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(Stroke. 2000;31:2093.)
© 2000 American Heart Association, Inc.
Original Contributions |
From the Department of Public Health (C.L.H.) and West of Scotland Cancer Surveillance Unit, Department of Public Health (D.J.H.), University of Glasgow, Glasgow, UK, and the Department of Social Medicine (G.D.S.), University of Bristol, Bristol, UK.
Correspondence and reprint requests to Carole Hart, Department of Public Health, University of Glasgow, 1 Lilybank Gardens, Glasgow G12 8RZ, UK. E-mail c.l.hart{at}udcf.gla.ac.uk
| Abstract |
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MethodsThe analysis was based on a large cohort study of 5765 working men, from 27 workplaces in Scotland, who were screened between 1970 and 1973. Stroke was defined as having a hospital admission with a main diagnosis of stroke or dying of stroke in the 25-year follow-up period.
ResultsThere were 416 men who had a stroke. Men with manual occupations when screened, on first entering the workforce, men with manual occupations, and men whose fathers had manual occupations had significantly higher rates of stroke than men in the nonmanual categories. Men who left full-time education at age 16 years or under also had significantly higher rates of stroke. Men living in more deprived areas had higher rates of stroke, but the rates were not statistically significant. The most marked difference was in relation to fathers social class, and although adjusting for risk factors for stroke attenuated the relative rates, men whose fathers were in manual social classes had higher relative rates of stroke than men whose fathers were in nonmanual classes (adjusted relative rate for fathers social class III manual was 1.37 [95% CI 1.03 to 1.81] and for fathers social class IV or V was 1.46 [1.09 to 1.96]). Men who were upwardly mobile (fathers social class manual, own social class nonmanual) had a rate of stroke similar to that of stable manual men.
ConclusionsPoorer socioeconomic circumstance was associated with greater stroke risk, with adverse early-life circumstances of particular importance.
Key Words: cerebrovascular disorders epidemiology prospective studies social class
| Introduction |
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65
years were caused by stroke in 1997.3 Scotland also had
higher stroke mortality rates than the rest of the United
Kingdom.3 Stroke morbidity is a leading cause of
disability, particularly among the elderly, so reducing the burden of
stroke has implications for quality of life as well as for
health-service planning.4 Socioeconomic differences in stroke risk have been seen in many countries,5 but it is not clear whether these differences can be accounted for by differences in risk factors. In the present study we present an analysis of stroke risk in a large, prospective cohort study of employed men in Scotland, which recorded several socioeconomic measures. In particular, occupational social class in adulthood and in childhood (as measured by fathers main occupation) were available. Members of the cohort underwent screening in the early 1970s, and a large number of risk factors for stroke were measured. Previous findings from this cohort, using 21 years of mortality, highlighted deaths from stroke and stomach cancer as being particularly related to socioeconomic circumstances during childhood.6 We now report on follow-up data for stroke deaths and hospital admissions for stroke over a 25-year period.
| Subjects and Methods |
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The physical examination included measurement of blood pressure, height, forced expiratory volume in 1 second (FEV1) and a 6-lead ECG. The questionnaire collected information about smoking, alcohol consumption, angina (from the Rose angina questionnaire),8 age leaving full-time education, home address, and occupation.
Blood pressure was measured with the subject seated, and diastolic pressure was recorded at the disappearance of the fifth Korotkoff sound. The adjusted FEV1 was defined as the actual FEV1 as a percentage of the expected FEV1 (obtained from a linear regression of age and height from a healthy subset of the study).7 A 6-lead ECG recording was made with the subject seated. The ECG was coded according to the Minnesota system, with the codes 1.1 to 1.3, 4.1 to 4.4, 5.1 to 5.3, and 7.1 considered evidence of ischemia, encompassing diagnoses of definite myocardial infarction, myocardial ischemia, and left bundle branch block.9 10 Angina was considered present if the definite or possible criteria of the Rose angina questionnaire were met.11 Severe chest pain was defined as a participants admitting to ever experiencing a severe pain across the front of the chest lasting half an hour or more.11 Preexisting CHD was defined as a participants having angina, ECG-documented ischemia, or severe chest pain. Units of alcohol consumed per week were calculated from responses to the questionnaire about usual weekly consumption of beer, spirits, and wine.12
The home address at the time of screening was retrospectively assigned a postal code, which enabled a deprivation category, as defined by Carstairs and Morris,13 to be ascertained. This measure is an area-based measure of deprivation, obtained from 4 census variables: male unemployment, overcrowding, car ownership, and the proportion of heads of households in social classes IV and V. A deprivation score for each postal code sector is obtained, which is then converted to 7 categories ranging from 1 (least deprived) to 7 (most deprived).
The questionnaire asked for the main occupation of the participants
father, the participants own first regular occupation (excluding
temporary work), and the participants occupation at the time of
screening. Social class was coded according to the Registrar Generals
Classification14 for each of the 3 occupations. For this
analysis, social class was defined as either nonmanual (classes
I, II, and III nonmanual) or manual (classes III manual, IV and V) for
each of the 3 occupations. Fathers social class was subsequently also
defined in 3 groupings (nonmanual, III manual, IV and V). Age leaving
full-time education was categorized as >16 years or
16 years.
Deprivation category was defined as 1 to 4 (high) or 5 to 7
(low). Social mobility from fathers to own social class was
analyzed in 4 categories both nonmanual (stable nonmanual),
father nonmanual and own class manual (downwardly mobile), father
manual and own class nonmanual (upwardly mobile) and both manual
(stable manual).15
The analysis was based on 5765 men aged between 35 and 64 years at screening, who had not embarked from Britain during the follow-up period. Data were missing for the following categories: 9 men for social class, 87 men for first social class, 112 men for fathers social class, 8 men for age leaving full-time education, and 9 men for deprivation. Missing data were excluded from the relevant analysis.
Study participants were flagged at the National Health Service Central Register in Edinburgh. Dates of death up to the end of 1998 and their causes were provided. In addition, a computerized link with acute hospital discharges in Scotland provided records of all main diagnoses of stroke between 1972 and 1998.16 Stroke was defined as either having a hospital admission with a main diagnosis of stroke in the 25-year follow-up period after screening or dying of stroke in the 25-year follow-up period. Stroke was defined as International Classification of Diseases (ICD)-8 or ICD-9 codes 430 to 438, and as ICD-10 codes I60 through I69 and G45. Coxs models17 were used to calculate proportional hazards regression coefficients for each socioeconomic variable separately (social class, first social class, fathers social class, age leaving full-time education, and deprivation category). The exponentiated proportional hazards regression coefficients are referred to as relative rates. Survival time was taken from the date of screening until either the date of hospital admission for stroke or the date of death from stroke if no hospital admission for stroke was found. Adjustments were made for risk factors related to stroke risk by including them in the models.18
| Results |
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Men with manual occupations when screened, men with manual occupations
on first entering the workforce, and men whose fathers had manual
occupations all had significantly higher relative rates of stroke than
men in the nonmanual categories (Table 1
). Men who left full-time education at
16 years or under had a significantly higher relative rate of stroke
than men who left at older ages. Men living in the most deprived areas
had a nonsignificantly raised relative rate of stroke compared with men
living in more affluent areas. Exclusion of men with preexisting
coronary disease did not affect the results. For own social
class, first social class, age leaving full-time education, and
deprivation categories, the relative rates were
non-significant when adjusting for other risk factors for stroke. For
fathers social class, the relative rate for manual compared with
nonmanual remained significant when adjustments were made for other
risk factors. A more detailed analysis by fathers social
class in 3 categories was undertaken. The 3 categories, chosen to be of
adequate size, were (1) nonmanual (I, II, and III nonmanual), (2) III
manual, and (3) IV and V. Table 2
presents the age-adjusted relative rates of stroke by fathers
social class and shows the effect of adjusting for risk factors
individually and, finally, together. Men with fathers in the 2 manual
social classes had significantly higher rates of stroke than men with
fathers in nonmanual classes when adjusted for age only. Men with
fathers in social classes IV and V had the highest rate. Adjustment for
each risk factor individually attenuated the relative rates, with
height and systolic blood pressure being the only ones having a
substantial effect. Adjustment for all the risk factors attenuated the
relative rates considerably, although both manual categories remained
at significantly higher risk than the nonmanual category. The trend
across the 3 fathers social class groups also remained significant at
statistically conventional levels (P<0.05). The survival
curves for the 3 groups are shown in the Figure
.
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Men whose fathers were in manual social classes and whose own
social class was manual (stable manual) had almost double the rate of
having a stroke than men in the stable nonmanual group when adjusted
for age only (Table 3
). The fairly large
upwardly mobile group also had a significantly increased rate (1.60
[95% CI 1.16 to 2.21]). The rate of the downwardly mobile group was
not significantly different from that of the stable nonmanual group,
but since the number of events was small, this could have resulted from
chance. Adjusting for all the risk factors showed the stable nonmanual
and the downwardly mobile groups to have a similar rate of stroke in
contrast to the upwardly mobile and the stable manual groups, which had
an almost-identical higher rate of stroke. This would suggest that it
is fathers social class which has the strongest influence on stroke
risk, and this risk is not dependent on social class obtained in
adulthood.
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| Discussion |
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Other studies have found relationships with adulthood social class. In an international overview, Kunst et al5 found that stroke mortality was higher in manual than nonmanual men in all countries investigated (European countries and the United States). They also found that socioeconomic inequalities were substantially larger for stroke than for CHD mortality. The Whitehall study19 of male civil servants in the London area found an inverse relationship between occupational grade and age-adjusted stroke mortality. Men whose own occupation was manual in the British Regional Heart Study had almost twice the risk of fatal or nonfatal stroke as men in nonmanual social classes, adjusting for age only.20 Additional adjustment for systolic blood pressure and smoking accounted for half the excess. In an Australian study,21 men in manual occupations were 60% more likely to die of stroke than men in professional occupations. The Rotterdam study of elderly women found that with socioeconomic status defined as the occupation of the head of the household, professionals had a significantly lower risk of stroke than manual workers.22 High socioeconomic groups, defined by education and household income, had the lowest stroke risk.22 Adjustments for risk factors had small effects.
In another study in the West of Scotland (the Renfrew/Paisley study23 ), we found that stroke (again defined as hospital admission for stroke or death from stroke) was strongly related to adulthood social class for men and women. Adjustment for risk factors accounted for most of the difference between social classes.
There have been fewer studies of stroke and deprivation category. Gradients in stroke mortality between the most affluent and the most deprived have been described for Scotland.13 An ecological study in England24 found significant positive correlations between deprivation and stroke mortality at ward level. An ecological analysis of the 22 districts of the Scottish Heart Health Study found that hospital admission rates for stroke were significantly associated with deprivation score.25 In the Renfrew/Paisley study,23 deprivation category was strongly related to stroke risk in men and women. Adjustment for risk factors explained some, but not all, of the difference between the stroke risk for men and women living in affluent and deprived areas. However in the current study, deprivation category was the weakest of all the socioeconomic factors investigated.
Fathers social class, taken to be an indicator of childhood socioeconomic circumstances, is measured less frequently than adult social class in cohort studies. We previously showed6 that adverse socioeconomic circumstances in childhood have a specific influence on mortality from stroke and stomach cancer. Other causes of death were influenced by both childhood and adulthood circumstances (CHD and respiratory disease mortality), or predominantly by adulthood circumstances (lung cancer, other cancer, and accidents and violence). With 25 years of mortality follow-up, together with 25 years of information on hospital admissions for stroke, we have now shown that fathers social class is the strongest of the socioeconomic indicators for stroke risk. Adjustment for risk factors for stroke explained some of the difference in risk of stroke between nonmanual and manual fathers social classes, but significantly elevated risks remained for men with fathers in manual social classes. We found that men who were upwardly mobile (fathers social class manual, adult social class nonmanual) had risks similar to those of men who remained in manual social classes. Improving ones social position does not, therefore, appear to greatly improve ones stroke risk. This emphasizes the strong influence of early-life socioeconomic circumstances on stroke risk.
No association was seen between fathers social class and prevalence of nonfatal stroke in the British Regional Heart Study, but this related only to those who had survived to 1992, so selective survival differences may have influenced the findings.26 No association was seen between fathers social class and fatal and nonfatal stroke in a large cohort of middle-aged female nurses27 ; however, due to selection into this profession, nurses with manual and nonmanual social class backgrounds will not be representative of the source population in ways that could distort underlying associations. A strong association was seen between fathers social class and stroke mortality in the Boyd Orr cohort,28 and adjustment for the Townsend deprivation index of area of residence in adulthood did not affect this relationship.
All of the risk factors could explain some variation in stroke risk by
fathers social class (Table 2
). Adjustment for height produced
the greatest decrement in risk, and this suggests the involvement of
early-life factors, since poor childhood circumstances (for example,
lack of food and childhood illness) can lead to reduced stature in
adulthood. Other studies have shown inverse relationships of height
with stroke mortality or events.29 30 31 Blood pressure is
the most important identified risk factor for stroke, but the
differences in blood pressure according to childhood social
circumstances are relatively small: <2 mm Hg difference between
men with fathers in social classes I and II and those with fathers in
social classes IV and V.32 Even accepting that this
adulthood measure is only a proxy for lifetime blood pressure, it is
unlikely that residual confounding by blood pressure could explain the
association between fathers social class and stroke risk. Blood
pressure will also proxy for unmeasured nutritional factors (such as
salt consumption) that may influence stroke risk through blood
pressure. Similarly, adjusting for cholesterol did not
affect the association; so, to the degree to which dietary factors are
indexed by blood lipids or blood pressure, it is unlikely that such
nutritional factors generate the association between early-life
deprivation and stroke. Clearly, unmeasured nutritional
factors that are not related to blood lipids or blood pressure could be
involved, with antioxidants being an example of these. Several other
potential mechanisms could relate childhood deprivation to
later stroke risk.33 Low birthweight is strongly socially
patterned and also predicts adulthood stroke,34 which
suggests that suboptimal fetal development increases susceptibility.
Poor growth in childhood may also predispose to adulthood stroke
risk,35 as could chronic infections acquired in childhood.
Limitations of this study, as in similar prospective cohort studies,
are that risk factors and socioeconomic measures were measured at
baseline and may have changed during the follow-up period.
To conclude, we have shown that poor socioeconomic experience is associated with increased risk of stroke. Adverse circumstances in early life are particularly important, and adulthood behavioral risk factors such as smoking and alcohol consumption can explain only some of the difference in stroke risk. Improving ones social position does not remove the effect of adverse social circumstances in childhood on stroke risk. If we want to reduce both overall levels of stroke and socioeconomic differentials in stroke in the future, we must recognize the importance of providing the best environment in which children can grow up.
| Acknowledgments |
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Received March 27, 2000; revision received June 9, 2000; accepted June 9, 2000.
| References |
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