. The membrane potential is a shot noise process for which statistical properties are derived with a Gaussian approximation. The statistics of the output spike train are obtained by using the property that a pool of a large number of output spike trains can be modeled by an inhomogeneous Poisson process. It is shown that, under certain conditions, the addition of uncorrelated input spikes, i.e. noise, can enhance the transmission of periodic temporal information. This phenomenon, called stochastic resonance, is demonstrated analytically and supported by computer simulations. Results are compared with those obtained from the traditional leaky integrate-and- fire neuron receiving a continuous waveform input. The shot-noise property of the membrane potential, which implies that its variance is de facto modulated by the input stimulus, is shown to enhance the phenomenon of stochastic resonance. Indeed, for a given average noise level, a modulated noise gives a higher output signal-to-noise ratio than an unmodulated noise with the same average amplitude. The derivation is then extended to certain polyperiodic stimuli mimicking vowel sounds. The fact that the addition of uncorrelated input spikes can enhance the transmission of information is discussed in the context of cochlear implants. The results provide supportive evidence to the postulate that a cochlear implant speech coding strategy that elicits stochastic firing neural activity might benefit the user.
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ItemThe protective effects of systemic steroids on residual hearing post cochlear implantation( 2012)The aim of this study is to assess residual hearing after cochlear implantation with administration of systemic dexamethasone. In accordance with previous studies carried out by the hearing protection team at Royal Victorian Eye and Ear Hospital the purpose of this study is to observe the relationship between a single dose of intra-venous dexamethasone administration and residual hearing protection. This thesis will address whether hearing protection is sustained by observing functional outcomes in guinea pigs for three months after single dose of intravenous dexamethasone administration. Seventeen normal hearing guinea pigs were randomly allocated to receive an intravenous injection of (2 mg/kg) dose of dexamethasone 60 min prior to cochlear implantation or topical (Round window) application of normal saline pad (control) 30 min prior to cochlear implantation. Auditory brainstem response (ABR) threshold shifts at 2, 8,16, 24 and 32 kHz were measured and a comparison between pre- and 12 weeks post-operative levels was carried out.
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ItemAn evaluation of speech perception when introducing ADRO to adults who use both a cochlear implant and a contralateral hearing aid( 2011)Speech perception outcomes obtainable with cochlear implants have improved over time, so cochlear implantation is now routinely offered to adults with residual hearing who gain benefit from using a hearing aid in their contralateral ear. To maximize the overall sound perception abilities for these patients, we need to consider optimizing the fittings of both the cochlear implant and the hearing aid. One means of optimizing a person's speech perception is to allow them to trial sound processing schemes and evaluate their effects on speech recognition. Adaptive Dynamic Range Optimization (ADRO) is one such scheme that is available in Cochlear Limited's speech processors and has been shown to offer speech perception benefits for adult and paediatric cochlear implant recipients. More recently, ADRO has been implemented in hearing aids and shown to offer some speech perception benefits over other hearing aid amplification algorithms. The aim of this study is to evaluate the ADRO sound processing scheme when implemented in both a cochlear implant speech processor and a hearing aid in a group of adults who would normally wear both (bimodal) devices. Following a period of take-home experience with all device combinations, speech perception measures using words presented at 50dB SPL and 60dB SPL and sentences presented with competing noise were evaluated for the participants using their devices with and without ADRO activated, and the results compared. Participant preferences for the bimodal device combinations were also obtained using a take-home questionnaire. The results from this study show that adults can obtain significant improvement in speech perception when listening in quiet environments when ADRO is activated in both their hearing aid and cochlear implant. The greatest benefit is seen when listening to softer levels of speech. There is no detrimental effect on speech perception when using ADRO in the bimodal device condition in noisy environments. Whist statistically significant differences in speech perception scores were observed between the bimodal-ADRO and no-ADRO device combinations, the differences were not large. This is reflected in the participants indicating no overall preference for either device combination. The outcomes of this study suggest that adults who routinely use a cochlear implant in one ear and a hearing aid in the other could benefit from having ADRO implemented in both devices.
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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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ItemPrinciples of inner ear pharmacotherapy and its application to residual hearing protection in cochlear implantation: an animal model of residual hearing protection using locally delivered dexamethasone( 2011)BACKGROUND: There is growing experimental evidence to support that targeted application of steroid to the inner ear can ameliorate hearing loss resulting from the trauma of cochlear implantation in experimental animals. However, it is yet to be elucidated which factors in cochlear pharmacodynamics are more important in influencing protection, and whether the hearing protection achieved is long-term. This experimental project was designed to firstly confirm previous experimental observation that a single dose of steroid applied locally to a guinea pig cochlea can protect hearing, and then to assess whether such protection is sustained over a long term by following the animals up for 3 months. Furthermore, the study groups were designed to assess which variable between the duration of the application and the size of the applied dose is more important in therapeutic efficacy in terms of better and broader hearing protection. The results may be useful in helping to design a more clinically appropriate and adaptable treatment protocol in cochlear implantation surgery. METHODS: Dexamethasone or saline (control) was absorbed into a disc of polymeric sponge (Seprapack TM) and applied to the round window of adult guinea pigs prior to electrode implantation. The treatment groups were 2% (w/v) dexamethasone applied for 120 minutes; and 20% (w/v) applied for 30 minutes prior to the electrode implantation. Auditory sensitivity was determined pre-operatively and at 1 week, 4 weeks, 8 weeks and 12 weeks for all animals using pure-tone auditory brainstem response audiometry (2 – 32 kHz). Cochlear implantation was performed via a cochleostomy drilled into the basal turn of the cochlea, into which a miniature Sialistic electrode array was inserted using soft-surgery techniques. RESULTS: The implantation resulted in an increase in hearing thresholds across all frequencies examined (2 – 32 kHz). In all groups, after initial loss, auditory thresholds improved after the first week and stabilised by 4 weeks and remained stable until the end of the trial period. No mean progressive loss was evident in this experimental model beyond 4 weeks. Thresholds were less elevated after dexamethasone treatment with maximal protection occurring at 32 kHz. Much greater protection was achieved when dexamethasone was applied for longer period even at a lower loading dose. Higher concentration of dexamethasone applied for shorter duration resulted in protection over basal frequencies but no significant protection was afforded to more apical regions of cochlea. CONCLUSION: A single dose of steroid may be applied to cochlear round window to achieve protection of residual hearing against implantation related trauma, using a locally applied delivery vehicle such as a bio-degradable bead. Protection is both immediate and long-term. This effect occurs in a manner dependent upon an interplay between the loading dosage and the duration of the application, with the latter likely to be more important variable than the size of the dosing in achieving more potent and broader frequency protection. However, the requirement for such lengthy duration of treatment poses practical challenges for the clinical translation of this experiment.