Risk of ischemic and hemorrhagic stroke in relation to cold spells in four seasons

Cold winter weather increases the risk of stroke, but the evidence is scarce on whether the risk increases during season-specific cold weather in the other seasons. The objective of our study was to test the hypothesis of an association between personal cold spells and different types of stroke in the season-specific context, and to formally assess effect modification by age and sex. We conducted a case-crossover study of all 5396 confirmed 25-64 years old cases with stroke in the city of Kaunas, Lithuania, 2000-2015. We assigned to each case a one-week hazard period and 15 reference periods of the same calendar days of other study years. A personal cold day was defined for each case with a mean temperature below the fifth percentile of the frequency distribution of daily mean temperatures of the hazard and reference periods. Conditional logistic regression was applied to estimate odds ratios (OR) and 95% confidence intervals (95% CI) representing associations between time- and place-specific cold weather and stroke. There were positive associations between cold weather and stroke in Kaunas, with each additional cold day during the week before the stroke increases the risk by 3% (OR 1.03; 95% CI 1.00-1.07). The association was present for ischemic stroke (OR 1.05; 95% CI 1.01-1.09) but not hemorrhagic stroke (OR 0.98; 95% CI 0.91-1.06). In the summer, the risk of stroke increased by 8% (OR 1.08; 95% CI 1.00-1.16) per each additional cold day during the hazard period. Age and sex did not modify the effect. Our findings show that personal cold spells increase the risk of stroke, and this pertains to ischemic stroke specifically. Most importantly, cold weather in the summer season may be a previously unrecognized determinant of stroke.