Graeme Clark Collection

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    Chronic electrical stimulation of the auditory nerve at high stimulus rates: a physiological and histopathological study
    XU, JIN ; Shepherd, Robert K. ; Millard, Rodney E. ; Clark, Graeme M. ( 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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    Chronic electrical stimulation of the auditory nerve using non-charge-balanced stimuli
    Shepherd, Robert K. ; Linahan, N. ; Xu, J. ; Clark, Graeme M. ; Araki, S. ( 1999)
    This study was designed to evaluate the pathophysiological response of the cochlea following long-term intracochlear electrical stimulation using a poorly charge-balanced stimulus regime, leading to direct current (DC) levels >0.1 µA. Four normal-hearing adult cats were bilaterally implanted with scala tympani electrode arrays and unilaterally stimulated for periods up to 2200 h. Stimuli consisted of 50 µs monophasic current pulses presented at 2000 pulses per second (pps) per channel, and resulted in DC levels of 0.4-2.8 µA. Both acoustic and electrical (EABR) evoked potentials were periodically recorded during the stimulation program. Frequency-specific stimuli indicated that an extensive and widespread hearing loss occurred over the 4-24 KHz region in all stimulated cochleae, although the 2 KHz region exhibited thresholds close to normal in some animals, despite long-term implantation and chronic stimulation. Longitudinal EABRs showed a statistically significant increase in threshold for three of the four animals. Histopathological evaluation of the cochleae revealed a highly significant reduction in ganglion cell density in stimulated cochleae compared with their controls. Spiral ganglion cell loss was significantly correlated with the degree of inflammation, duration of electrical stimulation, and the level of DC. In conclusion, the present study highlights the potential for neural damage following stimulation using poorly charge-balanced stimuli.
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    Improved and simplified methods for specifying positions of the electrode bands of a cochlear implant array
    Cohen, Lawrence T. ; XU, JIN ; Xu, Shi Ang ; Clark, Graeme M. ( 1996)
    Objective: To develop techniques for measuring the positions of the individual electrodes of a multiple channel cochlear implant and for estimating associated characteristic frequencies. Background: Information concerning the positions of the individual electrodes of a cochlear implant array is important for analyzing speech perception or psychophysical data and for optimizing speech-processing strategies. This study presents two techniques for obtaining such information from postoperative plain film radiographs. Methods: A template spiral shape, derived from analysis of the radiographs of 30 cochlear implant patients, is used to obtain measurements of the angular positions of the electrode bands within scala tympani. A research technique measures angular positions and estimates characteristic frequencies for all electrode bands but requires that the positions of two cochlear landmarks and all electrode bands be digitized. A clinical technique provides similar angle and frequency estimates but requires a minimum of information to be extracted visually from the radiograph. The lateral positions of the bands are estimated, in the research technique, using mean outer and inner wall functions obtained from 11 Silastic molds of scala tympani. Results: The mean position of the implanted array relative to the mean scala tympani outer wall function was consistent with published histologic observations of implanted temporal bones. Measured angles did not vary greatly with experimenter or with rotation of the cochlea relative to the radiographic beam by up to 20°. Conclusions: The techniques described allow, principally, measurement of the longitudinal positions of the bands of a cochlear implant in scala tympani and estimation of corresponding characteristic frequencies.
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    Precurved electrode array and insertion tool
    Treaba, C. G. ; Xu, J. ; Xu, S-A. ; Clark, Graeme M. ( 1995)
    Future improvements in the performance of patients using cochlear implants largely depend on improving the electrode array design. In particular, it is necessary to produce an array that would lie in the desired portion of the scala tympani while minimizing insertion trauma to the cochlea.
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    Investigations on a curved intracochlear array
    Donnelly, M. J. ; Cohen, L. T. ; Xu, J. ; Xu, S-A. ; Clark, Graeme M. ( 1995)
    The electrode array of a multiple-channel cochlear implant lies against the outer wall of the scala tympani. From this position electrical current spreads to excite residual neural elements, particularly spiral ganglion cells within the modiolus. It is not clear whether the spread of current from the outer wall is optimal for multiple-channel speech processing, but placement closer to the target nerves could result in lower thresholds. This could have benefits through the use of shorter pulse durations and extended battery life. Computer modeling studies and animal experiments have suggested that for localized current the optimal electrode position is adjacent to the modiolus. At the University of Melbourne it was felt that an electrode with a curve matching the internal cochlear spiral would remain close to the modiolus after insertion. A curved electrode was developed and an inserting tool was designed and produced (Treaba et al, this suppl, this section). Preliminary studies suggested that the electrode array did indeed remain close to the modiolus. Before further development of this type of electrode design, it was necessary to determine whether modifications to the surgical technique for its insertion were required. It was also important to ensure that the curved electrode fabricated for clinical trial would lie closer to the modiolus than to the outer wall of the scala tympani. This study was undertaken to examine these issues.
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    Cochlear implantation in young children: histological studies on head growth, leadwire design, and electrode fixation in the monkey model
    Burton, M. J. ; Shepherd, R. K. ; Xu, S. A. ; Xu, J. ; Franz, B. K-H. G. ; Clark, Graeme M. ( 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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    Comparison of half-band and full-band electrodes for intracochlear electrical stimulation
    Xu, Shi-Ang ; McAnally, Ken I. ; Xu, Jin. ; Clark, Graeme M. ( 1993)
    It was hypothesized that intracochlear stimulating electrodes oriented toward the modiolus would require a lower stimulus current to elicit a threshold neural response than longitudinal band electrodes. Electrically evoked auditory brain stem responses (EABRs) and electrode impedances were recorded with full-band and oriented half-band scala tympani electrodes in anesthetized, deafened cats. To elicit a threshold EABR, the stimulus current required for stimulation through half-band electrodes oriented toward the modiolus was not significantly different from the current required for stimulation through full-band electrodes. The impedances of full-band electrodes were significantly lower than those of half-band electrodes. Considering the significantly higher impedance and current density of half band electrodes in comparison to full-band electrodes, as well as the critical orientation of half-band electrodes during implantation, we believe that full-band electrodes have advantages over half-band electrodes for scala tympani implantation.
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    Paediatric cochlear implantation: radiologic observations of skull growth
    XU, JIN ; Shepherd, Robert K. ; Xu, Shi-Ang ; Seldon, H. Lee ; Clark, Graeme M. ( 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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    Radiologic evaluation of multichannel intracochlear implant insertion depth
    Marsh, Michael A. ; XU, JIN ; Blamey, Peter J. ; Whitford, Lesley A. ; Xu, Shi-Ang ; Silverman, Julianna M. ; Clark, Graeme, M. ( 1993)
    Postoperative plain film x-rays are necessary in all multichannel cochlear implant patients to confirm intracochlear position, detect possible electrode kinking, and provide a reference if postoperative slippage occurs. In addition, precise documentation of multichannel intracochlear electrode insertion depths is necessary for comparison of speech recognition results among patients and may be of use for future speech processing strategies. In the present study, a method has been devised, using a modified Stenver's view, to more accurately document insertion depths of the electrode array and location of individual electrodes on 50 multichannel cochlear implant patients. Surgical estimates of insertion depth are shown to have great variability in regard to distance along the basilar membrane when compared with x-ray documentation. Additionally, there is preliminary evidence that insertion depth, as determined by x-ray studies, has a strong correlation with open-set speech discrimination.