MELBOURNE, Nov. 4 (Xinhua) -- Australian researchers have discovered that the age at which symptoms of multiple sclerosis (MS) first start is strongly linked to latitude.
A large international study led by the University of Tasmania's Menzies Institute for Medical Research found that people who lived a long way from the equator were more likely to first exhibit signs of MS than those who lived in the tropics.
The lead author of the study, Menzies' neuroscientist and clinician Professor Bruce Taylor, said on Friday that each 10 increase in distance from the equator was associated with a 10-month earlier onset of symptoms.
MS is thought to be caused by a complex interplay of genetic and environmental factors, including latitude and/or exposure to sunlight and vitamin D levels.
But until now it was not clear whether latitude might also affect the age at which symptoms first start. In an effort to find out, the research team drew on an international database of more than 22,000 MS patients from 52 centers in 21 countries in Europe, North and South America, Asia Minor, South Asia and Australia.
The latitude of each of the centers was divided into lower than 40, 40-50, and higher than 50.
The average amount of winter ultraviolet B (UVB) sunlight - the type involved in vitamin D manufacture in the skin - was calculated from information supplied by the Solar Radiation Database service.
More than 80 percent of the patients in the study were from the northern hemisphere, with around two-thirds (67 percent) from Europe. Around one in six (just under 16 percent) were from Australia, including around 300 people with MS from Tasmania.
The average age at which symptoms first appeared was around 32. But after taking account of potentially influential factors, it emerged that each 10 increase in latitude was associated with a 10 month earlier start of symptoms, with those furthest from the equator starting their symptoms almost two years earlier than those closest to the equator.
While the authors pointed out that their study was purely observational, it did "strongly suggest" that the latitudinal factors associated with MS risk also affected MS disease severity (the timing of the onset of the disease).
In common with known data on the incidence and prevalence of MS, nearly three out of four of the participants were women.
The Menzies team's research has been published online in the Journal of Neurology Neurosurgery & Psychiatry.
