Graeme Clark Collection

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http://hdl.handle.net/11343/375

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Author: Clark, Graeme × Author: Shepherd, Robert × Author: Xu, Jin × End date: 1999 × Start date: 1990 × Type: Conference Item × Subject: EABR × Subject: otolaryngology ×

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    Physiological and histopathological effects of chronic monopolar stimulation on the auditory nerve using very high stimulus rates [Abstract]
    Linahan, N. ; Tykocinski, R. K. ; Shepherd, Robert K. ; Clark, Graeme M. ( 1999)
    Speech-processing strategies using high stimulus rates are used in some cochlear implant systems. While some data suggests that electrical stimulation of the auditory nerve at rates of 2000 pps per channel is safe, there is little data concerning higher rates. The present study was designed to evaluate the safety of a rate of 5000 pps per channel. Under anaesthesia, (ketamine (20 mg/kg. i.v.) and xylazine (3.8 mg/kg. i.v.)), four normal hearing cats were bilaterally implanted with a three channel platinum (Pt) scala tympani electrode array and a return Pt-electrode placed within the temporalis muscle. Each animal was stimulated unilaterally for durations of up to 2700 h using 25μגs per phase charge-balanced biphasic current pulses. The stimuli were delivered at 5000 pps per channel at mid-dynamic range intensities. Acoustically-evoked auditory brainstem responses (ABRs) were recorded during the stimulation regime to monitor the animals' residual hearing. Electrically-evoked auditory brainstem responses (EABRs) were periodically recorded to monitor the status of the auditory nerve and to ensure stimulus intensity remained above threshold. ABRs typically showed poor recovery in the stimulated ear. Longitudinal EABRs recorded from all animals remained relatively stable for the duration of stimulation. Electrode impedances were calculated from daily monitoring of current and voltage waveforms. Two animals that exhibited the highest electrode impedance throughout the duration of stimulation were found to have significant amounts of new bone growth and fibrous tissue in the basal region of the cochlea. However, as one of these animals showed a similar response in the contralateral, unstimulated, implanted cochlea, this response can not be attributed to electrical stimulation per se. There was no statistically significant difference in spiral ganglion cell density in the stimulated cochleae when compared to corresponding regions in controls (p?0.2, Mann-Whitney Rank. Sum Test). These initial results indicate that chronic monopolar stimulation of the cochlea at a rate of 5000 pps per channel does not have an adverse effect on spiral ganglion cell density.
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    Chronic electrical stimulation of the auditory nerve using non-charge balanced stimuli [Abstract]
    Linahan, N. ; Shepherd, Robert K. ; Xu, J. ; Araki, S. ; Clark, Graeme M. ( 1998)
    Cochear implants use charge balanced biphasic current pulses and electrode shorting between current pulses to minimise potentially damaging direct current (DC). In the present study we evaluated the effectiveness of the electrode shorting technique using a non-charge balanced stimulus regime. Under general anaesthesia (ketamine (20 mg/kg. i.m.) and xylazine (3.8 mg/kg. i.m)), eight normal hearing cats were bilaterally implanted with two channel platinum scala tympani electrodes. Each animal was stimulated unilaterally for 500 to 2200 h using 50 μs monophasic current pulses. The stimuli were delivered at rates of 500 or 2000 pulses per channel continuously at mid-dynamic range intensities. Electrically-evoked auditory brainstem responses (EABR) were periodically recorded to monitor the status of the auditory nerve and to ensure stimulus intensity remained above threshold. At a stimulus rate of 500 pulses/s, electrode shorting effectively reduced DC levels to ≤ 0.3µA. Longitudinal EABR's recorded from these animals , remained relatively stable over the stimulus duration. These cochleae showed minimal tissue response and there was no statistically significant difference in spiral ganglion cell density when compared with controls (p=0.21, Mann-Whitney U-test). Chronic stimulation at 2000 pulses/s resulted in increased DC levels (0.6-2.8µA). These cochleae exhibited a highly significant reduction in spiral ganglion cell density when compared with controls (p<0.0001), and their EABR's typically displayed an elevation in threshold as a function of stimulus duration. The present findings indicate that continuous non-charge balanced stimuli at rates of 2000 pulses/s can result in significant loss of spiral ganglion cells, presumably as a result of increased DC levels.