Graeme Clark Collection
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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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ItemEvoked responses in the auditory cortex of the congenitally deaf white cat following electrical stimulation of the cochlea [Abstract]( 1995)Knowledge of the functional status of central auditory structures is important when estimating possible benefits of cochlear implantation in the congenitally deaf. Thus far the performance of prelingually deaf adults following cochlear implantation has been disappointing. We have used congenitally deaf cats as a model for prelingual deafness. These animals are deaf from an early age as shown by longitudinal recordings of auditory brainstem responses. They were studied as adults (age 2 years). Under general anaesthesia the cochleae were electrically stimulated using the NUCLEUS-22 banded scala tympani electrode array. Recordings were made from the contralateral auditory cortex and inferior colliculus. Gross potentials, together with multi� and single-unit activities were recorded. Here we confine ourselves to gross potential recordings from the auditory cortex. The skull was opened over the auditory area and the cortex photographed. A computer-controlled 3-axes microdrive provided precise and reproducible positioning of the monopolar recording electrode. Gross potentials were evoked by electrical stimulation of the auditory nerve using bipolar electrodes 1/2, 7/8 or 1/8 (electrode 1 being the most apical). These potentials were recorded from both the cortical surface and at depths of up to 4 mm, amplified and band pass filtered (10 Hz to 10 kHz). The stimuli (0.2 ms biphasic pulses) evoked middle latency responses (10 - 20 ms) over the primary and secondary auditory areas. Thresholds were lowest using electrodes 1/8 (-24 dB re. 1 mApp). Narrower electrode configurations (1/2 and 7/8) were up to 15 dB less effective. The potentials evoked were mono-, bi- or triphasic in shape, depending on recording site. We observed little evidence of tonotopic cortical mapping of stimulation site (1/2 vs. 7/8). If present at all, potentials were considerably smaller when the recording electrode was placed outside the auditory areas. Moreover, threshold currents were far higher (40 dB). It is concluded that the auditory cortex of congenitally deaf animals receives specific information via stimulation of the auditory nerve.
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ItemCochlear implantation in children: the risk of pneumococcal otitis media [Abstract]( 1992)Pneumococcal otitis media is most frequent in young children and is a matter of concern in cochlear implantation. In the course of the 'implantation surgery the physiological barrier between the middle ear and inner ear is broken down by incising the round window membrane or by fenestration of the cochlear wall. It is feared that the insertion of an electrode array into the scala tympani could provide a pathway for microorganisms and toxins to enter the cochlea, resulting in labyrinthitis. To assess the actual risk of, secondary inner ear infection post implantation we developed a cat animal model of otitis media. In addition we examined the, effectiveness of different sealing strategies compared to the alternative of leaving the electrode entry point unprotected. For sealing of the cochlea fibrous tissue or gelatine foam was wrapped around the electrode in the round window niche. 22 kittens (44 ears) were used for this study 32, ears were implanted at 2 months of age, and all 44 ears were inoculated after 2 months with a broth of, streptococcus pneumoniae and the animals sacrificed one week later. The bullae of the animals were swabbed and the cochleas processed and examined under light microscopy. Histological analysis of the cochleas showed ,the� highest incidence (45%) of labyrinthine spread of infection in the unimplanted control group. Suppurative or serous labyrinthitis was found in only one third (33.3%) of the implanted and unsealed cochleas. In contrast only one of 16 sealed cochleas, (6.2 %) showed labyrinthine signs of acute inflammation. Experimental pneumococcal otitis media could be reliably established in all animals and proved to be a valuable animal model for the testing of the intracochlear spread of infection. We conclude that a cochlear implant electrode inserted via the round window does not increase the risk of tympanogenic abyrinthitis. Our results indicate that grafting of the electrode entry point results in significant protection of the inner ear against labyrinthine spread of ototis media along the leadwire of the cochlear implant.
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ItemCochlear implantation in young children: studies on head growth, leadwire design and electrode fixation in the monkey model [Abstract]( 1992)For the safety of cochlear implantation in children under two, the implant assembly must not adversely effect the tissue of compromise head growth. Furthermore, growth changes and tissue responses should not impair functioning of the device. Dummy receiver-stimulators, interconnect plugs and leadwire-lengthening systems have been implanted for periods of 40 months in the young monkey to most effectively model the implantation of the young human child. The results show that implanting a receiver-stimulator package has no effect on skull growth or brain tissue under the package. The system for fixing the electrode at the fossa includes proved effective. There was marked osteoneogenesis in the mastoid cavity and this also resulted in fixation of the leadwire outside the cochlea. This study provides evidence for the safety of cochlear implantation in young children.
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ItemCochlear implantation in young children: long-term effects of implantation on the skull and underlying central nervous system tissues in a primate model [Abstract]( 1992)Recent independent studies reporting results obtained by profoundly deaf children implanted with the Melbourne 22-channel cochlear implant have provided further impetus for assessing the feasibility of implanting children under two. Studies in appropriate animal models must first establish the safety of this procedure.