Biomedical subjects

S Zeger

Publications and source records attributed to S Zeger.

At least 19 recordsLinked to original sources

PubMed1998

Does weather confound or modify the association of particulate air pollution with mortality? An analysis of the Philadelphia data, 1973-1980.

Because weather has the potential to confound or modify the pollution-mortality relationship, researchers have developed several approaches for controlling it in estimating the independent effect of air pollution on mortality. This report considers the consequences of using alternative approaches to controlling for weather and explores modification of air pollution effects by weather, as weather patterns could plausibly alter air pollution's effect on health. We analyzed 1973-1980 total mortality data for Philadelphia using four weather models and compared estimates of the effects of TSP and SO2 on mortality using a Poisson regression model. Two synoptic categories developed by Kalkstein were selected--the Temporal Synoptic Index (TSI) and the Spatial Synoptic Classification (SSC)--and compared with (1) descriptive models developed by Schwartz and Dockery (S-D); and (2) LOESS, a non-parametric function of the previous day's temperature and dew point. We considered model fit using Akaike's Information Criterion (AIC) and changes in the estimated effects of TSP and SO2. In the full-year analysis, S-D is better than LOESS at predicting mortality, and S-D and LOESS are better than TSI, as measured by AIC. When TSP or SO2 was fit alone, the results were qualitatively similar, regardless of how weather was controlled; when TSP and SO2 were fit simultaneously, the S-D and LOESS models give qualitatively different results than TSI, which attributes more of the pollution effect to SO2 than to TSP. Model fit is substantially poorer with TSI. This pattern was repeated in analyses of summer and winter months, which included SSC. In summary, using synoptic weather categories in regression models does not meaningfully change the association between mortality and air pollution indexes. We also found little evidence that weather conditions modified the effect of pollution, regardless of the approach used to represent weather.

Air Pollution↗

PubMed1997

Function and visual impairment in a population-based study of older adults. The SEE project. Salisbury Eye Evaluation.

PURPOSE: The Salisbury Eye Evaluation (SEE) project investigates the impact of vision on functional status in a population-based sample of elderly persons. The prevalence of self-reports of functional status and the association with visual acuity loss are described. METHODS: A random sample of men and women 65 to 84 years of age from Salisbury, Maryland were recruited for home interviews and clinic examinations. Of the eligible sample, 78% responded to the home questionnaire and 65% responded to the questionnaire and the clinic examination. Binocular visual acuity of each person was measured using ETDRS charts and protocols. Questions were asked concerning activities of daily living, instrumental activities of daily living, physical function, social interaction, and activities of daily vision. Analyses of the association of presenting visual acuity worse than 20/40 with the measures of function were carried out and adjusted for age, race, and sex. RESULTS: All measures of functional status showed a decline with age; women and blacks were more likely to report difficulties. The age-adjusted proportions of those with visual impairment were not significantly different between men and women (7.2% versus 6.7%; P = 0.60). Black persons had almost twice the rate (10.4%) of white persons (5.6%; P < 0.001). Age, race, gender, and visual impairment were all significantly related to declines in functional status in multivariate models. CONCLUSIONS: Data on functional status in the project population were similar to national data and confirmed higher rates of disability in women and blacks. Binocular visual acuity worse than 20/40 appeared to have an impact on all the self-report measures of functional status. Further analyses on the interaction of vision with other comorbid conditions on function status clearly are indicated.

Activities of Daily Living↗

PubMed1997

A comprehensive assessment of visual impairment in a population of older Americans. The SEE Study. Salisbury Eye Evaluation Project.

PURPOSE: The Salisbury Eye Evaluation Project is a longitudinal study of risk factors for age-related eye diseases and the impact of eye disease and visual impairment on physical disability. In this article, the authors report the prevalence of visual impairment in their population and explore the relations among the various measures of visual function. METHODS: A population-based sample of 2520 residents of Salisbury, Maryland, between the ages of 65 and 84 years were enrolled in the study. Twenty-six percent of participants were black. Vision tests included best-corrected Early Treatment Diabetic Retinopathy Study acuity, Pelli-Robson contrast sensitivity with and without glare, Randot stereoacuity, and 60 degrees Humphrey visual fields. RESULTS: Visual function decreased linearly with age for the acuity, contrast sensitivity, glare, and visual field tests. Stereoacuity remained constant into the mid-70s and declined at an accelerating rate thereafter. Black participants had lower contrast sensitivity, reduced stereoacuity, and worse visual fields, at all ages compared to white participants; however, white participants were more sensitive to glare. The overall prevalence of visual acuity impairment in blacks was 5.6% versus 3.0% for whites, using the traditional United States definition (worse than 20/40 to better than 20/200) and 3.3% for blacks versus 1.6% for whites, using the World Health Organization definition (worse than 20/60 to 20/400). Acuity was correlated moderately with contrast sensitivity, stereoacuity, and visual fields (Spearman rho = 0.50, 0.35, and 0.34, respectively). The correlation between acuity and glare sensitivity was low (rho = 0.12). CONCLUSIONS: Many aspects of visual function, not just acuity, decline with age. Black participants have more visual impairement than do white participants for all tests except glare sensitivity. The prevalence of visual acuity impairement in the Salisbury Eye Evaluation population is lower than that reported by other studies using similar test procedures. Low-to-moderate correlations among vision test scores suggest that several different dimensions of visual function are being assessed.

Aged↗

PubMed1996

Classifying visual field data.

We develop a prediction model for classifying an eye according to glaucoma status based on its visual field. We develop measures of both diffuse and localized defects in the visual field as potential predictors of glaucoma. To identify predictors of abnormal fields, we must describe the variability in the fields of normal eyes, hence we first model the mean, variance and correlation structures of normal fields with use of generalized estimating equations. The best measures of diffuse loss include a field's mean level, contrasts of the upper and lower halves, and contrasts of the nasal and temporal halves. Local loss is measured by the depth, area, volume and location of the field's largest defect. We develop logistic regression models to classify eyes as having glaucoma or not. We present ROC curves of the results that are highly competitive with current clinical methods of classification.

Adult↗

PubMed1996

Pattern of growth in children with cerebral palsy.

OBJECTIVE: To provide a growth reference standard for children with quadriplegic cerebral palsy (QCP). Growth references specific to children with cerebral palsy would facilitate uniformity in clinical appraisal of their growth and nutritional status and would simplify comparative interpretation of growth data. DESIGN: Weights and lengths were obtained according to standardized procedures. Measurements were taken at time of visit to an orthopedic clinic and from retrospective review of charts. SUBJECTS: Three hundred sixty children with QCP. Growth data were based on 1,630 observations. ANALYSIS: Growth curves representing the 10th, 50th, and 90th percentiles were estimated using a smoothing splines technique. Statistical bootstrapping was performed to confirm significant differences from the growth charts of the National Center for Health Statistics (NCHS). RESULTS: Growth charts were constructed for boys and girls, aged 0 to 120 months, depicting length for age, weight for age, and weight for length. Average differences in length for age, weight for age, and weight for length between children with QCP and NCHS standards were -2.3 z, -2.4 z, -1.3 z for boys, and -2.1 z, -2.1 z, -1.1 z for girls. Children with QCP fell progressively behind in stature and weight. Compared with their NCHS counterparts, they were 5% shorter at 2 years of age and more than 10% shorter at 8 years of age. APPLICATION: Charts representing the pattern of growth in children with QCP should be accessible to parents and health care professionals to facilitate evaluation and monitoring of nutritional status.

Anthropometry↗

PubMed1996

Statistical issues in prevention and therapeutic trials of Alzheimer disease.

Relatively little attention has been given to the proper design and statistical analysis of prevention and therapeutic trials of Alzheimer disease (AD). A number of important methodological issues arise in studies of AD that do not typically arise in studies of other chronic diseases. These issues include: the definition of the primary study endpoint that is simple to measure and easy to interpret; proper design of trials that take into account that the patient population is very old; and the proper statistical analysis that addresses the high drop out rate and the multiple ways of measuring disease severity. Addressing these issues will speed the progress of the evaluation of promising compounds to prevent and treat AD.

Alzheimer Disease↗

PubMed1994

Prospective study of central nervous system function in amateur boxers in the United States.

Active amateur boxers from six US cities were studied in 1986-1990 to determine whether changes in central nervous system function over a 2-year interval (as evaluated by tests of perceptual/motor function, attention/concentration, psychomotor speed, memory, visuoconstructional ability, and mental control, measures of ataxia and brain-stem auditory evoked potentials, and electroencephalography) were associated with degree of participation in amateur boxing. A total of 484 participants were examined at baseline; 393 (81.2%) were examined 2 years later. At baseline, 22% of the participants had not yet competed in a bout; 9% had never competed in a bout by the second examination. Exposure was defined by number bouts, sparring-years, and sparring with a professional boxer. Very few statistically significant odds ratios were found between exposure and change in function. Significant tests of trend were found between the total number of bouts incurred before the baseline examination and changes in memory, visuoconstructional ability, and perceptual/motor ability. The significant trends for change in function in the latter two domains were primarily due to performance on the Block Design test, which was common to both test domains. No statistically significant associations were found between more recent bouts (after the baseline visit) and any functional domains, nor between bouts or sparring and any other outcome measures. The significant trends with past bouts, but not more recent bouts, may reflect the need for a long latency period before effects are manifest. Alternatively, given changes in safety practices, the observed association may be related to more severe exposure from bouts that occurred before 1986, when new safety measures were imposed.

Adolescent↗

PubMed1994

Altered cortical blood flow in HIV-seropositive individuals with and without dementia: a single photon emission computed tomography study.

OBJECTIVE: To quantitatively demonstrate the pattern of cerebral perfusion abnormalities in HIV-1-infected individuals described as 'patchiness' or inhomogeneity in previous qualitative emission tomographic imaging studies. DESIGN: We aimed to create a quantitative measure of inhomogeneity in HIV-infected individuals. High-frequency variance in cortical profiles is an indication of inhomogeneity in the distribution of radiotracer in the cerebral cortex. Therefore, the study analysis was designed to enable the estimation of variance frequencies in cortical profiles. METHODS: Regional cerebral blood flow was examined in nine mildly demented and 10 cognitively normal HIV-1-seropositive individuals and eight seronegative normal controls using single photon emission computed tomography with the radiotracer [I-123]-N-isopropyl-p-iodoamphetamine. Quantitative analysis was performed using circumferential profiles of cerebral cortical perfusion. Fourier transform power spectra of the profiles were examined as an index of patchiness in tracer distribution. RESULTS: Normal controls were characterized by strong middle frequency and weak high-frequency power. Both HIV-1-infected groups showed a significant power shift from middle to high frequencies. CONCLUSIONS: Increased high-frequency variations in both HIV-1-infected groups indicates diffuse cortical perfusion changes compared with normal controls. This study suggests that there are cerebral bloodflow abnormalities in HIV-1-infected individuals both with and without clinically severe dementia.

AIDS Dementia Complex↗

PubMed1994

Appropriate statistical methods to account for similarities in binary outcomes between fellow eyes.

PURPOSE: Many ocular measurements are more alike between fellow eyes than between eyes from different individuals. To make appropriate inferences using data from both eyes rather than the best or worst eye, statistical methods that account for the association between fellow eyes must be used. METHODS: Marginal and conditional regression models account for the association between fellow eyes in different ways. The authors compare and contrast these methods using data from a series of patients with retinitis pigmentosa in whom the primary object is to identify risk factors, some of which are subject specific and some of which are eye specific, for visual acuity loss (as a binary outcome) among affected subjects. RESULTS: Odds ratios for age, gender, presence of posterior subcapsular cataract, and genetic type of retinitis pigmentosa obtained from the marginal model were all larger than those from the conditional model. Familial aggregation of visual acuity loss was statistically significant in the marginal, but not in the conditional, model. CONCLUSIONS: The estimates and interpretation of the association between an ocular outcome and risk factors can differ significantly between these two approaches. The choice of model depends on the scientific questions of interest rather than on statistical considerations. Computer programs are available for implementing both models.

Baltimore↗

PubMed1992

Lymphohematopoietic cancer in styrene-butadiene polymerization workers.

1,3-Butadiene and styrene are suspected carcinogens and common chemicals used in the synthesis of rubber. To investigate any potential human hazards from exposure to these chemicals, a case-control study of 59 lymphohematopoietic cancers was conducted within a cohort of male workers employed between 1943 and 1982 in eight North American styrene-butadiene rubber polymer-producing plants. A total of 193 controls were matched to the cases by plant, age, year of hire, duration worked, and survival to time of death of the case. Each job was assigned an estimated exposure rank, and each worker's cumulated rank score was calculated on the basis of the time spent in each job throughout his employment. "Exposure" as a dichotomous variable was defined as a log rank score above the mean of the log scores for the total population of cases and controls within a subtype of cancer. Matched-pair analysis identified a strong association between leukemia and butadiene, with an odds ratio of 9.36 (95% confidence interval 2.05-22.9) and an association between styrene and leukemia (odds ratio = 3.13, 95% confidence interval 0.84-11.2) that did not achieve statistical significance. When exposure to both styrene and butadiene was included in a conditional logistic regression model, the odds ratio for butadiene remained high (odds ratio = 7.39), but the estimated association of leukemia with styrene was small. The results of this study support the hypothesis that exposure to butadiene is associated with the risk of leukemia. There also appears to be an additional risk from work in specific subdivisions of the industry.

Butadienes↗

PubMed1991

Daily diaries of respiratory symptoms and air pollution: methodological issues and results.

Daily diaries of respiratory symptoms are a powerful technique for detecting acute effects of air pollution exposure. While conceptually simple, these diary studies can be difficult to analyze. The daily symptom rates are highly correlated, even after adjustment for covariates, and this lack of independence must be considered in the analysis. Possible approaches include the use of incidence instead of prevalence rates and autoregressive models. Heterogeneity among subjects also induces dependencies in the data. These can be addressed by stratification and by two-stage models such as those developed by Korn and Whittemore. These approaches have been applied to two data sets: a cohort of school children participating in the Harvard Six Cities Study and a cohort of student nurses in Los Angeles. Both data sets provide evidence of autocorrelation and heterogeneity. Controlling for autocorrelation corrects the precision estimates, and because diary data are usually positively autocorrelated, this leads to larger variance estimates. Controlling for heterogeneity among subjects appears to increase the effect sizes for air pollution exposure. Preliminary results indicate associations between sulfur dioxide and cough incidence in children and between nitrogen dioxide and phlegm incidence in student nurses.

Air Pollution↗

PubMed1990

Passive smoking, air pollution, and acute respiratory symptoms in a diary study of student nurses.

A cohort of approximately 100 student nurses in Los Angeles was recruited for a diary study of the acute effects of air pollution. Smoking histories and presence of asthma and other allergies were determined by questionnaire. Diaries were completed daily and collected weekly for as long as 3 yr. Air pollution was measured at a monitoring location within 2.5 miles of the school. Incidence and duration of a symptom were modeled separately. Pack-years of cigarettes were predictive of the number of episodes of coughing (p less than 0.0001) and of bringing up phlegm (p less than 0.0001). Current smoking, rather than cumulative smoking, was a better predictor of the duration of a phlegm episode (p less than 0.0001). Controlling for personal smoking, a smoking roommate increased the risk of an episode of phlegm (odds ratio [OR] = 1.41, p less than 0.001), but not of cough. Excluding asthmatics (who may be medicated), increased the odds ratio for passive smoking to 1.76 (p less than 0.0001). In logistic regression models controlling for temperature and serial correlation between days, an increase of 1 SD in carbon monoxide exposure (6.5 ppm) was associated with increased risk of headache (OR = 1.09, p less than 0.001), photochemical oxidants (7.4 pphm) were associated with increased risk of chest discomfort (OR = 1.17, p less than 0.001) and eye irritation (OR = 1.20 p less than 0.001), and nitrogen dioxide (9.1 pphm) was associated with increased risk of phlegm (OR = 1.08 p less than 0.01), sore throats (OR = 1.26, p less than 0.001), and eye irritation (OR = 1.16, p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Acute Disease↗

PubMed1989

Diet and opiate addiction: a quantitative assessment of the diet of non-institutionalized opiate addicts.

The diet, weight and height of 38 non-institutionalized chronic opiate addicts have been assessed and compared to those of a random sample of the 20-35 year old general population of Canton Geneva, Switzerland. A history questionnaire was administered by specially trained dietitians. Results show that, after adjustment for age and alcohol intake, male addicts have the same caloric intake as non-addicts. However, addicts tend to replace foods that are rich in fat and proteins with foods rich in sucrose and relatively poor in vitamins and minerals. Beer is the major component of their much larger alcohol consumption. On the other hand, the body mass index (kg/m2) of the male addicts is within the normal range, suggesting this may not be a good indicator of their unbalanced diet. This study gives a quantitative assessment, in terms of nutrient intake, of the typical craving for sweet described by opiate addicts.

Adult↗

PubMed1989

Evaluation of human immunodeficiency virus seroprevalence in population surveys using pooled sera.

The pooling of individual serum samples to determine human immunodeficiency virus (HIV) seropositivity was examined to assess whether testing pooled sera was technically feasible, cost-effective, and accurate for estimating seroprevalence in large population surveys. The sensitivities and specificities of three commercially available HIV enzyme-linked immunosorbent assay (ELISA) kits were tested using 65 serum pools of 15 individual serum samples each (975 total serum samples) at two different dilutions. With pooled sera, the Organon Teknika Bio-EnzaBead ELISA at half the dilution recommended by the manufacturer showed the best agreement with ELISA and Western blot results of individual sera. In subsequently testing 92 pools, each containing 15 individual serum samples from a population of American patients attending a sexually transmitted diseases clinic, the estimated seroprevalence was 5.27 compared with 4.93% in a test of 1,380 individual serum samples and 5.19% in a test of 4,028 individual serum samples from the same population. In an evaluation of 1,380 African patients using 10 serum samples per pool, the estimated seroprevalence was 5.79 compared with 6.16% in a test of individual sera. These results indicate that ELISA testing with pooled sera is highly sensitive and specific and appears to be a cost-effective means for estimating HIV seroprevalence in large population-based surveys.

Baltimore↗

PubMed1988

Callosal and association neurons in the cortical space: a spectral analysis approach.

The tangential distributions of callosal neurons of area 5 projecting homotopically to the contralateral hemisphere and of association neurons of areas 4 and 6 projecting to ipsilateral area 5 were determined in the macaque monkey by using neuroanatomical methods based on the retrograde transport of horseradish peroxidase. Both distributions were studied qualitatively through 2-dimensional reconstructions of the cortical areas of origin and quantitatively through a spectral analysis. This approach facilitated the characterization of the spatial periodicities contained in these distributions revealing that, in area 5, callosal neurons were organized in bands of various shapes and width; these bands were composed of more discrete clusters of cells. In the frontal lobe, association neurons projecting to ipsilateral area 5 were arranged similarly. This study suggests that a common principle underlies the tangential organization of both callosal and association projecting cells in different cortical areas and emphasizes a basic similarity of interhemispheric and intrahemispheric connections.

Animals↗

PubMed1988

Sensory threshold estimation from a continuously graded response produced by reflex modification audiometry.

This article describes the use of reflex modification to determine sensory detection thresholds. The method is based upon the finding that low-intensity sensory stimuli presented shortly before a reflex eliciting stimulus are able to modify the amplitude of the reflex. The extent of such modification is related to the intensity of the initial low-intensity stimulus. In contrast to earlier reported procedures for threshold estimation, the method described in this article consists of fitting a smooth function to the relationship between startle response amplitude and the intensity of the inhibiting stimulus. The method entails fitting a cubic spline function to the medians of the square-root reflex amplitude at each prestimulus intensity. The resulting audiometric curves closely approximate audiometric data obtained from traditional operant methods both in sensitivity and shape. Parametric data are also presented that allow for optimizing stimulus presentation so as to obtain reliable thresholds using a minimal number of test trials. The procedures developed in this article may prove useful in other situations involving the estimation of a threshold effect from a continuously graded response.

Animals↗

PubMed1985

Corticocortical efferent systems in the monkey: a quantitative spatial analysis of the tangential distribution of cells of origin.

The laminar and tangential distributions of association neurons projecting from areas 4 and 6 of the frontal lobe to area 5 of the superior parietal lobule were studied in macaque monkeys by using horseradish peroxidase histochemistry. In both areas 4 and 6 association neurons were medium-large pyramidal cells of layers II and III, and pyramidal and fusiform cells of layers V-VI. Tangentially, they were distributed unevenly over the cortical surface occupying only certain parts of areas 4 and 6, including the dorsomedial part of area 6, the proximal arm region of Woolsey's M1 map, parts of the postarcuate cortex, and the supplementary motor area. Within these projection zones, the number of projection cells waxed and waned in a periodic fashion. Quantitative methods, including spectral analysis techniques, were used to characterize precisely spatial periodicities along the rostrocaudal dimension. The same quantitative analyses were used to determine the nature of the tangential distribution of corticocallosal cells of area 5 projecting to contralateral area 5. Both association and callosal spectra contained a strong component in the range of low spatial frequencies, corresponding to periods greater than 2 mm. Moreover, a consistent peak was observed in both spectra at spatial frequencies corresponding to periods ranging from 0.85 to 1.28 mm. This peak is in accord with the hypothesis of a modular organization of the cells of origin of these projection systems.

Animals↗

PubMed1984

The effect of a keto acid-amino acid supplement to a restricted diet on the progression of chronic renal failure.

We treated 24 patients who had chronic renal failure with a low-phosphorus diet containing 20 to 30 g of mixed-quality protein, supplemented by amino acids and their keto analogues. Seventeen patients had well-defined rates of progression before treatment, as assessed by serial determinations of serum creatinine levels. By extrapolating these rates of progression, we found that 10 of the 17 (59 per cent) had a clinically important slower rise in creatinine levels during long-term treatment (average, 20 months) than predicted; none had a faster rise than predicted. Seven of the 17 patients began treatment before creatinine reached the level of 8 mg per deciliter; in six of the seven, followed for an average of 22 months, creatinine has remained at or below the level at the start of treatment. Nutrition, as assessed by body weight, nitrogen balance, serum albumin, and serum transferrin, has been well maintained. This regimen slowed or arrested the rise in creatinine levels and thus must have slowed or halted the progression of renal insufficiency in a majority of cases, especially when treatment was initiated before creatinine had reached the level of 8 mg per deciliter. The mechanism underlying this effect remains to be determined.

Adult↗