Researchers also say warmer temperatures and solar radiation also seem to be linked to increased activity in MS patients.
Scientists at Brigham and Women's Hospital in Boston and Washington University in St. Louis compared magnetic resonance imaging (MRI) scans of 44 people with untreated MS, taken from 1991 to 1993, to weather data from the same time period.
Each person had eight weekly scans, then eight scans every other week, followed by six monthly checkups. The average number of MRI scans was 22 per person.
The scientists examined daily temperatures, solar radiation, and precipitation measures for the Boston area.
After a year, 310 new lesions that cause MS symptoms were found in 31 patients. No new lesions were found in 13 participants in the study.
"Our results showed that the appearance of lesions on brain scans was two to three times higher in the months of March to August, compared to other months of the year," study researcher Dominik Meier, PhD, of Brigham and Women's Hospital, says in a news release. "Not only were more lesions found during the spring and summer seasons, our study also found that warmer temperature and solar radiation were linked to disease activity."
No connection was found between brain lesions and precipitation.
The study is published in Neurology, the medical journal of the American Academy of Neurology.
Role of Environment in MS
Anne Cross, MD, of the Washington University School of Medicine in St. Louis, says in an accompanying editorial that the Meier study is important because it analyzes records from the early 1990s, before medications were approved for relapsing MS patients, "so medicines likely could not affect the outcome."
Cross says future studies should explore how and why environmental factors play a role in disease activity in MS patients.
Meier and colleagues conclude that their study "documents evidence of a strong seasonal pattern" in MS activity.
"Our results agree with seasonality of clinical variables from studies in Japan, Sweden, and the United States," the researchers say. "Relapsing and progressive MS exhibited different seasonal patterns, with peak prevalence shifting toward spring for progressive MS."
Two of Meier's study colleagues at Washington University disclose pharmaceutical company support: K.E. Balashov, MD, from Bayer Schering Pharma, and H.L. Weiner, MD, from several drug companies.