Graeme Clark Collection
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ItemExperimental study on extracochlear electric stimulation [Abstract]( 1992)The efficiency and feasibility of chronic extracochlear implantation and electric stimulation were studied in two adult cats and four 2-month kittens. The first electrode was placed on the round window by fixing the leadwire on the bridge of aditus between the middle ear and bulla cavity; the second electrode was placed on the surface of the tympanic promontory; the third was inserted into the temporal muscle out of the bulla and the forth fixed in transverse sinus with dental cement. ABRs and EABRs were recorded pre-and postoperatively and during electric stimulation.
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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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ItemScanning electron microscopy of platinum scala tympani electrodes following chronic stimulation in patients( 1991)Platinum (Pt) electrodes from three auditory prostheses (Cochlear Pty Ltd) were examined for evidence of corrosion following implantation periods of up to 1000 days. These devices were used for periods ranging from 1600 to 10 400 h and developed maximum charge densities of 0.257 µC mm^-2 geom. per phase. Scanning electron microscopy of the surface of the 66 stimulated electrodes examined showed no evidence of definitive Pt corrosion. Their surface features were essentially identical to control (unstimulated) electrodes. In addition, there was no evidence of any change in the surface morphology of the Silastic® carrier adjacent to the stimulating electrodes. These results indicate that Pt is a suitable electrode material for neural prostheses that use relatively large surface area electrodes (0.1-1.0 mm^2 and low to moderate charge densities (0.01-0.26 µC mm^-2 geom. per phase).
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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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ItemAnimal models of human disease: otitis media( 1991)Otitis media is an inflammation of the middle ear, which may or may not be of microbial origin. Genetic, immunologic, allergic conditions, antecedent viral respiratory infections, and mastoid size are contributing factors for middle ear disease. Dysfunction of the eustachian tube predisposes to acute otitis media. Collection of fluid within the middle ear cavity is part of the disease process and is equally observed in infectious and noninfectious middle ear disease. Streptococcus pneumonia, Haemophilus influenza, Streptococcus pyogenes, and Streptococcus aureus are the most common organisms that cause acute infectious otitis media.
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ItemSurgical and safety considerations of multi-channel cochlear implants in children( 1991)Multi-channel cochlear implants have become a viable surgical treatment for profoundly deaf individuals. With the Food and Drug Administration’s (FDA’s) June 1990 approval for the release of the 22-channel implant for children aged 2 to 17, more hearing impaired young people than ever before will be able to benefit from the auditory sensations provided by the device. The surgical procedure, the complications experienced, and safety issues are somewhat different for children than they are for adults. This report describes the modifications required in the surgical procedure and discusses the complications associated with cochlear implants in children. It then addresses issues related to the safety of the device, such as the prevention of middle ear infection. Finally, it summarizes new research conducted at the University of Melbourne and supported by the National Institutes of Health concerning skull growth, explantation/reimplantation, and sealing the electrode entry point into the cochlea.
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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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ItemThe histopathology of the human temporal bone and auditory central nervous system following cochlear implantation in a patient: correlation with psychophysics and speech perception results( 1988)Cochlear implantation has become a recognised surgical procedure for the management of a profound-total hearing loss, especially in patients who have previously had hearing before going deaf (postlingual deafness). Nevertheless, it is important for progress in the field that patients who have had a cochlear implant, bequeath their temporal bones for research. This will then make it possible to further assess the safety of the procedure, and the factors that are important for its effectiveness. Biological safety has been assessed in a number of studies on animals, in particular, the biocompatibility of the materials used (1,2), the histopathological effects of long-term implantation on the cochlea (3, 4, 5, 6, 7, 8), and the effects of chronic electrical stimulation on the viability of spiral ganglion cells (9, 10, 11, 12). In studying the temporal bones of deceased cochlear implant patients it is possible to help establish that the animal experimental results are applicable to Man. Surgical trauma has been most frequently evaluated by inserting electrodes into cadaver temporal bones. It is important, however, to examine bones that have been previously implanted surgically to ensure that the cadaver findings are applicable to operations on patients. The effectiveness of cochlear implantation can be studied by correlating the histopathological findings, the dendrite and spiral ganglion cell densities, in particular, with the psychophysical and speech perception results. Other benefits also accrue, for example, establishing the accuracy of preoperative X-rays and electrical stimulation of the promontory in predicting cochlear pathology and spiral ganglion cell numbers. For the above reasons it has been especially interesting to examine both the temporal bones and central nervous system from one of our patients (patient 13) who participated in the initial clinical trial of the Cochlear Proprietary Limited (a member of the Nucleus group) multiple-electrode cochlear prosthesis, and who died due to a myocardial infarction following coronary bypass surgery.
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ItemThe University of Melbourne/Nucleus cochlear prosthesis( 1988)This is a review of research to develop the University of Melbourne/Nucleus cochlear prosthesis for patients with a profound-total hearing loss. A more complete review can be obtained in Clark et al. A prototype receiver-stimulator and multiple-electrode array developed at the University of Melbourne was first implanted in a postlingually deaf adult patient with a profound-total hearing loss on 1 August 1978. A speech processing strategy which could help this patient understand running speech, especially when combined with lipreading was developed in 1978 following initial psychophysical studies. A prototype wearable speech processor was fabricated in 1979, that could provide significant help for the first two patients in understanding running speech when used in combination with lipreading compared with lipreading alone, and it also enabled them to understand some running speech when using electrical stimulation alone. An implantable receiver-stimulator and wearable speech processor embodying the principles of the prototype devices were then produced for clinical trial by the Australian biomedical firm, Nucleus Ltd, and its subsidiaries, Cochlear Pty Ltd and Cochlear Corporation. This cochlear implant was initially clinically trialled on six patients at The Royal Victorian Eye & Ear Hospital in 1982, and shown to give similar results to those obtained with the prototype device. In view of these findings a clinical trial was carried out for a Premarket Approval Application to the US Food and Drug Administration (FDA), and extended to a number of centres in the US, Canada, and West Germany. This clinical trial confirmed that patients could understand running speech when electrical stimulation was combined with lipreading, and that some patients could also understand running speech when using electrical stimulation alone. Today, more than 600 patients world-wide are using cochlear implants developed from the research described in this paper.
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