Graeme Clark Collection
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ItemElectrophonically driven single unit responses of the anteroventral cochlear nucleus in cat [Abstract]( 1996)Electrical stimulation of the cochlea results in both direct and electrophonic excitation of auditory nerve fibres. It has been proposed that electrophonic stimulation results from the creation of a mechanical disturbance on the basilar membrane which has properties similar those resulting from acoustic stimuli. Auditory nerve compound action potential (CAP) forward masking studies1 show the level of frequency specific electrophonic stimulation is highly correlated with the spectral energy of the electrical stimulus waveform. The level of spectral energy in pulsatile biphasic electrical stimuli decreases toward low frequencies suggesting the level of electrophonic stimulation will be diminished in the low frequency region of the cochlea.
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ItemResults for children and adolescents using the multichannel cochlear prosthesis [Abstract]( 1992)The first adolescent to use the 22-electrode cochlear prosthesis was Implanted In Melbourne in 1985 and the first child (less than 10 years), the following year. Since then, over 100 children have received the cochlear prosthesis in Australia and over 1200 worldwide. Detailed assessment of 200 children in the U.S.A., Australia and Germany lead to the market approval of the prosthesis by the U.S. Food and Drug Administration in July 1990. The analysis of results for these children has proven to be difficult due to the use of different tests in different places, the lack of appropriate assessment tools for young children, the wide range of performance, and the problems of cooperation for young children. Despite these problems, some trends are beginning to emerge in the speech perception results for implanted children. Children with a greater amount of auditory experience before becoming profoundly deaf tend to perform better, as do children with more experience with the cochlear prosthesis. Those with a greater number of electrodes in use also perform better, a result supported by adult studies. Although older prelinguistically deafened children do not perform as well as postlinguistically deafened adults, there appears to be little difference between results for pre-and post-linguistically deafened young children. These trends In speech perception results will be discussed in more detail.
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ItemEvaluation of expandable leadwires for paediatric cochlear implants( 1993)The development of cochlear implants for use in very young children (1-2 years old) will require techniques designed to accommodate temporal bone growth. Previous anatomic studies have shown that the leadwire of a cochlear implant must be capable of expanding up to 20 mm between the round window and the implanted receiver-stimulator in response to skull growth. In the present study morphologic and biomechanical evaluation of five expandable leadwire designs was conducted following their implantation in young cats. Two helical shaped leadwire designs frequently exhibited extensive fibrous tissue adhesions and broke during long-term implantation. In contrast, thin, flexible Silastic envelopes were effective in minimizing tissue adhesions. Residual V- and Z-shaped leadwires, placed in these envelopes, showed little evidence of fibrous tissue adhesions following implantation periods of up to 2 years. Moreover, these leadwires readily expanded both during the growth of the animal and when biomechanical expansion studies performed at the completion of the implant period. These expandable leadwire designs appear to be appropriate candidates for use in pediatric cochlear implants.
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ItemEffect of chronic electrical stimulation on cochlear nucleus neuron size in normal hearing kittens( 1993)Very young cochlear-implant candidates may have undetected islands of residual hearing. Would the maturation of these functioning auditory neurons be affected by chronic cochlear stimulation? This was tested by examining neuron sizes in the cochlear nuclei of young, normal hearing kittens with and without chronic cochlear stimulation. Six animals received bilateral intra-or extracochlear implants and were electrically stimulated unilaterally for periods of 1,000-1,500 hours. After sacrifice, cross-sectional areas of approximately 11,000 neuron somata in the cochlear nuclei were measured with an image-analysis system. There were statistically significant differences between stimulated and unstimulated nuclei, especially the posteroventral cochlear nucleus (PYCN), in individual cats, but the directions of the differences were inconsistent. Overall, there was no significant effect of electrical stimulation on soma size. These results indicate that chronic electrical stimulation of the auditory nerve has no positive or negative trophic effects on otherwise innervated, maturing cochlear nucleus neurons.
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ItemThe histological and physiological effects of the auditory brainstem prosthesis of the auditory pathway(Monduzzi Editore, 1997)To rehabilitate profoundly deaf patients who cannot benefit from the cochlear implant due to bilateral interruption of the auditory nerve, particularly from neurofibromatosis II, the histological and physiological effects of an auditory brainstem prosthesis on the cochlear nuclei of guinea pigs were examined in order to establish the safety and the efficacy of this prosthesis.
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ItemThe histological and physiological effects of the auditory brainstem prosthesis of the auditory pathway [Abstract]( 1997)The cochlear implant can successfully rehabilitate the majority of profoundly deaf patients. However, some of them cannot benefit from the cochlear implant due to bilateral interruption of the auditory nerve, particularly from neurofibromatosis II. These patients can be stimulated directly with an auditory brainstem prosthesis on the cochlear nucleus. To examine the safety and the efficacy of this prosthesis, the cochlear nuclei of guinea pigs were implanted unilaterally with bipolar surface electrodes, and stimulated acutely using charge-balanced, biphasic current pulses at rates of 250, SOO or 1000 Hz and charge intensities of 1.8, 2.8, 3.5 or 7.1?C/phase/cm2.
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ItemEffects of chronic electrical stimulation on cochlear nuclear neuron size in deaf kittens [Abstract]( 1996)It is now well recognized that normal afferent innervation is necessary for the development. This study investigated the effect of chronic electrical stimulation of the auditory nerve on the maturation of cochlear nucleus soma area of the neonatally deafened kittens. Eight kittens were deafened using kanamycin and ethacrynic acid, received a stimulated intracochlear implant in the left side and a dummy implant in right side, and classified two groups - the stimulated and the control group.
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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 implant surgery [Abstract]( 1992)The operation in children is similar to that in adults, but special care needs to be taken with the skin flap, the anchoring of the electrode array and the sealing of .the cochleostomy. Research into the effects of head growth and otitis media in an implanted ear indicates that these should not be a problem. Surgical complications are also similar to those in adults, with the most common being related to the skin flap. The major complication rate at 4 % is slightly less than that in adults and most of these get effective implant function.
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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.