Graeme Clark Collection
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ItemCochlear pathology following chronic electrical stimulation of the auditory nerve. I: Normal hearing kittens( 1992)The present study examines the histopathological effects of long-term intracochlear electrical stimulation in young normal hearing animals. Eight-week old kittens were implanted with scala tympani electrode arrays and stimulated for periods of up to 1500 h using charge balanced biphasic current pulses at charge densities in the range 21-52 µC cm^-2 geom. per phase. Both click and electrically evoked auditory brainstem responses were periodically recorded to monitor the status of the hair cell and spiral ganglion cell populations. In addition, the impedance of the stimulating electrodes was measured daily to monitor their electrical characteristics during chronic implantation. Histopathological examination of the cochleas showed no evidence of stimulus induced damage to cochlear structures when compared with implanted, unstimulated control cochleas. Indeed, there was no statistically significant difference in the ganglion cell density adjacent to the stimulating electrodes when compared with a similar population in implanted control cochleas. In addition, hair cell loss, which was restricted to regions adjacent to the electrode array, was not influenced by the degree of electrical stimulation. These histopathological findings were consistent with the evoked potential recordings. Finally, electrode impedance data correlated well with the degree of tissue growth observed within the scala tympani. The present findings indicate that the young mammalian cochlea is no more susceptible to cochlear pathology following chronic implantation and electrical stimulation than is the adult.
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ItemA physiological investigation of chronic electrical stimulation with scala tympani electrodes in kittens( 1992)A physiological investigation of cochlear electrical stimulation was undertaken in six two-month-old kittens. The scala tympani electrodes were implanted and electrically stimulated using biphasic balanced electrical pulses' for periods of 1000-1500h in four ears. Four ears received implants for same period but without electrical stimulation. The other two ears served as normal control. The results indicated: 1) Chronic electrical stimulation of the cochlea within electrochemically safe limits did not influence the hearing of kittens and the normal delivery of impulses evoked by acoustic and electrical signals on the auditory brainstem pathway. 2) The wave shapes of EABRs were similar to those of ABRs. The aptitudes of EABRs showed a significant increase following chronic electrical stimulation, resulting in a leftward shift in the input/ output function. The absolute latencies and interwave latencies of waves II-III , III -IV and II -IV were significantly shorter than those of ABRs. These results imply that there was no adverse effect of chronic electrical stimulation on the maturing auditory systems of kittens using these electrical parameters and the mechanism of electrical hearing should be further studied.
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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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ItemCochleotopic selectivity of a multichannel scala tympani electrode array using the 2-deoxyglucose technique( 1992)The 2-deoxyglucose (2-DG) technique was used to study the cochleotopic selectivity of a multichannel scala tympani electrode array in four cats with another acting as an unstimulated control. Each animal was unilaterally deafened and a multichannel electrode array inserted 6 mm into the scala tympani. Thresholds to electrical stimulation were determined by recording electrically evoked auditory brainstem responses (EABRs). Each animal was injected with 2-DG, and electrically stimulated using bipolar electrodes located either distal or proximal to the round window. The contralateral- ear was stimulated with acoustic tone pips at frequencies that matched the electrode place. Stimulation of both distal and proximal bipolar electrodes at 3X EABR threshold, evoked localized 2-DG labelling in both ipsilateral cochlear nucleus (CN) and the contralateral inferior colliculus (IC), which was very similar in orientation and breadth to labelling evoked by the contralateral tone pips. The cochleotopic position of labelling to proximal stimulation was located in the 24-26 kHz region of each structure, whereas the distal labelling was located around 12 kHz. Distal stimulation at 10 X EABR threshold produced very broad 2-DG labelling in IC centered around the 12 kHz place. The present 2-DG results clearly illustrate cochleotopic selectivity using multichannel bipolar scala tympani electrodes. The extent of this selectivity is dependent on electrical stimulus levels. The 2-DG technique has great potential in evaluating the efficacy of new electrode array designs.
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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.