Graeme Clark Collection
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ItemMeningitis after cochlear implantation: the risk is low, and preventive measures can reduce this further( 2007)Since the 1980s, more than 80 000 people have received cochlear implants worldwide. These implants are designed to enable people who are severely or profoundly deaf to experience sound and speech. Since 1990, implantation has become standard treatment for people who cannot communicate effectively despite well fitted hearing aids. Children who are deaf when they are born can perceive sound and learn to speak if they receive cochlear implants at a young age (ideally under 18 months). The use of cochlear implants has been thought to be safe. But since 2002 the number of patients with meningitis related to cochlear implantation has increased worldwide. Mortality and neurological complications after meningitis are high. We need to investigate the reasons for this and look at measures to reduce them.
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ItemThreshold shift: effects of cochlear implantation on the risk of pneumococcal meningitis( 2007)Unavailable due to copyright.
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ItemEffects of inner ear trauma on the risk of pneumococcal meningitis( 2007)Objective: To examine the risk of pneumococcal meningitis in healthy rats that received a severe surgical trauma to the modiolus and osseous spiral lamina or the standard insertion technique for acute cochlear implantation. Design: Interventional animal studies. Subjects: Fifty-four otologically normal adult Hooded- Wistar rats. Interventions: Fifty-four rats (18 of which received a cochleostomy alone; 18, a cochleostomy and acute cochlear implantation using standard surgical techniques; and 18, a cochleostomy followed by severe inner ear trauma) were infected 4 weeks after surgery with Streptococcus pneumoniae via 3 different routes (hematogenous, middle ear, and inner ear) to represent all potential routes of bacterial infection from the upper respiratory tract to the meninges in cochlear implant recipients with meningitis. Results: Severe trauma to the osseous spiral lamina and modiolus increased the risk of pneumococcal meningitis when the bacteria were given via the middle or inner ear (Fisher exact test, P<.05). However, the risk of meningitis did not change when the bacteria were given via the hematogenous route. Acute electrode insertion did not alter the risk of subsequent pneumococcal meningitis for any route of infection. Conclusions: Severe inner ear surgical trauma to the osseous spiral lamina and modiolus can increase the risk of pneumococcal meningitis. Therefore, every effort should be made to ensure that cochlear implant design and insertion technique cause minimal trauma to the bony structures of the inner ear to reduce the risk of pneumococcalmeningitis.
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ItemAssessment of the protective effect of pneumococcal vaccination in preventing meningitis after cochlear implantation( 2007)Objectives: To examine if a 23-valent pneumococcal capsular polysaccharide vaccine (PPV23) reduces the risk of meningitis in healthy rats after cochlear implantation. Design: Interventional animal study. Interventions: Thirty-six rats (18 immunized and 18 unimmunized) received cochlear implantations and were then infected with Streptococcus pneumoniae via 3 different routes (hematogenous, middle ear, and inner ear) in numbers sufficient to induce meningitis. Results: The rats with implants that received PPV23 were protected from meningitis when the bacteria were delivered via the hematogenous and middle-ear routes (Fisher exact test P<.05). However, the protective effect of the vaccine in the rats with implants was only moderate when the bacteria were inoculated directly into the inner ear. Conclusions: Our animal model clearly demonstrates that immunization can protect healthy rats with a cochlear implant from meningitis caused by a vaccine-covered serotype. This finding supports the notion that all current and future implant recipients should be vaccinated against S pneumoniae.
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ItemHistopathology of the binaural cochlear implant subject [Abstract]( 2001)Binaural hearing improves speech reception in noise, and is necessary for sound localisation. Normal hearing subjects use both interaural time, and intensity, differences to localise sound. This study investigates why sound localisation in bilateral cochlear implantees is insensitive to interaural time differences (Hoesel 1993). We looked for evidence of neural degeneration in the auditory brainstem involved in binaural sound localisation, since this may have degraded the neural circuitry required to accurately code interaural time delays. Method: The brainstem of a bilateral cochlear implantee was prepared for light microscopy by embedding it in paraffin, sectioning at 10 mm and staining sections with thionine or Luxol fast blue (LFB). The histological sections were digitised with NIH Image and 3-dimensional reconstructions made of the cochlear nucleus (CN) and superior olivary complex (SOC) with AnalysePC. Within the CN and the SOC, cell number and size were estimated by the physical dissector technique following thionine staining, and myelination of the nerve fibres was estimated using the optical density method following LFB staining. Results: A reduction in cell size (from thionine staining) and myelination (from LFB staining) was seen in both the CN and the SOC. Conclusions: These finding are consistent with neural degeneration within the auditory pathways. This may have lead to a degradation of the neural circuitry required to accurately detect interaural time delays.
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ItemThe effects of stochastic neural activity in a model predicting intensity perception with cochlear implants: low-rate stimulation( 1999)Most models of auditory nerve response to electrical stimulation are deterministic, despite significant physiological evidence for stochastic activity. Furthermore, psychophysical models and analyses of physiological data using deterministic descriptions do not accurately predict many psychophysical phenomena. In this paper we investigate whether inclusion of stochastic activity in neural models improves such predictions. To avoid the complication of interpulse interactions and to enable the use of a simpler and faster auditory nerve model we restrict our investigation to single pulses and low-rate (<200 pulses/s) pulse trains. We apply signal detection theory to produce direct predictions of behavioural threshold, dynamic range and intensity difference limen. Specifically, we investigate threshold versus pulse duration (the strength-duration characteristics), threshold and uncomfortable loudness (and the corresponding dynamic range) versus phase duration, the effects of electrode configuration on dynamic range and on strength-duration, threshold versus number of pulses (the temporal-integration characteristics), intensity difference limen as a function of loudness, and the effects of neural survival on these measures. For all psychophysical measures investigated, the inclusion of stochastic activity in the auditory nerve model was found to produce more accurate predictions.
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ItemA stochastic model of the electrically stimulated auditory nerve: pulse-train response( 1999)The single-pulse model of the companion paper [1] is extended to describe responses to pulse trains by introducing a phenomenological refractory mechanism. Comparisons with physiological data from cat auditory nerve fibers are made for pulse rates between 100 and 800 pulses/s. First, it is shown that both the shape and slope of mean discharge rate curves are better predicted by the stochastic model than by the deterministic model. Second, while interpulse effects such as refractory effects do indeed increase the dynamic range at higher pulse rates, both the physiological data and the model indicate that much of the dynamic range for pulse-train stimuli is due to stochastic activity. Third, it is shown that the stochastic model is able to predict the general magnitude and behavior of variance in discharge rate as a function of pulse rate, while the deterministic model predicts no variance at all.
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ItemA stochastic model of the electrically stimulated auditory nerve: single-pulse response( 1999)Most models of neural response to electrical stimulation, such as the Hodgkin-Huxley equations, are deterministic, despite significant physiological evidence for the existence of stochastic activity. For instance, the range of discharge probabilities measured in response to single electrical pulses cannot be explained at all by deterministic models. Furthermore, there is growing evidence that the stochastic component of auditory nerve response to electrical stimulation may be fundamental to functionally significant physiological and psychophysical phenomena. In this paper we present a simple and computationally efficient stochastic model of single-fiber response to single biphasic electrical pulses, based on a deterministic threshold model of action potential generation. Comparisons with physiological data from cat auditory nerve fibers are made, and it is shown that the stochastic model predicts discharge probabilities measured in response to single biphasic pulses more accurately than does the equivalent deterministic model. In addition, physiological data show an increase in stochastic activity with increasing pulse width of anodic/cathodic biphasic pulses, a phenomenon not present for monophasic stimuli. These and other data from the auditory nerve are then used to develop a population model of the total auditory nerve, where each fiber is described by the single-fiber model.
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ItemResponses from single units in the dorsal cochlear nucleus to electrical stimulation of the cochlea(Karger, 1993)An aim of the electrical stimulation strategy of a cochlear implant is to mimic the response of the auditory system to acoustic stimuli, so that hearing sensations generated by the implant can be recognisable and useful to the implantee. To help improve our understanding of how the brain responds to electrical stimulation of the auditory nerve we have examined the responses of dorsal cochlear nucleus (DCN) units to both acoustic and electrical stimulation of the cochlea in a hearing animal. This work extended our previous studies which have compared the responses to electrical and acoustic stimulation in the auditory nerve [1] and the ventral cochlear nucleus [2]. Our studies addressed two questions: (1) What are the responses of DCN units to electrical stimulation of the auditory nerve? (2) Was it possible to identify acoustic and electrical stimuli which generated similar responses from individual DCN units? By answering questions 1 and 2, it may be possible to deduce the electrical stimulus parameters which should be employed in cochlear implant speech processing strategies to mimic acoustic-like responses from neurons of the dorsal cochlear nucleus. The generality of observations from the cochlear nucleus could then be tested at other nuclei within the central auditory pathways.
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ItemCurrent distributions in the cat cochlea: a modelling and electrophysiological study( 1985)The current distribution of bipolar electrodes implanted into the scala tympani of the cat cochlea was investigated using a two-electrode masking technique. Two electrode masking is a non-Invasive technique which requires two electrically independent electrodes and relies upon the forward masking of the electrically evoked brainstem response to a probe stimulus by that of a preceding test stimulus. The technique was described in terms of a model, which enabled an approach for estimating the scala tympani length constant to he established. Model results have shown good agreement with electrophysiological results. Application of the model confirmed the scala tympani length constant within the basal turn of the cochlea to lie between 3 and 4 mm.