Graeme Clark Collection
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ItemCochlear pathology following chronic electrical stimulation of the auditory nerve: II Deafened kittens( 1994)The present study examines the effects of long-term electrical stimulation of the auditory nerve on cochlear histopathology and spiral ganglion cell survival in young sensorineural deafened cats. Eight kittens were deafened using kanamycin and ethacrynic acid, and implanted with bipolar or monopolar scala tympani electrodes. Following recovery from surgery the animals were unilaterally stimulated using charge balanced biphasic current pulses for 450-1730 hours over implant periods of up to four months. Charge densities varied from 0.6-0.9 µC.cm ^-2 geom. per phase for monopolar electrodes to 12-26 µC.cm ^-2 geom. per phase for the bipolar electrodes. Electrically-evoked auditory brainstem responses (EABRs) were periodically monitored during stimulation to confirm that the stimulus levels were above threshold, and to monitor any change in the response of the auditory nerve. Following completion of the stimulation program cochleae were prepared for histological examination. EABRs exhibited relatively stable thresholds for both stimulated and implanted, unstimulated control cochleae for the stimulus duration. While the growth in response amplitude as a function of stimulus current remained stable for the bipolar control and monopolar stimulated cochleae, the five cochleae chronically stimulated using bipolar electrodes exhibited a moderate to large increase in response amplitude. These increases were associated with a more widespread fibrous tissue response which may have altered the current distribution within these cochleae. Implanted control cochleae exhibited significantly less tissue response within the scala tympani. Importantly, we observed no statistically significant difference in the spiral ganglion cell density associated with chronic electrical stimulation when compared with unstimulated control cochleae. While the present study supports the safe application of cochlear implants in young profoundly deafened children, it does not corroborate previous studies that have reported electrical stimulation providing a trophic effect on degenerating auditory nerve fibres.
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ItemPartial hearing loss in the macaque following the co-administration of kanamycin and ethacrynic acid( 1994)Co-administration of kanamycin (KA) with the loop diuretic ethacrynic acid (EA) rapidly produces a profound hearing loss in the cat while maintaining normal renal function [Xu et al., Hear. Res. 70, 205-215 (1993)]. In the present paper we have applied this deafening procedure to the old world monkey Macaca fascicularis (macaque). Following the co-administration of KA and EA, the hearing loss in the macaque developed far slower than we observed in the cat. Moreover, unlike the cat, there was evidence of a partial recovery in the animal’s hearing, resulting in a bilaterally symmetrical high frequency hearing loss. The extent of this hearing loss was dependent on the dose of the EA administered. Finally, the most unexpected result of the present study was the degree of acute nephrotoxicity experienced by these animals following the drug administration. The sensitivity of this species to renal failure restricted the dose of EA that could be safely administered. In conclusion, the co-administration of KA and EA cannot reliably produce a profound hearing loss in the macaque. While it can produce a dose dependent high frequency hearing loss the animal will also experience acute renal failure that requires careful management.
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ItemCochlear pathology following chronic electrical stimulation using non charge balanced stimuli( 1991)During the course of a chronic intracochlear electrical stimulation study using charge balanced biphasic current pulses, one animal inadvertently received a short period of direct current (DC) stimulation at a level of approximately 1 µA. Subsequent, the animal was chronically stimulated using a poorly charge balanced waveform that produced a DC level of approximately 2 µA. Extensive pathological changes were observed within the cochlea. These changes included widespread spiral ganglion cell loss and new bone growth that extended throughout all turns of the cochlea. Significant changes in the morphology of the electrically evoked auditory brainstem response (EABR) were associated with these pathological changes. EABRs recorded prior to the DC stimulation exhibited a normal waveform morphology. However, responses recorded during the course of the DC stimulation were dominated by a short latency response believed to be vestibular in origin. The response thresholds were also significantly higher than levels recorded before the DC stimulation. In contrast, the contralateral cochlea, stimulated using charge balanced stimuli, showed no evidence of adverse pathological changes. Furthermore, EABRs evoked from this cochlea remained stable throughout the chronic stimulation period. Although preliminary, the present results illustrate the adverse nature of poorly charge balanced electrical stimuli. These results have important implications for both the design of neural prostheses and the use of DC stimuli to suppress tinnitus in patients.
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ItemEvaluation of a sealing device for the intracochlear electrode entry point( 1991)Experimental evidence in animals indicates that middle ear infection in the presence of an intracochlear electrode may result in widespread cochlear damage due to the passage of organisms or products of inflammation through the electrode entry point. In this paper, results are presented of a study undertaken to test the efficacy of a titanium electrode entry point seal designed by the principal author, to protect the implanted cochlea from the pathological effects of experimentally induced pneumococcal otitis media in five cats. lntracochlear electrodes were inserted into both cochleas of each cat, one side sealed with the device and the other side left unsealed, as is current operative practice in human cochlear implantation, as a control. After a minimum of twelve post-operative weeks, pneumococcal otitis media was successfully inoculated in all but one (control) middle ear, which was not inoculated due to accidental removal of the electrode. One week after inoculation the animals were sacrificed and cochleas removed for histological examination. Results of histological examination of the cochleas are presented together with bacteriological data. The results of microscopic examination of the bond interface between otic capsule bone and the titanium seal are presented.
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ItemElectrical stimulation of the auditory nerve in deaf kittens: effects on cochlear nucleus morphology( 1991)The present study examines the effects of long-term electrical stimulation of the auditory nerve on the morphology of neurons in the cochlear nucleus in young, sensorineural deaf animals. Kittens, systemically deafened using kanamycin and ethacrynic acid, received bilateral cochlear implants and were stimulated unilaterally for periods of up to four months. After sacrifice, cross-sectional areas of neuron somata were measured with an image-analysis system and compared using nonparametric statistics. The areas of cell somata within the anteroventral cochlear nucleus (AVCN) on the stimulated side were significantly larger than those of corresponding somata on the control, unstimulated side (P < 0.001). However, there was no statistically significant difference among dorsal cochlear nucleus (DCN) neurons. These results indicate that long-term electrical stimulation of the auditory nerve can at least partially negate some effects of early postnatal auditory deprivation at the level of the cochlear nucleus.
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ItemMultichannel cochlear implantation in children: a summary of current work at The University of Melbourne( 1991)This paper summarizes research work relating to multichannel cochlear implantation in children at the University of Melbourne. Ongoing safety studies relating to the implantation of young children are discussed. Results of these studies suggest that special design considerations are necessary for a prosthesis to be implanted in children under the age of 2 years. Results of clinical assessment of implanted children and adolescents are also discussed in terms of speech perception, speech production, and language development, and some possible predictive factors are suggested. Preliminary data suggests that a high proportion of young children can achieve open-set speech perception with the cochlear implant given appropriate training and support. Initial results with adults using new speech processing hardware and a new coding scheme are also presented. These suggest that improved speech perception in quiet and competing noise is possible with the new system.
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ItemEvaluation of leadwire fixation for paediatric cochlear implants [Abstract]( 1994)A paediatric cochlear implant should include a leadwire system that can readily expand in the presence of tissue adhesions and can be effectively fixed at a site close to the cochlea to ensure that the electrode array is not displaced during skull growth. In this study, leadwires were implanted in six young animals for a period of five months. During explantation, the mean force � standard deviation required to expand individual leadwire was found to be 12.5 � 5.0g. In order to evaluate the efficacy of leadwire fixation techniques, four fixation procedures were initially developed in human temporal bones and subsequently used to fix leadwires implanted in the temporal bones of eight animals for a period of four months. Leadwires were fixed by platinum wires at the fossa incudis or by platinum wires with a titanium barbed nail at the mastoid. The biomechanical evaluation revealed that the forces required to displace the leadwire from fixation points were 70.6 � 33.5g. Significantly, the forces required to withdraw a chronically implanted electrode array from an animal cochlea were 1.5 � 0.4g. The present results highlight the importance of an effective leadwire fixation technique for paediatric cochlear implants, particularly in preventing the displacement of an electrode array from the cochlea during skull growth.
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ItemElectrical stimulation of the auditory nerve: comparison of half-band with full-band scala tympani bipolar electrodes( 1993)The Melbourne/Cochlear auditory prosthesis uses an intracochlear electrode array containing 22 circumferential full-band electrodes mounted on a Silastic carrier. It could be hypothesized that half-band electrodes, oriented towards the modiolus, would produce lower stimulus thresholds than conventional full-band electrodes. This hypothesis is based on the assumption that, compared with full-band electrodes, half-band electrodes would produce an electrical field in which a greater proportion of the current would excite a defined group of neurons. In order to verify this hypothesis we recorded electrically evoked auditory brainstem responses (EABRs) for both full- and half-band electrodes inserted in the scala tympani of deafened cats. EABR thresholds for half-band electrodes oriented towards the modiolus were not significantly different from thresholds evoked using full-band electrodes (p>0.05, paired t-test), whereas thresholds evoked using half-band electrodes oriented towards the outer scala wall were significantly higher (p<0.01) than either the modiolar half-band or the full-band electrodes. These physiological results suggest that the electrical field generated within the auditory nerve by modiolar oriented half-band electrodes does not differ significantly from that produced by full-band electrodes. On the basis of these results, together with the fact that half-band electrodes would have higher current densities and electrode impedances, and would require careful orientation during implantation, we consider that there is no benefit in incorporating half-band electrodes in the design of scala tympani electrode arrays.
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ItemMultichannel cochlear implants in children: an overview of experimental and clinical results [Abstract]( 1991)During the last decade there has been great progress in the clinical management of profound, postlinguistically deafened adults through the use of multichannel cochlear implants. The device developed by Cochlear Pty. Ltd. in association with the University of Melbourne, electrically stimulates selective regions of the residual auditory nerve using an array of 22 Pt electrodes located within the scala tympani. A speech processing strategy has been developed to provide patients with both voice pitch, and first and second formant information. Following experimental safety studies and successful clinical trials, this device was approved for use in adults by the United States FDA in 1985. In 1990, following further miniaturization of the implant, the FDA approved the device for use in profoundly deafened children above the age of two years. The present paper presents an overview of our recent biological safety studies and clinical experience with cochlear implants in children, and discusses the likely future development of these devices. Our biological safety studies were designed to evaluate the safety and design requirements of cochlear implantation in children, and more recently has focussed on issues for implantation in very young children (< 2 years old). These studies included the measurement of growth in the human temporal bone and the development of lead wires that can accommodate such growth, the development of an electrode fixation technique close to the cochlea, the effect of cochlear implantation on skull growth, the effect of long-term electrical stimulation on the maturing auditory system and the stimulating electrodes, and the effect of middle ear infection on cochlear implantation. Our clinical experience is based on twenty-five children that have now been implanted in our clinic. They include (i) postlinguistically deafened children; (ii) congenitally or early-deafened young children; and (iii) congenitally or early deafened adolescents. Clinical testing has shown improvements in speech perception, speech production and language in all three groups. Postlinguistically deafened children show similar speech perception results to postlinguistically deafened adults. For the congenitally deaf, younger children tend to show better results than the adolescents. Significantly, these clinical results are consistent with results from 142 children obtained from clinics throughout the world. These experimental and clinical results support the use of cochlear implants in young children. Further clinical improvements can be expected in the future with advances in both hardware and speech processing strategies.
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ItemCochlear implant safety studies [Abstract]( 1991)We have examined a number of safety issues associated with cochlear implantation in both adults and children and confirm that it is safe. The results of these studies may be summarized as follows: 1) The insertion of a free-fit scala tympani array into the human cochlea produces minimal damage to cochlear structures provided insertion is stopped at the point of first resistance. 2) Chronic intracochlear implantation and electrical stimulation using charge balanced biphasic current pulses does not result in neural degeneration or evoke an adverse tissue reaction within the cochlea. 3) The use of fascia to seal the implanted round window results in an effective barrier to the spread of infection into the cochlea. 4) Temporal bone studies have shown that the distance between the round window and the fossa incudis remains essentially unchanged from birth. Therefore, the fossa incudis provides a suitable fixation point for the electrode array in young children. This study also showed that the distance from the round window to the implanted receiver-stimulator undergoes significant change in children of up to two years of age. These children would require an expanding leadwire system. 5) Finally, long-term effects of cochlear implant surgery on skull growth have shown that this should not be a problem when implanting very young children. This work was supported by the NIH (NOI-NS-7-2342).