Effect of Socioeconomic Status and Underlying Disease on the Association between Ambient Temperature and Ischemic Stroke

Abstract

Purpose: Inconsistent findings have been reported regarding the effect of ambient temperature on ischemic stroke. Furthermore, little is known about how underlying disease and low socioeconomic status influence the association. We, therefore, investigated the relationship between ambient temperature and emergency department (ED) visits for ischemic stroke, and aimed to identify susceptible populations.

Materials and methods: Using medical claims data, we identified ED visits for ischemic stroke during 2005-2009 in Seoul, Korea. We conducted piecewise linear regression analyses to find optimum ambient temperature thresholds in summer and winter, and estimated the relative risks (RR) and 95% confidence intervals (CI) per a 1°C increase in temperature above/below the thresholds, adjusting for relative humidity, holidays, day of the week, and air pollutant levels.

Results: There were 63564 ED visits for ischemic stroke. In summer, the risk of ED visits for ischemic stroke was not significant, with the threshold at 26.8°C. However, the RRs were 1.055 (95% CI, 1.006-1.106) above 25.0°C in medical aid beneficiaries and 1.044 (1.007-1.082) above 25.8°C in patients with diabetes. In winter, the risk of ED visits for ischemic stroke significantly increased as the temperature decreased above the threshold at 7.2°C. This inverse association was significant also in patients with hypertension and diabetes mellitus above threshold temperatures.

Conclusion: Ambient temperature increases above a threshold were positively associated with ED visits for ischemic stroke in patients with diabetes and medical aid beneficiaries in summer. In winter, temperature, to a point, and ischemic stroke visits were inversely associated.

Keywords: Temperature; cardiovascular diseases; diabetes mellitus; ischemic stroke.

Fig. 1. Relationship of average temperature to daily number of ED visits for ischemic stroke in Seoul, Republic of Korea. (A) All seasons, (B) spring, (C) summer, (D) fall, and (E) winter. ED, emergency department.