Geomagnetic Storms Can Trigger Stroke

Although research linking cardiovascular disorders to geomagnetic activity during the past 40 years is accumulating, robust evidence for the effect of geomagnetic activity (the earth’s magnetic field) on stroke occurrence is lacking and remains a matter of controversy. This is because most studies to date are complicated by significant methodological limitations: high likelihood of stroke misclassification and selection bias (eg, official mortality data are particularly inappropriate for studying determinants of stroke occurrence, or use of hospital-based data, including both incident and recurrent strokes in the analysis, inaccurate data on stroke onset, poor neuroimaging verification of stroke types [there are significant differences in the pathogenesis and determinants of different stroke pathological types]), arbitrary selection of a time lag between exposure and outcome (spurious associations), and small sample sizes (type I error). No reliable data exist on effects of geomagnetic activity on the risk of occurrence of different stroke pathological types in various population groups Background and Purpose—Although the research linking cardiovascular disorders to geomagnetic activity is accumulating, robust evidence for the impact of geomagnetic activity on stroke occurrence is limited and controversial. Methods—We used a time-stratified case-crossover study design to analyze individual participant and daily geomagnetic activity (as measured by Ap Index) data from several large population-based stroke incidence studies (with information on 11 453 patients with stroke collected during 16 031 764 person-years of observation) in New Zealand, Australia, United Kingdom, France, and Sweden conducted between 1981 and 2004. Hazard ratios and corresponding 95% confidence intervals (CIs) were calculated. Results—Overall, geomagnetic storms (Ap Index 60+) were associated with 19% increase in the risk of stroke occurrence (95% CI, 11%–27%). The triggering effect of geomagnetic storms was most evident across the combined group of all strokes in those aged <65 years, increasing stroke risk by >50%: moderate geomagnetic storms (60–99 Ap Index) were associated with a 27% (95% CI, 8%–48%) increased risk of stroke occurrence, strong geomagnetic storms (100–149 Ap Index) with a 52% (95% CI, 19%–92%) increased risk, and severe/extreme geomagnetic storms (Ap Index 150+) with a 52% (95% CI, 19%–94%) increased risk (test for trend, P<2×10). Conclusions—Geomagnetic storms are associated with increased risk of stroke and should be considered along with other established risk factors. Our findings provide a framework to advance stroke prevention through future investigation of the contribution of geomagnetic factors to the risk of stroke occurrence and pathogenesis. (Stroke. 2014;45:1639-1645.)