Seasonal variation in incidence of acute myocardial infarction in a sub-Arctic population: the Tromsø Study 1974-2004.

Background: A seasonal pattern with higher winter morbidity and mortality has been reported for acute myocardial infarction (MI). The magnitude of the difference between peak and nadir season has been associated with latitude, but results are inconsistent. Studies of seasonal variation of MI in population-based cohorts, based on adjudicated MI cases,are few.We investigated the monthly and seasonal variation in first-ever nonfatal and fatal MI in the population of Tromsø in northern Norway, a region with a harsh climate and extreme seasonal variation in daylight exposure.

Design: Prospective population-based cohort study.

Methods: A total of 37 392 participants from the Tromsø Study enrolled between 1974 and 2001 were followed throughout 2004. Each incident case of MI was validated by the review of medical records and death certificates. MI incidence rates for months and seasons were analyzed for seasonal patterns with Poisson regression and the Cosinor procedure. All analyses were stratified by sex, age and smoking status.

Results: A total of 1893 first-ever MIs were registered, of which 592 were fatal. There was an 11 % (95% confidence interval: 1.00-1.23, P=0.04) increased risk of incident MI during winter (November-January) compared with non-winter seasons, with no statistically significant interaction with sex, age, smoking or calendar year. Other seasonal modelling gave similar but not statistically significant results.

Conclusion: We found a small increase in risk of incident MI during the darkest winter months. Populations living in sub-Arctic areas may be adapted to face climate exposure during winter through behavioural protection.