The very elderly are hit hardest by seasonal increases in deaths due to acute myocardial infarctions and stroke, new research indicates.
Besides confirming previous studies' findings that people are more likely to die of heart attack or stroke during the winter, Dr. Tej Sheth of the Hamilton General Hospital and colleagues identified a pronounced age effect in the seasonal variations in death rates (J Am Coll Cardiol 1999;33:1916-9), based on a huge database of deaths in Canada. Seniors over age 85 had up to 16% higher rates of MI deaths in winter than in summer, and 19% higher stroke rates.
MI deaths were highest in January and lowest in September. The seasonal variation in MI deaths (winter versus summer) climbed with age: 5.8% in the under-65 age group, 8.3% in the 65-to-74 group, 13.4% in 75-to-84 group and 15.8% for people over 85.
Similarly, stroke mortality peaked in January and was lowest in September. As with MI, seasonal variation in stroke mortality also increased with age. Researchers identified no seasonal variation in people under age 65, but an 11.6% variation in the 65-to-74 group, 15.2% in the 75-to-84 group and 19.3% in those over age 85.
Sheth believes winter-related changes in clotting, immune system activity and blood pressure - all factors that make atherosclerotic plaques unstable and vulnerable to rupture - may be more pronounced in the elderly and lead to a higher incidence of MI. The limited data available suggest that winter's colder temperatures trigger these physiologic changes. "This is only a hypothesis, though," says Sheth. "We will need to test it in careful studies to confirm whether this is the case."
The Canadian-American research team analysed seasonal variations by month and for the 4 seasons. For each of 4 age groups, scientists labelled the magnitude of seasonal variation as the difference in mortality between seasons with highest and lowest frequency of deaths. Researchers looked at 300 000 deaths from the Canadian Mortality Database for the years 1980 to 1982 and 1990 to 1992. It is the largest North American study to date on seasonal mortality.
The findings, says Sheth, open up a new avenue for research into the prevention of cardiovascular disease.