A new US study that compared brain scans of people with multiple sclerosis to weather data over a two year period found that disease activity varied with the seasons, with spring and summer months showing predominantly the highest rates of activity, but with increased temperature and solar activity also showing a strong link. The researchers said designers of drug trials that use brain scans to measure results should also consider the possible influence of seasonal effects.

The study, by co-author Dr Dominik Meier, of Brigham and Women's Hospital in Boston, and colleagues, is published in the 31 August issue of Neurology.

Meier told the press that:

"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."

For the study, Meier and colleagues used data taken between 1991 and 1993 from 44 people aged 25 to 52 with untreated MS who were living in Boston.

The participants underwent a total of 939 MRI brain scans in that period, with an overall average of 22 scans each. At first, each participant had one scan a week for eight weeks, then eight scans every other week, and after that, one follow up scan every six months.

The researchers used the brain scan results to compare the likelihood and intensity of disease activity to time of year (season) and regional weather data from the same period, including daily temperature, solar radiation and rainfall.

The brain scan results showed that after one year, there was a total of 310 new lesions in 31 of the participants, while scans for the other 13 participants showed no new lesions.

Meier said that not only did they find more new lesions during the spring and sumer seasons, their study also found that warmer temperatures and solar radiation were linked to disease activity, while no such link was found between precipitation and new lesions.

He and his colleagues concluded that:

"The observed seasonality in MS disease activity has implications for trial design and therapy assessment. The observed activity pattern is suggestive of a modulating role of seasonally changing environmental factors or season-dependent metabolic activity."

In an editorial in the same issue of the journal, Dr Anne Cross from the Washington University School of Medicine in St. Louis, and a member of the American Academy of Neurology, stressed this study was important because it used data from the early 1990s, before medications for relapsing MS were approved, so drugs could not account for the results.

"A study like this probably won't be able to be repeated," said Cross.

"Future studies should further explore how and why environmental factors play a role in MS," she added.

This study also raises an important question for research in general: should designers of clinical trials that use MRI brain scans be more aware of seasonal effects?

Clinical trials that use MRI brain scans to assess drug effectiveness often last between 6 and 12 months. If they ran from spring to winter, and the results showed lesions lessening over the period, the investigators might conclude this was because of the drug, whereas it could just be a seasonal effect. Conversely, a trial that ran from winter to spring and summer, might have the opposite pattern, which could also be due to seasonal influences and not necessarily the drug.