Otolaryngology - Theses
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ItemIntraoperative monitoring during cochlear implantation and correlations to preservation of hearing( 2019)Cochlear implants having been widely used over the past few decades to restore or provide hearing to profoundly deaf individuals. Recently the benefit of combining electrical stimulation from a cochlear implant with low frequency natural hearing or acoustically amplified hearing have been recognised. Benefits include better speech understanding in noise and better appreciation of music (Gantz et al., 2005). Unfortunately inserting a cochlear implant presents significant risk to a patient’s natural hearing therefore, although there are great potential benefits, there is also risk. There are numerous causes of hearing loss after cochlear implant surgery (Rowe et al., 2016). One of those causes is physical damage to the fine structures in the cochlear during array insertion. Electrophysiological monitoring is used to maintain the integrity of important structures for many operations such as facial nerve monitoring in many major ear or parotid gland operations and recurrent laryngeal nerve monitoring in thyroid gland surgery. Electrophysiological monitoring of hearing has been used when resecting tumours near the auditory nerve (Cueva, 2012). Monitoring hearing during cochlear implantation seems a logical extension of a surgeon’s arsenal of monitoring tools. This thesis explores the role of intraoperative monitoring of hearing during cochlear implantation with a goal to reduce postoperative hearing loss by avoiding implant related trauma at the time of surgery. This thesis takes a pragmatic approach focussing only on measurement modalities and methodologies which would be feasible to be use in a day-to-day clinical setting. This work involved: 1. Developing a novel electrophysiological recording system leveraging the telemetry capabilities of commercial cochlear implants. 2. A small animal study to technically validate the function of the system. 3. A small human study to validate the practical use of the system and define stimulus parameters and expected response patterns. 4. A larger human study demonstrating the correlation between adverse intraoperative measurements and failure to preserve operative hearing.
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ItemAn advanced cochlear implant hearing prosthesis for profound to total deafness( 1988)This thesis describes the design and application of an advanced cochlear implant for a hearing prosthesis. When used with a suitable external speech processor, the device is intended to convey more auditory information to deaf implantees than existing prostheses. The implant comprises an electrode array, a capsule containing a receiver-stimulator integrated circuit, and a coil to which radio frequency (RF) signals are transmitted from the speech processor. The electrode array includes 20 platinum bands mounted on a flexible carrier. It is surgically inserted into the scala tympani of the cochlea (inner ear) to bring the electrodes into proximity with surviving auditory neurons. Each electrode is connected individually to the receiver-stimulator. It is the task of the receiver-stimulator to demodulate the RF signals picked up by the receiving coil and to deliver stimulating current pulses to the electrodes under control of the received data. The entire device is implanted beneath the skin, and derives an internal power supply by rectification of the RF signal transmitted from the speech processor. The receiver-stimulator chip, which contains approximately 8000 transistors in digital and analogue circuits, was custom designed. On receipt of each data sequence, it can generate three pulsatile stimuli controlled independently in onset time, duration and current level. Each stimulus can be delivered to any two electrodes selected from the array and configured as a bipolar pair. Bipolar stimulation is used to reduce the spread of neural excitation distributions so as to minimize undesirable interactions between stimuli. The stimuli can be presented sequentially or effectively simultaneously. In the simultaneous stimulation mode, a novel current waveform is generated which was developed to ensure that chronic stimulation would be biologically safe. The implant also contains a telemetry system which enables a selected electrode voltage waveform to be conveyed to an external receiver for display and analysis. The receiver-stimulator chip was fabricated externally using a low-power high-reliability process. Subsequently, the complete advanced cochlear implant was constructed and tested intensively. The device has been implanted in two patients so far. In initial audiological evaluations, both implantees are achieving good results.
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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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ItemThe cochlear prosthesis: safety investigations( 1986)The present research used both physiological and histological techniques to assess the effects of chronic intracochlear electrical stimulation on the residual auditory nerve population in cats. Stimuli consisted of charge balanced biphasic current pulses presented at 500 pps. Stimulus levels were in the range 0.5 - 0.9 mA, and 200 us per phase, and developed charge densities of 18 - 32 uC.cm^-2 geom. per phase. These stimulus levels are within the range used clinically. The animals were stimulated for periods of up to 2000 hours during which time electrically evoked auditory brainstem responses (EABRs) were periodically recorded. At the conclusion of the stimulus program spiral ganglion cell survival was assessed for both stimulated and control cochleas; comparison of the two groups showed no statistically significant difference in ganglion cell population. A number of cochleas exhibited various degrees of cochlear pathology in association with a general inflammation reaction. Severe inflammation, observed in four of the 20 cochleas examined, was attributed to the presence of infection and resulted in significant and widespread neural degeneration. The histopathological changes were correlated with changes in the EABR input-output functions and confirmed the physiological viability of these cells. The results of this study indicates that long-term intracochlear electrical stimulation, using carefully controlled biphasic pulses, does not adversely affect the auditory nerve population. However, widespread infection can result in severe loss of auditory nerve fibres and care must be taken in this regard during implant surgery. Finally, the correlation between cochlear histopathology and EABR recordings suggests that the EABR may be a useful physiological tool in determining auditory nerve survival in patients. The impedance of these scala tympani electrodes were monitored throughout the chronic stimulation program and were compared with impedance data from similar electrodes chronically stimulated in inorganic saline. The changes in impedance of the in vivo stimulated electrodes generally correlated with the degree of fibrous tissue reaction adjacent to the electrode surface. These scala tympani electrodes were examined for evidence of corrosion using a scanning electron microscope. The surface of these in vivo electrodes were compared with in vivo control electrodes and in vitro electrodes stimulated in inorganic saline using similar stimulus parameters. The in vitro stimulated electrodes showed evidence of platinum dissolution at high charge densities (36 uC.cm^-2 geom. per phase) and aggregate charge (270 C). Significantly, the in vivo stimulated electrodes showed no evidence of stimulus induced corrosion. Indeed, their surfaces were similar to the in vivo control electrodes. Previous in vitro electrochemical studies have demonstrated that proteins play a significant role in the inhibition of platinum dissolution. The present study has demonstrated an inhibitory effect in vivo which may be due to the presence of proteins. The results from these studies reflect the biocompatible nature of this neural stimulator. Finally, temporary and permanent reductions in the excitability of the auditory nerve were observed following acute stimulation at intensities and rates above the range used clinically. The extent of these changes correlated with the degree of stimulus evoked neural activity. Furthermore, these stimulus induced changes were metabolically active. These findings suggest that the changes in neural excitability were a result of long-term metabolic changes in the stimulated neural population. Moreover they indicate upper functional operating ranges for auditory prostheses using this form of stimulus regime.