Biomedical subjects

M L Thompson

Publications and source records attributed to M L Thompson.

At least 37 records · Page 2Linked to original sources

PubMed1999

Neuropsychological function in solvent-exposed South African paint makers.

Neuropsychological and neurological function were investigated in 228 organic solvent exposed paint manufacturing workers in two factories. Solvent exposure was expressed as both American Conference of Governmental Industrial Hygienists 1990 Threshold Limit Value equivalents and total hydrocarbon parts per million. The World Health Organization (WHO) neurobehavioral core test battery, the Neurobehavioural Evaluation System--2 (NES-2), and the UNISA Neuropsychological Assessment Procedure (UNAP) were used to measure outcomes, and a Vibratron II was used to measure peripheral vibration sensation. Exposures were generally below the Threshold Limit Value, and no exposure effect was found. Possible explanations for this finding are discussed, including low exposure, exposure, and outcome misclassification, together with inappropriateness, nonvalidity, and relative insensitivity of neuropsychological tests. Only age predicted Vibratron score. Some tests were clearly inappropriate for use in developing countries. Neuropsychological tests were sensitive to educational level, age, alcohol consumption, and cultural indicators. The WHO tests in particular showed good construct validity for neuropsychological functions and should be more formally validated for use in developing countries. Nevertheless, a major problem of cultural dependence is evident for all neuropsychological tests used in this study, and methods for bypassing this problem in less developed settings are discussed.

Adult↗

PubMed1998

A model for the prediction of time to union in fractures of the tibia.

BACKGROUND AND PURPOSE: This report covers the second arm of a trial, the main focus of which was to assess the effect of interferential currents (IFCs) on time to union in tibial fractures. No significant improvement was found with the use of IFCs for the parameters employed (Fourie & Bowerbank, 1997). METHODS: The focus of this study is the development of logistic regression models which used subject characteristics to predict non-union of fracture within 24, 32 and 40 weeks. Such models could be used to identify clients for consideration of alternative interventions, for example, electric current stimulation, bone grafting or the injection of bone morphogenic proteins (BMPs). The usefulness of such models would depend on their ability to correctly identify subjects whose fractures do or do not unite. These models were validated with respect to their sensitivity and specificity to predict non-union for a separate data set. RESULTS: The results indicated, for instance, that use of a model to predict non-union of fracture within 24 weeks would lead to 27% of subjects being correctly classified (as union or non-union), 51% of subjects whose fractures did not unite within 24 weeks were identified and 65% of subjects (diagnosed as non-union) whose fractures did not unite within 24 weeks (the closer these values are to 100% the more accurate the model). CONCLUSIONS: The conclusion reached was that the models may have value for low-cost, non-invasive interventions, but that they could not be used to predict those cases where surgical involvement would be necessary, due to the high false-positive rate.

Adult↗

PubMed1998

The usefulness of a weight gain spurt to identify women who will develop preeclampsia.

OBJECTIVE: The usefulness of adaptive centiles for weight gain as well as sudden weight gain spurts in identifying women who will develop preeclampsia was assessed. METHODS: A study sample of 99 women who developed preeclampsia and a control sample of 675 women with normal pregnancies, were randomly selected. Weight gain spurts were identified by the upper bound of adaptive maternal weight centiles. Average changes in weight between successive clinic visits were also assessed. RESULTS: Weight gain exceeding the 90th percentile of the adaptive centiles resulted in estimated sensitivity and specificity of 52% and 66% and weight gain >0.9 kg per week between visits in 76% and 29% respectively. Of women who would develop preeclampsia identified by the adaptive chart, 62.0% exhibited abnormal weight gain prior to the onset of abnormal clinical findings. CONCLUSIONS: A sudden weight gain spurt is a far from reliable sign of impending preeclampsia. This reason for weighing women at antenatal visits appears to be unfounded.

Adult↗

PubMed1998

Meta-analysis and occupational epidemiology.

Meta-analysis is increasingly appearing in the epidemiologic literature. Although originally performed in the context of experimental or randomized controlled study designs and with regard to problems that are amenable to these designs, the method has also been applied to studies in observational epidemiologic settings. This movement has generated considerable debate about the validity of meta-analysis in observational studies, which can be said to be rather shaky. Meta-analysis is also appearing in occupational epidemiology, and a recent application to studies of industrial cohorts exposed to organic solvents has illustrated many of the arguments against meta-analysis of observational study. The propensity of meta-analysis for disarming critical approaches to individual study methods such as the standardized morbidity or mortality ratio (SMR) as effect measure, and to application of commonly used biostatistical techniques is illustrated. A cautionary note is sounded.

Healthy Worker Effect↗

PubMed1998

Prevalence odds ratio or prevalence ratio in the analysis of cross sectional data: what is to be done?

OBJECTIVES: To review the appropriateness of the prevalence odds ratio (POR) and the prevalence ratio (PR) as effect measures in the analysis of cross sectional data and to evaluate different models for the multivariate estimation of the PR. METHODS: A system of linear differential equations corresponding to a dynamic model of a cohort with a chronic disease was developed. At any point in time, a cross sectional analysis of the people then in the cohort provided a prevalence based measure of the effect of exposure on disease. This formed the basis for exploring the relations between the POR, the PR, and the incidence rate ratio (IRR). Examples illustrate relations for various IRRs, prevalences, and differential exodus rates. Multivariate point and interval estimation of the PR by logistic regression is illustrated and compared with the results from proportional hazards regression (PH) and generalised linear modelling (GLM). RESULTS: The POR is difficult to interpret without making restrictive assumptions and the POR and PR may lead to different conclusions with regard to confounding and effect modification. The PR is always conservative relative to the IRR and, if PR > 1, the POR is always > PR. In a fixed cohort and with an adverse exposure, the POR is always > or = IRR, but in a dynamic cohort with sufficient underlying follow up the POR may overestimate or underestimate the IRR, depending on the duration of follow up. Logistic regression models provide point and interval estimates of the PR (and POR) but may be intractable in the presence of many covariates. Proportional hazards and generalised linear models provide statistical methods directed specifically at the PR, but the interval estimation in the case of PH is conservative and the GLM procedure may require constrained estimation. CONCLUSIONS: The PR is conservative, consistent, and interpretable relative to the IRR and should be used in preference to the POR. Multivariate estimation of the PR should be executed by means of generalised linear models or, conservatively, by proportional hazards regression.

Chronic Disease↗

PubMed1998

Effects of long-term organophosphate exposures on neurological symptoms, vibration sense and tremor among South African farm workers.

OBJECTIVES: This study assessed the relationship between long-term exposure to organophosphate insecticides and neurological symptoms, vibration sense, and motor tremor after control for the effect of past poisoning and acute exposure. METHODS: This cross-sectional study included 164 pesticide applicators and 83 nonspraying reference workers on deciduous fruit farms. The workers were tested on the Vibratron II, on tests of dynamic and static tremor, and for a set of neurological and "dummy" symptoms. Exposure was derived with the use of a job-exposure matrix for pesticides in agriculture. RESULTS: Compared with nonapplicators, current applicators reported significantly more dizziness, sleepiness, and headache and had a higher overall neurological symptom score. This association remained statistically significant after multiple logistic regression analyses controlling for a range of confounders and effect modifiers [odds ratio (OR) 2.25, for current applicators having high neurological score, 95% confidence interval (95% CI) 1.15-4.39]. The average lifetime intensity of organophosphate exposure was nonsignificantly associated with both neurological (OR 1.98, 95% CI 0.49-7.94) and "dummy" symptoms (OR 2.37, 95% CI 0.54-10.35). Previous pesticide poisoning was significantly associated with the neurological scores (OR 4.08, 95% CI 1.48-11.22) but not with the "dummy" symptoms. Vibration sense outcomes were associated with age and height, but not with the organophosphate exposure measures. In the multiple linear regression modeling for tremor intensity in the dominant hand, recent organophosphate exposure in the past 10 days was a significant predictor (partial correlation coefficient = 0.04), but none of the long-term organophosphate exposure measures were significant. CONCLUSIONS: Strong evidence was found for an association between symptom outcomes and past organophosphate poisoning and between symptom outcomes and current spray activity. In contrast to symptoms, there was no association between either past poisoning or current spray activity and vibration sense or tremor outcome. Long-term organophosphate exposure did not appear to predict symptoms, vibration sense, or tremor outcome.

Adult↗

PubMed1998

Health status among farm workers in the Western Cape--collateral evidence from a study of occupational hazards.

MOTIVATION: Farm workers are a marginalised occupational group whose poor living and working conditions may place them at increased risk for occupational and non-occupational morbidity and mortality. Research into the health status of farm workers has been neglected in the past. AIMS AND OBJECTIVES: As part of an investigation into the neurological and neurobehavioural effects of exposure to organophosphate insecticides, this study describes the demographics, life histories, risk factors for chronic illness and selected indicators of general health status among a group of farm workers in the Western Cape. STUDY DESIGN: Cross-sectional study conducted in the deciduous fruit farming industry in 1993. SUBJECTS: 164 pesticide applicators and 83 non-spraying controls (all men) from 73 farms, frequency matched for age and education. MEASUREMENTS: A structured questionnaire, venous blood sample for serum and erythrocyte cholinesterase, albumin, gamma glutamyl transferase and haemoglobin assessment, height and weight measurement. RESULTS: Most farm workers were children of farm workers and had lived and worked on farms for most of their lives. The study found substantial levels of illiteracy (21-44%, depending on the definition) and innumeracy (4%) and evidence of a significant morbidity burden: high levels of alcohol intake and ongoing reported application of the 'dop' system, high levels of head injury (70% of subjects) and evidence of substantial adult undernutrition. Protective equipment was relatively well distributed and used, although certain work activities (such as acting as a human marker) continue to pose substantial exposure hazards. There were no differences in respect of cholinesterase levels between spray applicators and controls, although past poisoning was reported by 9% of subjects. CONCLUSION: Farm workers appear to be a closed community with a high disease burden. Their health needs pose substantial challenges to the public health authorities.

Adult↗

PubMed1997

Construction of multivariate centile charts for longitudinal measurements.

Reference centiles are commonly used as a means of identifying whether some measure of interest on an individual lies within a 'normal' range. Individuals outside the range may be at risk for some problem outcome. The information needed to screen for problem outcomes in often multivariate in nature. We develop a multivariate approach to centile estimation which allows for updating of centiles based on the prior path of the individuals as well as adjustments to the centiles according to individual covariate values. It is also possible to consider several variables jointly as a screen for problem outcomes.

Body Height↗

PubMed1997

Changes in corticomotor excitation and inhibition during prolonged submaximal muscle contractions.

Changes in motor evoked potential (MEP) amplitude, post-MEP silent period duration, and interpolated twitch torque were measured using transcranial magnetic (TMS) and electrical (TES) stimulation during a 20% maximum voluntary contraction of the elbow flexors sustained to exhaustion. TMS- and TES-induced MEP amplitude increased progressively over the contraction period up until the point of exhaustion. The TMS-induced silent period was prolonged only during the second half of the contraction period, the time course being different from that of the MEP responses, whereas the TES-induced silent period did not change. The findings indicate that corticomotor excitability increases during a sustained submaximal voluntary contraction and that, as fatigue develops, there is a progressive buildup of intracortical inhibition. This may represent a mechanism whereby corticomotor output is maintained at an appropriate level to preserve optimal motor unit firing frequencies during a fatiguing contraction.

Adult↗

PubMed1997

An investigation into neurologic and neurobehavioral effects of long-term agrichemical use among deciduous fruit farm workers in the Western Cape, South Africa.

Long-term exposure to organophosphates (OPs) in the absence of acute poisoning is increasingly suspected of causing chronic neurologic and neurobehavioral effects. A cross-sectional survey of 163 spray operators on deciduous fruit farms in the Western Cape, South Africa, and 84 nonspraying labororers was conducted in 1993 to investigate the relationship between long-term OP exposures and neurological and neurobehavioral outcomes. The study also sought to evaluate the performance of a set of neurobehavioral test batteries based on the information-processing theory of cognitive psychology, relative to the more established World Health Organization's Neurobehavioral Core Test Battery (WHO NCTB). These information-processing tests were designed for use in studies of subjects with little education, which are frequently conducted in developing countries in agriculture. They draw on experience from a previous South African study in which problems were encountered with a lack of cross-cultural validity of conventional test batteries. No evidence was found of a relationship between long-term OP exposure and loss of vibration sense. Small associations were found with the NCTB Pursuit-Aiming and Santa Ana (nondominant hand) subtests. The overall evidence of neurologic and neurobehavioral effects of long-term OP exposure was small; exposure misclassification may have contributed to this finding. Important confounders such as brain injury, alcohol consumption, and nutritional status were identified.

Agricultural Workers' Diseases↗

PubMed1997

Predictive value of conditional centile charts for weight and fundal height in pregnancy in detecting light for gestational age births.

OBJECTIVE: To estimate the predictive value of weight and symphysis fundal height (SFH) in pregnancy to detect light for gestational age (LiGA) births. STUDY DESIGN: New conditional centile charts for longitudinal monitoring of maternal weight and fundal height in pregnancy were developed based on the records of 676 women with singleton pregnancies and no complications. The records of these women and of a further sample of 102 women who had LiGA deliveries were used to assess the sensitivity and specificity of the charts in detecting LiGA during pregnancy. RESULTS: Maternal weight is a poor predictor of LiGA births. Fundal height shows moderate predictive ability. The simple cross-sectional chart for fundal height at the 20th percentile cutoff has sensitivity and specificity of 66% in detecting LiGA births. CONCLUSIONS: Monitoring via a cross-sectional chart of fundal height offers a potentially useful screening method. The results of this study add support to arguments that routine weighing in pregnancy should be abandoned.

Anthropometry↗

PubMed1997

Corticomotor representation of the sternocleidomastoid muscle.

The topography of the cortical motor projection to the sternocleidomastoid (SCM) muscles was investigated in 15 normal subjects using the technique of transcranial magnetic stimulation. Contrary to the long-held view that the representation of the neck muscles in the motor strip is close to that of the face, our findings indicate that the projection to both the ipsilateral and contralateral SCM muscles arises from an area of cortex high up on the cerebral convexity close to the trunk representation and at a comparable level to the sensory representation of the neck in the post-central cortex. The use of shielded intramuscular recordings showed that surface recordings detect activity not only from the SCM muscle but also from the overlying platysma muscle whose cortical representation is lateral to that of the hand muscles. Our findings confirm that the corticomotor projection to the SCM muscle follows both a contralateral monosynaptic pathway, and an ipsilateral pathway, and indicate that the latter pathway may be disynaptic.

Adult↗

PubMed1997

Characteristics of information resources preferred by primary care physicians.

Primary care physicians use patient data, medical knowledge, logistic information, and population statistics. They rely on their personal knowledge to care for their patients, their top priority. When they seek information beyond this personal knowledge base, they frequently want information in the context of the care of a specific patient. They also continually add to their personal knowledge base. Less frequently, they seek logistic information and population statistics. For patient-specific questions, physicians most often seek medical facts or medical opinions. A physician may be persistent in seeking information if the patient's problem is perceived to be urgent and the doctor believes a definitive answer exists. Information resources for answering clinical questions should be readily available, familiar, and quick to use. Lifelong learning activities should also be readily available, and they should require a minimum of time, effort, and expense. Minimal cost in time and effort is particularly important when knowledge is sought as a part of ongoing medical learning, since there is less immediate benefit to balance the time and effort invested in information seeking.

Humans↗

PubMed1997

The use (and abuse) of reference centiles with an application to weight gain in pregnancy.

Centile charts are commonly used in many areas of health research and practice, e.g. growth charts for children, Doppler ultrasonography in pregnancy and assessment of cholesterol levels at different ages. Yet there are a number of aspects of both their construction and application that are problematic and it is some of these issues that will be raised in this paper. The objective of the paper is to outline, in a non-technical way, some of the issues that need to be considered by the practitioner in estimating and using reference centile charts, but which frequently are either not known or ignored. These include: (i) the choice of reference population; (ii) how to estimate centiles; (iii) formally incorporating previous measurements on an individual, e.g. the interpretation of a child's weight that is on the 50th percentile for its age will be different if it has been moving along the 90th percentile at previous ages than if it has consistently been on the 50th percentile; and (iv) evaluation of centile charts used as a screen for problems. The concepts are introduced using an aspect of a study conducted at Tygerberg Hospital where centile charts for maternal weight gain in pregnancy were developed and assessed for their usefulness in detecting light-for-gestational-age (LiGA) births. The reference centile charts for maternal weight show poor discriminating ability between LiGA and normal births. These results support arguments in favour of abandoning the routine weighing of pregnant women.

Female↗

PubMed1995

Repeatability and validity of a field kit for estimation of cholinesterase in whole blood.

OBJECTIVES: To evaluate a spectrophotometric field kit (Test-Mate-OP) for repeatability and validity in comparison with reference laboratory methods and to model its anticipated sensitivity and specificity based on these findings. METHODS: 76 farm workers between the age of 20 and 55, of whom 30 were pesticide applicators exposed to a range of organophosphates in the preceding 10 days, had blood taken for plasma cholinesterase (PCE) and erythrocyte cholinesterase (ECE) measurement by field kit or laboratory methods. Paired blinded duplicate samples were taken from subgroups in the sample to assess repeatability of laboratory and field kit methods. Field kits were also used to test venous blood in one subgroup. The variance obtained for the field kit tests was then applied to two hypothetical scenarios that used published action guidelines to model the kit's sensitivity and specificity. RESULTS: Repeatability for PCE was much poorer and for ECE slightly poorer than that of laboratory measures. A substantial upward bias for field kit ECE relative to laboratory measurements was found. Sensitivity of the kit to a 40% drop in PCE was 67%, whereas that for ECE was 89%. Specificity of the kit with no change in mean of the population was 100% for ECE and 91% for PCE. CONCLUSION: Field kit ECE estimation seems to be sufficiently repeatable for surveillance activities, whereas PCE does not. Repeatability of both tests seems to be too low for use in epidemiological dose-response investigations. Further research is indicated to characterise the upward bias in ECE estimation on the kit.

Adult↗

PubMed1995

A centile chart for birth weight for an urban population of the Western Cape.

Evidence from large epidemiological studies has supported concern that being born light for gestational age (LiGA) may be detrimental. The incidence of LiGA babies is an important indicator of the health of women of reproductive age in deprived communities. In the assessment of LiGA in the Western Cape, centile charts constructed for populations in other parts of the world are generally used. These charts, however, may not be appropriate. Patients residing in the area served by the Tygerberg Hospital obstetric service, who booked early with singleton pregnancies, had their gestational age confirmed by early ultrasound and delivered between 1 March 1989 and 28 February 1990 were included in the study. The sample consisted of 3,643 patients. The mean birth weight was 2,995 g (SD 573 g) and the range 760-5,080 g. The distribution of birth weight at each week of gestation from 28 to 42 weeks was not normal. The 4-parameter Johnson family of densities was used to model the distribution of birth weight at each gestational age. A comparison of the distribution of birth weight in the study relative to the perinatal growth chart for international reference constructed by Dunn was also made. In addition to considering an overall chart, the sample was subdivided according to a number of characteristics (e.g. gender, firstborn and latter-born babies, smoking habit, hypertensive disorders and induction of labour) in order to explore their impact on the distribution of birth weight. Having explored the potential impact of all these factors, it was concluded that a single chart including all patients could be constructed.

Birth Weight↗

PubMed1994

Effect of chronic DDC treatment on LHRH and substance P amidation processes in the rat.

We examined the effects of chronic diethyldithiocarbamate (DDC) treatment on the concentrations of methionine-enkephalin, mature and unamidated forms (-Gly) of luteinizing hormone releasing hormone (LHRH) and substance P (SP) in various regions of the central nervous system (CNS). Chronic DDC treatment resulted in elevations of LHRH-Gly like immunoreactivity in the preoptic area (POA) and the medial basal hypothalamus (MBH), as well as elevations in SP-Gly like immunoreactivity in all areas of the CNS examined. Castration altered the ratios of SP-G-like/SP-like immunoreactivity in the pons, and LHRH-Gly like immunoreactivity in the MBH. Met-enkephalin concentrations were significantly elevated in the pons and medulla of intact DDC-treated animals, and in the POA of both intact- and castrated DDC-treated animals. Results demonstrate that it is possible to detect basal levels of unamidated LHRH and SP in many areas of the CNS, with ratios of unamidated/amidated peptides representing a unique and sensitive method for determining altered posttranslational processing of these transmitters, especially under altered endocrine states such as castration. Pharmacological blockade of terminal enzymatic processing of these peptides may be useful in studying upstream regulatory events in peptidergic neurons.

Amides↗

PubMed1994

Information-seeking coping and anxiety in school-age children anticipating surgery.

This study examined the relation between coping behavior and anxiety among school-age children anticipating an elective tonsillectomy and/or adenoidectomy. The demographic variables of age, gender, previous hospitalization, and birth order were also studied in relation to both coping and anxiety. Interview methods were utilized to measure information-seeking and information-limiting coping behaviors and feelings of anxiety. The results of this study indicate that children who used primarily information-seeking or information-limiting strategies were successful (as measured by low anxiety levels) in managing the stress of anticipated hospitalization. Children using information-seeking strategies for some, but not all, aspects of the hospitalization experience reported the greatest anxiety. The demographic variables were not related to either anxiety or coping.

Adaptation, Psychological↗