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.
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 nonwinter 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.
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