Mediation analyses of the mechanisms by which socioeconomic status, comorbidity, stroke severity, and acute care influence stroke outcome

BACKGROUND AND OBJECTIVES: Low socioeconomic status (SES) is associated with increased risk of death and disability after stroke, but interventional targets to minimize disparities remain unclear. We aim to assess the extent to which SES-based disparities in the association between low SES and death and dependency at three months after stroke could be eliminated by offsetting differences in comorbidity, stroke severity, and acute care. METHODS: This nationwide register-based cohort study included all 72 hospitals caring for patients with acute stroke in Sweden. All patients registered with an acute ischemic stroke in the Swedish Stroke Register in 2015-2016 who were independent in activities of daily living (ADL) at the time of stroke were included. Data on survival and SES the year before stroke were retrieved by cross-linkage with other national registers. SES was defined by education and income, and categorized into low, mid, and high. Causal mediation analysis was used to study the absolute risk of death and ADL-dependency at 3 months depending on SES, and to what extent hypothetical interventions on comorbidities, stroke severity, and acute care would equalize outcomes. RESULTS: Of the 25,846 patients in the study, 6,798 (26.3%) were dead or ADL-dependent three months after stroke. Adjusted for sex and age, low SES was associated with an increased absolute risk of 5.4% (95% CI: 3.9%-6.9%; p<0.001) compared to mid SES, and 10.1% (95% CI: 8.1%-12.2%; p<0.001) compared to high SES. Intervening to shift the distribution of all mediators among patients with low SES to those of the more privileged groups would result in absolute reductions of these effects by 2.2% (95% CI: 1.2%-3.2%; p<0.001), and 4.0% (95% CI: 2.6%-5.5%; p<0.001), respectively, with the largest reduction accomplished by equalizing stroke severity. DISCUSSION: Low SES patients have substantially increased risks of death and ADL-dependency three months after stroke compared to more privileged patient groups. This study suggests that if we could intervene to equalize SES-related differences in the distributions of comorbidity, acute care, and stroke severity, up to 40 out of every 1000 patients with low SES could be prevented from dying or becoming ADL-dependent