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Biomedical subjects

G P Clarke

Publications and source records attributed to G P Clarke.

14 recordsLinked to original sources
PubMed2001

Use of generalized linear mixed models in the spatial analysis of small-area malaria incidence rates in Kwazulu Natal, South Africa.

Spatial statistical analysis of 1994-1995 small-area malaria incidence rates in the population of the northernmost districts of KwaZulu Natal, South Africa, was undertaken to identify factors that might explain very strong heterogeneity in the rates. In this paper, the authors describe a method of adjusting the regression analysis results for strong spatial correlation in the rates by using generalized linear mixed models and variograms. The results of the spatially adjusted, multiple regression analysis showed that malaria incidence was significantly positively associated with higher winter rainfall and a higher average maximum temperature and was significantly negatively associated with increasing distance from water bodies. The statistical model was used to produce a map of predicted malaria incidence in the area, taking into account local variation from the model prediction if this variation was supported by the data. The predictor variables showed that even small differences in climate can have very marked effects on the intensity of malaria transmission, even in areas subject to malaria control for many years. The results of this study have important implications for malaria control programs in the area.

Climate↗
PubMed2000

A spatial statistical approach to malaria mapping.

BACKGROUND: Good maps of malaria risk have long been recognized as an important tool for malaria control. The production of such maps relies on modelling to predict the risk for most of the map, with actual observations of malaria prevalence usually only known at a limited number of specific locations. Estimation is complicated by the fact that there is often local variation of risk that cannot be accounted for by the known covariates and because data points of measured malaria prevalence are not evenly or randomly spread across the area to be mapped. METHOD: We describe, by way of an example, a simple two-stage procedure for producing maps of predicted risk: we use logistic regression modelling to determine approximate risk on a larger scale and we employ geo-statistical ('kriging') approaches to improve prediction at a local level. Malaria prevalence in children under 10 was modelled using climatic, population and topographic variables as potential predictors. After the regression analysis, spatial dependence of the model residuals was investigated. Kriging on the residuals was used to model local variation in malaria risk over and above that which is predicted by the regression model. RESULTS: The method is illustrated by a map showing the improvement of risk prediction brought about by the second stage. The advantages and shortcomings of this approach are discussed in the context of the need for further development of methodology and software.

Child↗
PubMed1997

Comparison of intraocular lens computations using a neural network versus the Holladay formula.

PURPOSE: To compare the accuracy of intraocular lens (IOL) calculations using Holladay personalized calculations and a new method of trained neural networks. SETTING: A private ophthalmic practice. METHODS: We developed and trained a neural network to predict IOL powers using a personalized Holladay program and clinical data from 200 consecutive cases of one surgeon's results with one IOL. Clinical data included preoperative axial length, both keratometry values, anterior chamber depth, and human lens thickness. The neural network was trained to produce the actual postoperative refractive error, and the Holladay surgeon factor was continuously refined using the same results. After the network was successfully trained against the clinical data, it was used to compute IOL power in a double-masked study. Ninety-five patients were randomized between the Holladay personalized calculation and the neural network computation. There were no significant differences in age or preoperative refractive errors between the two groups. Manifest refractions were obtained during the masked period at least 6 weeks after surgery. RESULTS: Mean postoperative error from predicted refraction was +0.271 diopters (D) for the neural network group and -0.217 D for the Holladay personal group. Mean absolute error from predicted refraction was +0.63 D for the neural network group and +0.93 D for the Holladay personal group. There was a significant difference in postoperative refractive errors and mean absolute error between the two groups (P < .022; nonparametric Mann-Whitney test). An error of less than +/- 0.75 D was obtained by 72.5% of the neural network group and 50.0% of the Holladay group. CONCLUSIONS: The neural network prediction formula can improve IOL implantation calculations by tightening the variance of errors.

Cataract Extraction↗
PubMed1997

Integrity testing of cochlear implants in the awake child.

Cochlear implantation is becoming a routine rehabilitation process for profoundly deaf adults and children. Often children are implanted at just two or three years of age and therefore the subsequent tuning of the device is challenging. Although some children demonstrate quick and reliable responses to electrical stimulation, there are others who do not respond consistently thus causing concern about the functioning of the device. It is therefore desirable to have an objective test of the integrity of the implanted electrodes. The principle of the integrity test is the measurement of voltages generated by the biphasic current pulses at the electrode array; this is accomplished using surface electrodes placed around the implanted ear, in conjunction with recording and averaging equipment typically used for evoked response testing. Traditional integrity testing usually requires a general anaesthetic in young children, however this study demonstrated a simple, quick and reliable method of obtaining results in the normally active child using ear-clip electrodes. Results are presented from 12 children tested in this way, and compared with results from 20 children who were tested in theatre using a different electrode configuration. The tests were performed in common ground stimulation mode, but some measurements were also made in bipolar + 1 and pseudo-monopolar modes. The three stimulation modes were compared, with the conclusion that common ground mode provides an efficient check of implant function in the awake child, while pseudo-monopolar mode may be preferable for anaesthetized patients. In addition, measurements were made in vitro using a functioning cochlear implant in a saline tank in order to investigate the current flow during stimulation. The standard procedure in this department is to perform a full intra-operative integrity test on all implanted children. The simplified technique is used to repeat the measurement post-operatively if required.

Child↗
PubMed1995

Interactive computer system for tuning the cochlear 22-channel implant in young children.

A computer system has been developed that has a games computer and a vibrotactile device linked into the Cochlear Diagnostic and Programming System. It uses a child's response to a computer game as a way of eliciting initial electrical thresholds and of maintaining attention, so that a large number of thresholds may be measured in a single session. Results have given times of 15 and 35 minutes for initial programming of all 22 electrodes in two 4-year-olds, and the system has been found to be useful in children from 3 years to early teens.

Child, Preschool↗
PubMed1994

Reliability and predictive value of the electrically evoked auditory brainstem response.

The reliability and predictive value of the low-pulse-rate electrical auditory brainstem response (EABR) threshold and the high-pulse-rate psychophysical threshold used in electrode mapping of adult cochlear implant patients was assessed. Eight patients, all implanted with the Nucleus multi-channel cochlear implant and experienced users, participated in this study. The relationship between like pulse rates of the EABR and psychophysical thresholds was assessed and showed high correlations of 0.90 or higher. When the relationship between low-pulse-rate EABRs and high-pulse-rate psychophysical thresholds was assessed the correlation was much poorer. The best correlation of 0.75 was shown between a 30-Hz EABR and the 500-Hz psychophysical threshold. This suggests that the low-pulse-rate EABR threshold is a poor predictor of the high-pulse-rate psychophysical threshold used in electrode mapping. Hence, if the EABR is to be a clinical tool for setting the electrodes it should be used cautiously and with the proper correction formula applied.

Adolescent↗
PubMed1993

Comparison of two methods of data presentation for the electrical brainstem response in cochlear implant patients.

Cochlear implants have become an acceptable method for management of the deaf patient, adult and child alike. Since the advent of implants there has been much research into objective methods for setting electrode values. The most common method is the electrical auditory brainstem response (EABR). The result of EABR threshold measurement is frequently reported as an arbitrary stimulus level set by the manufacturer. However, there has been recent discussion of whether an arbitrary stimulus level is acceptable or if the results should be reported in absolute units, either charge or current, particularly when combined data are used. This report compares two methods of data presentation for the EABR, arbitrary stimulus level and absolute units measured in nanoCoulombs (nC). Data collected from a previous study were used in this comparison. All patients were fitted with the Nucleus multi-channel device and Mini Speech Processor. The correlation between stimulus level and log charge (nC) was 0.990 with a slope of 0.987 which suggests the method of data reporting does not alter EABR results materially when using the Nucleus device.

Adolescent↗
PubMed1991

Direct referral hearing aid provision in the over sixties age group.

A prospective study was designed in which General Practitioners were issued with a standard referral letter for hearing aid provision. Unknown to the General Practitioner an independent otolaryngologist assessment was obtained at the time of first attendance. Three hundred consecutive patients selected by General Practitioners applying these guidelines were seen in a designated hearing aid clinic staffed by audiological technicians of senior grade or above. Referral pro formata were incomplete in 75 patients who were not assessed. Ninety-four patients (31 per cent) (95 per cent confidence interval 29-36 per cent) were accepted by the audiologist. Obstructing wax was the most common criterion failed. Clinical agreement between audiologist and otolaryngologist was 57 per cent greater than chance. None of the cases of clinical disagreement altered treatment. A direct referral system as proposed could have processed only 31 per cent of 300 referrals. However, experienced technical staff reliably detected otological pathology and with aural toilet facilities 91 per cent of 225 patients (confidence interval 88-94 per cent) were aided.

Aged↗
PubMed1981

Sarcoidosis of the lacrimal sac.

Two women with acute dacryocystitis were found to have noncaseating granulomatous inflammation of the lacrimal sac and nasal mucosa. One patient had chronic uveitis and the other had skin lesions and generalized lymphadenopathy histologically compatible with sarcoidosis. The diagnosis was supported by elevation of serum levels of angiotensin-converting enzyme. To our knowledge, these are the sixth and seventh histologically confirmed cases of sarcoidosis of the lacrimal sac. They differ from previously reported cases in the absence of classic roentgenographic changes of sarcoidosis. Analysis of all reported cases suggests an association between lacrimal sac and upper respiratory tract sarcoidosis.

Dacryocystitis↗
PubMed1979

Crossed acoustic response combined with visual and somatosensory evoked responses in the diagnosis of multiple sclerosis.

The crossed acoustic response, (CAR), a recently introduced test of brainstem function, has been studied in 66 patients with multiple sclerosis and 53 control subjects, and compared with conventional visual and somatosensory evoked responses (VER, SER). A latency abnormality was found in the CAR in 73% of patients, in the VER in 63%, and in the SER in 37%. Abnormalities have been related to the presence or absence of clinically detectable signs. All three responses detected subclinical lesions by showing abnormality in a proportion of multiple sclerosis patients who had no corresponding abnormal clinical signs (CAR 69%, VER 42%, SER 29%). The best diagnostic combination of responses was VER and CAR. Ninety per cent of patients had at least one of these two responses abnormal.

Acoustic Stimulation↗
PubMed1979

External electrical stimulation of the cochlea: clinical, psychophysical, speech-perceptual and histological findings.

Our progress towards the development of a particular form of cochlear implant for the totally deaf is described. A single channel stimulation at the round window or promontory is used. This involves a minimum of surgical intervention and infective risk, preserves the possibility of remission and allows the application of later developments. The signal used for stimulation is designed to be matched both to the deaf lip-reader's needs and to his new, restricted, auditory ability. This is done by concentrating on the acoustic pattern components of speech which carry intonation and voiced-voiceless information. Surgical electrophysical, psychoacoustic and speech perceptual aspects of our work with twelve patients are described. The tests involve responses, for example, relating to: threshold for sinusoids; frequency difference limens; periodic -aperiodic discrimination; stress placement; and consonant labelling using combined visual and electrical inputs. Relatively extensive measurements were made with six patients. Significant individual differences were found and the sets of responses provide an essential basis for an appraisal of the potential usefulness of our work to the individual patient. Possible reasons for the individual differences are discussed. A brief indication is given of the techniques which we have developed for the future speech training and speech production evaluation of patients with electro-cochlear voice monitoring. The final section of our paper mentions our histological investigation of the effects of this type of stimulation in the guinea pig.

Acoustic Impedance Tests↗
PubMed1977

Estimation of the prevalence of udder infection in dairy herds based on a single-quarter sample from each cow in the dairy herd.

Individual quarter milk samples were taken from 719 cows in 7 dairy herds and examined for mastitis-causing pathogens. Analyses were made to ascertain the prevalence of mastitis infection when the results from a single quarter were compared with those from all 4 quarters. There was a very close similarity between the 2 methods of assessment. When the number of samples is limited, sampling of a single quarter instead of all 4 quarters permits more cows to be examined.

Animals↗
PubMed1976

A single average crossed acoustic response.

Although the Crossed Acoustic Response is the simplest objective test of hearing acuity, as it can be used without an anaesthetic in the mobile child, a serious obstacle to its wide use has been cost. This was because two averagers have been necessary. This paper describes a modification of the equipment which is not only cheap but which can be used to adapt any existing electrocochleograph or cortical evoked response recorder.

Amplifiers, Electronic↗