The study in brief
This study aimed to study a large number of people from around the world to see if the number of relapses varied by season.
Using the information in the MSBase registry the researchers identified 32,762 relapses from 9,811 people with MS which were included in the study. The researchers also obtained data on the daily average UV radiation (an indicator of the amount of sunlight) for each month for each of the locations included in the study.
The study found that for both the northern and southern hemispheres relapses were most common in the spring and least common in the autumn. The study also identified a pattern related to latitude (distance away from the equator). Every 10 degree increase in latitude away from the equator resulted in the peak in the number of relapses in that area moving one month closer to the date of the lowest levels of sunlight.
The study showed a relationship between season and amount of sunlight and risk of relapse, but as it was an observational study it could not answer the question of what causes this relationship. The researchers suggest several theories as to what could be causing the number of relapses to vary by season.
These included vitamin D as levels tend to be at their highest after the summer, so may be protecting people from relapse over the autumn and as their levels fall through the winter, relapses become more likely in the spring. They also suggest it could be related to an infection, such as colds, which could be triggering relapses. However the researchers do note that the number of colds usually peak in the winter so this does not match the pattern for relapses being more common in the spring. The researchers suggest further studies are needed to test their theories.
The study in detail
Several previous studies have looked at the seasonal variation of relapses but have there have been conflicting results. Some have found a seasonal pattern, with relapses being more common at particular times of the year, and others have found no pattern. Most of these previous studies have looked at a small number of people or have studied people living in one area. This study aimed to study a large number of people from around the world to see if the number of relapses varied by season.
How this study was carried out
The study used information from the MSBase registry. This is a database that collects information from clinics around the world, including information on relapses, such as date, region of the body affected and corticosteroid treatment.
Using the MSBase information the researchers identified 32,762 relapses from 9,811 people with MS which were included in the study. 80% of the participants were diagnosed with relapsing remitting MS and 20% with secondary progressive MS. 89% of the relapses were from the 8,411 people with MS living in the northern hemisphere and the remaining 11% of relapses assessed were from the 1,400 people with MS in the southern hemisphere.
The researchers also obtained data on daily average UV radiation (an indicator of the amount of sunlight) for each month for each of the locations included in the study.
The study used this information to examine the dates that relapses occurred and the latitude at which each person experiencing a relapse was located. In the analysis age, EDSS score, DMT treatment and age at onset of MS were all also taken into account.
What was found
The study found that for both the northern and southern hemispheres relapses were most common in the spring and least common in the autumn.
The study also identified a pattern related to latitude (distance away from the equator). As day length gets shorter there is less sunlight, in the areas the furthest away from the equator where the days are the shortest in the winter, there was a shorter time period between the lowest seasonal level of sunlight and a peak in the number of relapses. Every 10 degree increase in latitude away from the equator resulted in the peak in the number of relapses in that area moving one month closer to the date of the lowest levels of sunlight.
What does it mean?
The study showed a relationship between season and amount of sunlight and risk of relapse.
The authors suggest several reasons that could explain why they found this relationship:
- The first was levels of Vitamin D, as this is produced in the skin when it is exposed to sunlight. Vitamin D levels tend to be at their highest after the summer, so may be protecting people from relapse over the autumn and as their levels fall through the winter relapses become more likely in the spring.
- The second was possibly some other, currently unidentified, effect that sunlight has on the immune system, which is not related to levels of vitamin D.
- Finally they suggested that infections such as colds, could be triggering relapses, however the researchers do note that the number of colds usually peak in the winter so this does not match the pattern they identified for relapses being more common in the spring.
As the study is based on observations it can only highlight a relationship and does not answer the question of what causes this relationship. The researchers suggest further studies are needed which measure levels of vitamin D, track other infections and measure levels of skin sunlight exposure so these can be analysed alongside the relapse information to test their theories.
Several studies which look at the effect of vitamin D on MS disease course are already underway.
Spelman T, Gray O, Trojano M, et al.
Seasonal variation of relapse rate in multiple sclerosis is latitude-dependent..
Ann Neurol. 2014 Oct 4. doi: [Epub ahead of print]
More about relapses
People call relapses by different names, some people refer to them as attacks, flare ups or episodes. A relapse in MS is defined as a sudden onset of symptoms or disability, which must last for at least 24 hours, but more commonly they go on for a number weeks. To be considered a new relapse, it must occur at least 30 days after the start of a previous episode and not be caused by infection or other cause.
Relapses are caused by the immune system attacking the myelin covering of nerves. This disrupts the messages passing along the nerves and gives rise to the symptoms. Which symptoms are experienced, depends on which part of the brain or spinal cord is affected. MS relapses are often treated with steroids. Studies have shown that steroids are effective in speeding up recovery from relapse but make no difference either to the degree of recovery or to the long-term progression of the condition.
Although this current study found that relapses seemed to be more common in the spring, it is not possible to predict when relapses will happen or how often. Every person's MS is different and so is every relapse. Some people experience several in a year but others will go for many years between relapses.
It can sometimes be difficult to tell if you are having a relapse or if you are having a bad patch of symptoms which is part of the everyday up and down pattern of MS. Information about Relapsing remitting MS: an introduction contains more information on relapses including how to tell if you are having a relapse and what you can do when you are having a relapse. This can be read online or ordered in a print or PDF version.
Vitamin D has several important roles in the body including regulating immune responses. Its possible role in multiple sclerosis is the focus of much debate and research. It is known that multiple sclerosis is more common in countries further from the equator. As vitamin D is made in the skin, this has led to the hypothesis that low sunlight exposure and consequent low vitamin D production triggers the development of MS. Vitamin D may also have a role in moderating relapses and disability in people who are already diagnosed with MS. There is some evidence that lower levels of vitamin D are associated with higher relapse rates and greater disability. Also, one study found that people with progressive forms of MS had lower levels than those with relapsing remitting MS. This current study also shows that vitamin D may also have an impact on when relapses occur.
Studies are underway to investigate both the role of vitamin D as a protective agent against the development of MS and as a treatment for people with the condition.
You can also read more about Vitamin D in the A to Z of MS.
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