Is high socioeconomic status a risk factor for multiple sclerosis?

Multiple sclerosis (MS) is a chronic disease of the central nervous system characterised by inflammation and neurodegeneration. An increased risk of the disease among those of high socioeconomic status (SES) was first observed over 50 years ago. This is in contrast to a more common pattern whereby adverse health outcomes are generally associated with low SES. Most MS risk factors, such as smoking, obesity, and a late age of EBV infection, vary in their prevalence by SES, and thus all provide pathways through which SES could affect disease risk. Alternatively, stress-related immune changes linked to SES could influence disease susceptibility. To establish the strength and nature of the association between SES and MS, two studies were performed as part of this manuscript-based thesis.The first manuscript is a systematic review of published cohort and case-control studies that examined the association between SES and MS risk. 21 articles were included. 5 studies, all from countries with higher levels of income inequality, reported an association between high SES and increased MS risk. 13 studies reported insufficient evidence of an association, and 2 studies reported an association with low SES; these largely came from more egalitarian nations. Few studies adequately controlled for all important mediators and confounders, precluding clear conclusions about the nature of the SES-MS association.The second manuscript is an original analysis of the association between SES and MS, using data from the multinational Environmental Risk Factors in MS (EnvIMS) case-control study. The study population comprised 2,144 cases and 3,859 controls, from Norway, Canada, and Italy. Multiple logistic regression was used to evaluate the association between SES and MS, with SES measured by parental education level. Analyses were adjusted for age, sex, sunlight exposure, history of infectious mononucleosis, smoking, obesity, and family size. In Canada, the OR (95% CI) for MS among individuals with university-educated parents relative to those whose parents had primary school education or below was 1.47 (1.03-2.09), with a statistically significant dose-response relationship across education levels (p for trend = 0.029). In Norway, this association was only present for those who grew up during a period of rising inequality (p for trend = 0.031). No evidence for an association was found in Italy.These two studies provide only partial support for an association between high SES and increased MS risk. Differing ages of EBV infection by social class, or stress-related immune changes linked to SES, are both possible explanations for the findings.

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