'Tis the season: Colder temps hurt hearts. Credit: istockphoto

When winter comes, it usually brings more than just snow. The frigid temperatures coincide with a predictable uptick in colds and influenza. Lately, though, researchers are finding some seasonal patterns in conditions one wouldn't necessarily expect to be tied to the calendar.

In August, scientists reported new insights into the seasonable variability of multiple sclerosis. They had examined 939 brain scans taken from 44 individuals with multiple sclerosis over a period of three years. The scans used magnetic resonance imaging to measure edema as an indicator of new lesions in the brain. The researchers discovered that between March and August, the subjects suffered brain lesions at a rate that was, on average, two to three times higher than during the rest of the year (Neurology 75, 799–806, 2010).

“It peaks in spring and then summer, and then falls off very rapidly, with much less activity in fall and winter,” says Dominik Meier, the lead author of the paper and a radiologist at Brigham and Women's Hospital in Boston.

Meier says there's a lot of speculation as to what environmental factors could contribute to the seasonal pattern. It's thought that stores of vitamin D from summer sun exposure may be depleted by the time spring rolls around. Vitamin D affects the immune system and may dampen the autoimmune response thought to drive neurodegeneration in multiple sclerosis.

It's not just disease symptoms that vary seasonally; responses to treatment have been shown to vary throughout the year as well. A clinical trial of more than 1,200 patients given chemotherapy treatments for breast cancer, conducted across several regions of Spain, reported in August that adverse reactions to the treatment—such as weaknesses and elevated levels of transaminases, which can be a sign of liver damage—occurred more frequently in spring and summer and in warmer climates overall (Breast Cancer Res. Treat. doi:10.1007/s10549-010-1136-0, 2010).

Sometimes it is variability within the climate, irrespective of the absolute temperatures, that's the deciding factor. Such is the case with heart attacks, according to a recent study from researchers at the London School of Hygiene and Tropical Medicine (BMJ 341, c3823, 2010). They obtained the records for more than 84,000 hospital admissions for heart attacks across England and Wales and compared the data to local weather reports. They found that a drop in temperature by just 1 degree Celsius increased the overall risk of heart attack by 2%, or about 200 extra heart attacks on that day.

“People were unsure whether it was the cold itself or the reduction in temperature,” says Liam Smeethe, the study's senior author. “But it didn't matter what temperature you started at. The increase in heart attacks was consistent.” Smeethe says these results could help hospitals to prepare for an influx of patients by simply paying attention to the weather forecast.