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Study objective: To analyse if socioeconomic characteristics in area of living affect the risk of myocardial infarction in a Swedish urban population, and to evaluate to what extent the contextual effect is confounded by the individual exposures.
Design: A population based case-referent study (SHEEP).
Setting: Cases (n=1631) were all incident first events of myocardial infarction during 1992-1994. The study base included all Swedish citizens aged 45-70 years, living in Stockholm metropolitan area during these years. The social context of all metropolitan parishes (n=89) was determined by routine statistics on 21 socioeconomic indicators. A factor analysis of the socioeconomic indicators resulted in three dimensions of socioeconomic deprivation, which were analysed separately as three different contextual exposures.
Main results: The main characteristics of the extracted factors were; class structure, social exclusion and poverty. Among men, there were increased relative risks of similar magnitudes (1.28 to 1.33) in the more deprived areas according to all three dimensions of the socioeconomic context. However, when adjusting for individual exposures, the poverty factor had the strongest contextual impact. The contextual effects among women showed a different pattern. In comparison with women living the most affluent areas according to the class structure index, women in the rest of Stockholm metropolitan area had nearly 70% higher risk of myocardial infarction after adjustment for individual social exposures.
Conclusions: The results suggest that the socioeconomic context in area of living increases the risk of myocardial infarction. The increased risk in only partially explained by individual social factors (the compositional effect).
An increasing number of studies analyse the health impact of the social context on local and national level. Since 1996 more than 20 scientific papers on individual health effects of socioeconomical contextual exposures have been published. (1-22)
Haan et al were among the first to conduct a study on contextual effects in small areas on mortality with control for individual characteristics. They reported that residents in a poverty area had a relative risk of 1.50 for all cause mortality over a nine year period, after adjustment for individual confounders including individual income. (23) More recently Anderson et al also showed an increased risk of all cause mortality in the US for inhabitants in census tract areas with a low median income after adjustment for family income. Other American and British studies have confirmed the relation between contextual characteristics on the small area level on both all cause and cardiovascular mortality. (2,3,12,13,15,21,22) Some studies have reported less favourable profiles of cardiovascular risk factors and increased prevalence of coronary heart disease in socioeconomically deprived areas.(7,2,19) Contextual effects on other outcomes like long term illness have also been published,(4,16,17) and recently Yen an d Kaplan reported results from Alameda county showing an excess risk of developing depressive symptoms and worse perceived health status for poverty area residents. (20)
The social context should be understood as the political, cultural, social and economic environment, which characterise a society. Area deprivation is a concept often used when studying contextual aspects operating at the level of the local community. It is claimed that it "may summarise an area's potential for health risk from ecological exposures such as from the concentration of poverty, unemployment, economic disinvestment, and social disorganisation". (1) Income distribution is another aspect of the social context. Associations between income inequality and mortality have been described in international comparison (24) and within the United States. (15,25-27) One hypothesis of the mechanisms behind this is that psychosocial factors--that is, perceived inequalities mediates the relation (28)--but it has been argued that these results are equally compatible with a mechanism where the actual material conditions of the relatively deprived have aetiological implications. (29) It is shown in the US, that wide income disparities coexist with disinvestment in public infrastructure like education, higher rates of unemployment, welfare dependency and medically uninsured, as well as more homicides and violent crimes. (25) Many of these material conditions are expressed in local communities, offering a link between the analyses of deprivation effects in small areas and income inequality in larger communities.
Recent research has pointed at the need for more focused attention to the meaning of neighbourhood quality and to the development of measures of these aspects. (21) A variety of indices and different methods have been developed to measure the social and material resources in the area. (2,4,21,30,31) Also single variables such as median income (1,3) and number of female headed households, (7) etc, have been used as indicators for the same purpose. Many of the indices have primarily been developed to identify health care needs in areas for resource allocation purposes,(32-34) but have anyway been applied in aetiological studies. (22)
The aim of this study was to analyse the contextual effect of social deprivation on risk of myocardial infarction among men and women separately, considering possible confounding from the corresponding individual social factors (the compositional effect). Another aim was to develop an index identifying and measuring specific socioeconomic contexts of parishes in the Stockholm metropolitan area.
SHEEP is a population based case-referent study of causes of myocardial infarction. (36 37) The study base included all Swedish citizens 45-70 years old who had not experienced myocardial infarction before and who were living in the Stockholm county. The cases were all first events of myocardial infarction that occurred in the studybase during 1992 to 1994. Non-fatal cases were identified through a special organisation at the 10 emergency hospitals. By screening of hospital discharge register missed cases were found. Fatal cases were identified from death certificates. All hospitalised cases were diagnosed according to standardised criteria using information on symptoms, ECG and enzymes. (38) Cases were included at the time of disease incidence. Simultaneously one referent per case was randomly selected from the study base after stratification for age, sex and hospital catchment area. More referents than cases were finally included, because sometimes the referent was already included when the case chose not t o participate. In addition, if a referent at first did not to participate another one was sampled, but sometimes they both ended up participating.
To fully explore the contextual exposure contrast in the whole region we did not want to adjust for hospital catchment area. As hospital catchment area is correlated with parish area characteristics the stratified sampling of referents would introduce confounding, biasing the relative risks towards unity. The sampling effect was corrected by giving the referents in each hospital catchment area weights, according to the proportion of person years in the study base. We did this separately in each age and sex stratum.
In total the SHEEP study included 2246 cases and 3206 controls. The questionnaire response rate among cases was 72% for women and 81% for men, while corresponding figures among controls were 70% and 75%. The subjects responded to the same extent in different age groups and were equally inclined to participate from the different catchment …