Graeme Clark Collection
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ItemChronic electrical stimulation of the auditory nerve at high stimulus rates: a physiological and histopathological study( 1997)A major factor associated with recent improvements in the clinical performance of cochlear implant patients has been the development of speech-processing strategies based on high stimulation rates. While these processing strategies show clear clinical advantage, we know little of their long-term safety implications. The present study was designed to evaluate the physiological and histopathological effects of long-term intracochlear electrical stimulation using these high rates. Thirteen normal-hearing adult cats were bilaterally implanted with scala tympani electrode arrays and unilaterally stimulated for periods of up to 2100 h using either two pairs of bipolar or three monopolar stimulating electrodes. Stimuli consisted of short duration (25-50 µs/phase) charge-balanced biphasic current pulses presented at 1000 pulses per second (pps) per channel for monopolar stimulation, and 2000 pps/channel for bipolar stimulation. The electrodes were shorted between current pulses to minimize any residual direct current, and the pulse trains were presented using a 50% duty cycle (500 ms on; 500 ms oft) in order to simulate speech. Both acoustic (ABR) and electrical (EABR) auditory brainstem responses were recorded periodically during the chronic stimulation program, All cochleas showed an increase in the click-evoked ABR threshold following implant surgery; however, recovery to near-normal levels occurred in approximately half of the stimulated cochleas 1 month post-operatively. The use of frequency-specific stimuli indicated that the most extensive hearing loss generally occurred in the high-frequency basal region of the cochlea (12 and 24 kHz) adjacent to the stimulating electrode. However, thresholds at lower frequencies (2, 4 and 8 kHz), appeared at near-normal levels despite long-term electrode implantation and electrical stimulation. Our longitudinal EABR results showed a statistically significant increase in threshold in nearly 40% of the chronically stimulated electrodes evaluated; however, the gradient of the EABR input/output (I/O) function (evoked potential response amplitude versus stimulus current) generally remained quite stable throughout the chronic stimulation period. Histopathological examination of the cochleas showed no statistically significant difference in ganglion cell densities between cochleas using monopolar and bipolar electrode configurations (P = 0.67), and no evidence of cochlear damage caused by high-rate electrical stimulation when compared with control cochleas. Indeed, there was no statistically significant relationship between spiral ganglion cell density and electrical stimulation (P = 0.459), or between the extent of loss of inner (IHC, P = 0.86) or outer (OHC, P=0.30) hair cells and electrical stimulation. Spiral ganglion cell loss was, however, influenced by the degree of inflammation (P=0.016) and electrode insertion trauma. These histopathological findings were consistent with the physiological data. Finally, electrode impedance, measured at completion of the chronic stimulation program, showed close correlation with the degree of tissue response adjacent to the electrode array. These results indicated that chronic intracochlear electrical stimulation, using carefully controlled charge-balanced biphasic current pulses at stimulus rates of up to 2000 pps/channel, does not appear to adversely affect residual auditory nerve elements or the cochlea in general. This study provides an important basis for the safe application of improved speech-processing strategies based on high-rate electrical stimulation.
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ItemChronic electrical stimulation of the auditory nerve at high rates: I. Effect on residual hearing [Abstract]( 1996)In addition to direct excitation of auditory nerve fibres, cochlear implant patients with small amounts of residual hearing may receive important additional auditory cues via electrophonic activation of hair cells 1. Before incorporating electrophonic hearing into speech processing strategies, the extent of hair cell survival following cochlear implantation must first be determined. We have recently demonstrated widespread survival of hair cells apical to electrode arrays implanted for periods of up to three years, the present report describes the effects of chronic electrical stimulation on hair cell survival.
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ItemChronic electrical stimulation of the auditory nerve at high rates: II. Cochlear pathophysiology [Abstract]( 1996)A major factor in the improved performance of cochlear implant patients has been the use of high stimulus rate speech processing strategies. While these strategies show clear clinical advantage, we know little of their long-term safety. Indeed, recent studies have indicated that high stimulus rates at intensities above clinical limits, can result in neural damage as a result of prolonged neuronal hyperactivity. The present study was designed to evaluate the effects of chronic electrical stimulation of the auditory nerve at high rates, using intensities within clinical limits.
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ItemChronic electrical stimulation of the auditory nerve at high stimulus rates: preliminary results( 1994)The present preliminary report describes the electrophysiological response of the cochlea during long-term stimulation. The data indicate that electrical stimulation at a rate of 1000 pulses per second does not appear to adversely affect the implanted cochlea.
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ItemCochlear implantation in young children: histological studies on head growth, leadwire design, and electrode fixation in the monkey model( 1994)For safe cochlear implantation in children under 2 years of age, the implant assembly must not adversely affect adjacent tissues or compromise head growth. Furthermore, growth changes and tissue responses should not impair the function of the device. Dummy receiver-stimulators, interconnect plugs, and leadwire-lengthening systems were implanted for periods of 36 months in the young monkey to effectively model the implantation of the young child. The results show that implanting a receiver-stimulator package has no adverse effects on skull growth or the underlying central nervous system. The system for fixing the electrode at the fossa incudis proved effective. There was marked osteoneogenesis in the mastoid cavity, resulting in the fixation of the leadwire outside the cochlea. This study provides evidence for the safety of cochlear implantation in young subjects.
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ItemPaediatric cochlear implantation: radiologic observations of skull growth( 1993)We investigated the effects of long-term implantation of auditory prostheses on skull growth in young animals. Four monkeys were implanted with dummy cochlear implants at 6 months of age. To simulate implantation in children, the bed for the receiver-stimulator or interconnecting plug was drilled across a calvarial suture down to the underlying dura. Plain skull oentgenograms were periodically taken to monitor head growth for up to 3 years after implantation. These longitudinal measurements revealed no significant asymmetric skull growth. Postmortem measurements using computed tomographic scans confirmed these results and showed no significant difference in the intracranial volumes between the implanted and control sides of each animal or between experimental and nonimplanted control monkeys. These results suggest that long-term cochlear implantation in very young children will not cause any significant deformity of the skull.
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ItemPeri-modiolar electrode arrays: a comparison of electrode position n the human temporal bone(Monduzzi Editore, 1997)This paper describes a radiologic evaluation of three types of peri-modiolar arrays, comparing their trajectory within the scala tympani with a standard Mini-22 electrode. All peri-modiolar arrays were found to lie closer to the modiolus for much of their insertion length compared with the standard array. While one design showed evidence for the potential of increased insertion trauma, two designs produced satisfactory results. Although further electrode development, temporal bone and histopathologic studies arE required, it would appear that the benefits of peri-modiolar electrode arrays will be realised clinically.
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ItemHigh rate electrical stimulation of the auditory nerve: physiological and pathological results [Abstract]( 1995)Previous experimental studies have shown that chronic electrical stimulation of the auditory nerve using charge balanced biphasic current pulses at rates of up to 500 pulses per second (pps) do not adversely affect the adjacent spiral ganglion population. More recently, a number of clinical trials have indicated that speech processing strategies based on high pulse rates (1000 pps and more), can further improve speech perception. In this paper we summarize our results following acute and chronic electrical stimulation of the auditory nerve using high pulse rates.
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ItemCochlear implants: high rate stimulation studies and the effect of electrode position [Abstract]( 1996)This paper summarizes our recent findings investigating the safety of high rate electrical stimulation, and reviews the effects of electrode position on auditory excitability. These studies used charge balanced biphasic pulses and electrode shorting between stimuli to minimize any residual charge or direct current. High rate (400-1000 pulses/s) electrical stimulation of the auditory nerve can result in significant stimulus induced reductions in auditory nerve excitability at stimulus levels well above those used clinically (1). The extent of this reduction was dependent on stimulus rate, intensity and duty cycle, implying that such changes were related to the degree of evoked activity.
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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.