To our knowledge, this is the first study in Japan to investigate the association of education and household income with CIMT. The current cross-sectional study showed that educational level, but not household income, is independently inversely related to both the prevalence of carotid wall thickening and the maximum CIMT in participants aged 70 years or older who are free of self-reported clinical cardiovascular disease. No measurable associations were observed between education or household income and carotid wall thickening or maximum CIMT in all age groups considered together, in participants younger than 60 years, or in those aged 60 to 69 years.
A cross-sectional study of 4524 African Americans (mean age 54 years, 64% female) showed that education, but not income, was significantly inversely associated with CIMT . In a prospective study of 1402 US women aged 42 to 52 years at baseline, no relationships were found between education or income over 12 years and CIMT . In a cross-sectional study of 5474 older French persons (mean age 73 years, 63% female), education was significantly inversely related to CIMT in men but not in women . In a Finnish cohort study, at baseline, there was no association between education and CIMT among 1813 subjects aged 24–39 years, but education was significantly inversely associated with changes in CIMT over 6 years . A cross-sectional study of 2042 US subjects (mean age 68 years, 57% female) showed significant inverse associations of education and income with CIMT . In the Multi-Ethnic Study of Atherosclerosis conducted in the USA, a significant inverse relationship was observed between education and CIMT among White participants (n = 2624), but not among Chinese (n = 803), Black (n = 1895), or Hispanic (n = 1492) participants, while there were no associations between income and CIMT regardless of race/ethnicity . A cross-sectional study of 4176 Swedish participants aged 46–68 years (59% female) found that, while education was not related to CIMT in men or women, it was significantly inversely associated with carotid stenosis (reduction of the luminal diameter by 15% or greater) in women but not in men . A significant inverse relationship was found between education, but not income, and CIMT in a cross-sectional study of 1140 Finnish participants . No association was shown between education and CIMT in a cross-sectional study of 3703 subjects from five European countries (median age 64 years, 52% female) each subject having at least three vascular risk factors but free from previous cardiovascular and cerebrovascular events . The current results are in partial agreement with these findings.
The mechanisms underlying the observed inverse association between education and CIMT only among those aged 70 years or older are likely numerous. Compared with income, education level is likely to be the more stable aspect of socioeconomic status because it is typically established at an early age and tends to remain the same over time . This might explain the inverse association between education level and CIMT, especially in those aged 70 years or older, through better health awareness in more educated individuals. As Table 2 shows, the proportion of low educational levels increased with age while the proportion of high educational levels decreased with age; the proportions of participants with low and high education levels were 2.1% and 58.9%, respectively, among those under 60 years of age; 15.2% and 32.6%, respectively, among those aged 60 to 69 years; and 30.5% and 19.7%, respectively, among those aged 70 years or older. The fact that each generation of residents of Japan has found it easier to achieve higher educational levels due to the nation’s economic growth might account for the weakness of the association between educational level and health findings at younger ages. The relative rarity of a high educational level among more aged persons might explain the stronger inverse association in this group, leading to our finding that the inverse association between education and CIMT was significant only among those aged 70 years or older. Alternatively, the observed inverse association might simply be a chance phenomenon.
The current study had methodological advantages in that participants were homogeneous with respect to their residential area, in that we used an automated onscreen carotid ultrasound system, and in that we adjusted for a variety of potential confounding factors.
Some weaknesses of the present study should be borne in mind. The present study design was cross-sectional; thus, our results should not be interpreted as a cause-effect association. Selection bias was unavoidable. The participation rate must have been low; additionally, the participation rate could not be estimated because the exact number of eligible subjects was not available. The present study subjects were probably not representative of the Japanese general population. For example, the educational levels of the present subjects were higher than those of the general population. According to a population census conducted in 2010 in Ehime Prefecture , the proportions of persons aged 60−69 years with low, medium, and high educational levels and an unknown educational level were 28.2%, 48.6%, 19.0%, and 4.2%, respectively, in men and 26.7%, 56.4%, 12.9%, and 4.0%, respectively, in women. The corresponding figures in the present study for persons aged 60−69 years were 13.2%, 52.7%, 34.1%, and 0.0%, respectively, in men and 16.3%, 51.9%, 31.8%, and 0.0%, respectively, in women.
We have no data on the validity of self-reported information on education and household income, but any potential non-differential exposure misclassification would result in a bias towards the null. A potential limitation of the automated onscreen carotid ultrasonography device used in the present study is the lack of quantified plaque present in the images. CIMT and plaque are phenotypically distinct findings that both indicate increased vascular risk, yet CIMT without plaque remains a significant marker of increased risk of vascular events and significantly predicts plaque occurrence . The lack of a significant inverse relationship between education and carotid wall thickening or maximum CIMT among the overall study subjects might be ascribed to insufficient statistical power. Residual confounding effects could not be overcome although several confounding factors were controlled for.