Graeme Clark Collection
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Item"Cochlear View" and its application in cochlear implant patients [Abstract]( 1994)Recent advances in multichannel intracochlear implantation have generated interests in correlating individual stimulating electrodes to pitch perception. An appropriate radiographic technique is required to precisely document the location of the implanted intracochlear electrode array. Anatomical studies, including the measurements of the temporal bone using high-resolution CT films and 3D reconstruction from the petrous bone sections, were conducted to define the spatial position of cochlea in the skull. Thus, a "Cochlear View" was designed and introduced for postoperative radiological evaluation of multichannel intracochlear implantation. In this paper, a detailed radiographic method and radiological interpretation of the "Cochlear View" are described. A plain radiograph of the "Cochlear View" was taken of 120 patients who had received the Nucleus multichannel implant. Studies have shown that a plain radiograph of the "Cochlear View" provides sufficient information to correctly evaluate the results of implantation, including the insertion depth and position of individual electrodes. It plays an important role in guiding the management of frequency mapping and acts as a useful reference for further research purposes.
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ItemDecrement in auditory nerve function following acute high rate stimulation in guinea pigs [Abstract]( 1995)Cochlear implants have been shown to successfully provide profoundly deaf patients with auditory cues for speech discrimination. Psychophysical studies suggested that speech processing strategies based on stimulus rates of up to 1000 pulses per second (pps) may lead to an improvement in speech perception, due to a better representation of the rapid variations in the amplitude of speech. However, "neural fatigue" has been known to occur following brief periods of electrical stimulation at rates high enough to ensure that stimuli occur within the neurons relative refractory period, and has been shown to depend on stimulus duration and rate of the evoked neural activity. Prolonged electrical stimulation at these high stimulus rates could, therefore, have an adverse effect on the neurons metabolism and result in cellular energy depletion.
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ItemExpandable leadwires for a paediatric cochlear implant [Abstract]( 1993)Anatomic studies of skull growth have shown an increase (about 20 mm) in the distance between the round window and the asterion where the receiver-stimulator is usually located. In order to accommodate the skull growth of young patients, an expandable leadwire connecting the receiver-stimulator and the electrode array is necessary. Several expandable leadwires were evaluated in experimental animals, including helical leadwires protected by Silastic tubes and leadwires, with "V" or "W"-shaped levels in a single phase, and protected by thin Silastic or Teflon bags. The leadwires together with their controls were implanted on young animal's scapulae, temporal and parietal bones and in subcutaneous tissue. The in vivo expansion of the leadwire was monitored by periodic x-ray examination and the force to expand the leadwire was measured at the completion of implantation. The results showed that helical leadwires weresurrounded by fibrous tissue and a large force was required to expand them. The V or W-shaped leadwires were able to expand up to 20 mm in vivo and only a moderate force was required to expand them. For most of the cases, there was none or little fibrous tissue in Silastic or Teflon bags. The results indicated that for a paediatric cochlear implant, leadwires with V or W-shaped levels could, expand and biocompatible envelopes could effectively protect the leadwires from being bound by fibrous tissue.
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ItemPaediatric cochlear implantation: radiological and histopathological studies of skull growth in the monkey( 1993)The human skull undergoes significant growth within the first two years of life (Dahm et aI, 1992). Therefore, before children under two can be considered candidates for cochlear implantation, the effects of the surgical procedure on subsequent skull growth must be well understood. To evaluate the effects of implantation on skull growth four macaque monkeys were implanted with dummy cochlear implants at six months of age. To model the procedure in the very young child, the bed for the receiver-stimulator was drilled across a calvarial suture down to the underlying dura and an electrode array inserted into the scala tympani via a mastoidectomy and posterior,tympanotomy. Plain skull radiographs were perioqical1y taken to monitor skull growth for periods of up to three years following implantation. Their longitudinal measurements revealed no evidence of asymmetrical skull growth when compared with unimplanted control animals. Computer tomographic scans taken at post-mortem confirmed these findings. Finally, subsequent histopathological evaluation of the receiver-stimulator package bed indicated that it becomes obliterated by hard tissue, resulting in a localized flattening of the vault under the receiver-stimulator. However, this tissue exhibited histological evidence of sutures, indicating that the surgical procedure should not lead to premature sutural closure. In conclusion, the present experimental results suggest that long-term cochlear implantation in very young children will not lead to any significant skull deformity.
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ItemInvestigation of curved intracochlear electrode arrays [Abstract]( 1992)It has been demonstrated that the Melbourne/Cochlear multi-channel cochlear implant is safe and effective for use in profoundly-totally deaf patients. Recent studies have highlighted the importance of deaf insertion and placing the electrodes closer to the spiral ganglion neurons. In order to improve the electrode insertion depth and proximity to the modiolus, we have investigated curved electrode arrays. Prototypes of such arrays and their accessory inserter have been made. Trial insertions were performed on skeletonized cochleae of human temporal bones. The preliminary results showed that, when compared with conventional straight electrode arrays, the curved arrays could be inserted deeper and located closer to the modiolus. These findings indicate that the curved --.~ electrodes currently under investigation should result in a reduction in stimulus threshold and improve pitch perception and may also result in the use of more channels of stimulation.
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ItemElectrical stimulation of the auditory nerve in deaf kittens: effects on the spiral ganglion [Abstract]( 1992)Cochlear pathology following the administration of ototoxic drugs results in a widespread and rapid loss of sensory hair cells followed by a gradual degeneration of auditory nerve fibres and their cell bodies, the spiral ganglion. Recently, two studies have described increased spiral ganglion cell survival in the cochleas of deafened animals following chronic electrical stimulation of the auditory nerve (Hartshorn et al., 1991; Leake et al., ]991). If electrical stimulation is shown to have a trophic effect on degenerating auditory nerve fibres, these findings will significantly influence the preoperative management of cochlear implant patients. The aim of the present study was to corroborate these earlier reports and to evaluate the general tissue response of deafened cochleae in young animals following chronic electrical stimulation.
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ItemMultichannel cochlear implants in children: an overview of experimental and clinical results at the University of Melbourne [Opening Lecture]( 1992)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 The University of Melbourne in association with Cochlear Pty Ltd, electrically stimulates selective regions of the auditory nerve using an array of 22 platinum (Pt) electrodes located in the scala tympani. Its development followed basic experimental studies and the development and evaluation of a prototype device in the 1970's. Following safety studies and a successful clinical trial, the Melbourne/Cochlear multichannel implant was approved for use in adults by the United States Food and Drug Administration (FDA) in 1985. More than 3000 patients throughout the world have since been implanted with this device, many being able to understand a significant amount of unfamiliar, connected speech without lipreading Following miniaturization of the implant, it became suitable for use with children. In 1990, after additional biological safety and clinical investigations, the FDA approved the use of the Melbourne/Cochlear multichannel implant for profoundly deaf children above the age of two years. And in 1991, the device received the medical device implantation approval certificate from the Japanese Government. The present paper presents an overview of our recent biological safety studies and clinical experience in children, and discusses the likely future development of these devices.
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ItemCochlear implantation in young children: long-term effects of implantation on normal hair cells and spiral ganglion cells in the monkey model [Abstract]( 1992)Recent independent results obtained by profoundly deaf children implanted with the Melbourne 22-channel cochlear implant (1) have provided further impetus. for examining thefeasibility of implanting children under two and children with profound deafness. Safety st1,ldies, in appropriate animal models, must first establish the safety of this procedure.
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ItemClinical results for children using the 22-channel cochlear prosthesis [Abstract]( 1991)Twenty five profoundly/totally hearing-impaired children aged between 2 and 18 years have been implanted with the 22-channel cochlear prosthesis (Cochlear Pty. Ltd.) at The University of Melbourne over the last five years. Speech perception, speech production and language development have been monitored for these children both pre- and postoperatively. Results have shown improvements for all children in speech perception, postoperatively. The younger children (< 12 years) and those with an acquired (postlinguistic) profound hearing loss, have demonstrated some open-set speech recognition without visual clues. In general, older children with a prelinguistic profound hearing loss have not attained this level of performance. Improvements in speech production and language have also been demonstrated and tend to occur faster in younger children. Importantly, a number of congenitally deaf young children have shown significant improvements in speech perception, production and language. Important factors in the success of cochlear implants in children appear to be: educational environment - it is vital to have a strong auditory component (i.e. non-signing); careful attention to correct programming and maintenance of the prosthesis; support of family and other professionals.