One in every 170 women in the islands has the degenerative neurological disease – and the incidence rate is rising

The Orkney islands in Scotland have the highest incidence of multiple sclerosis in the world, a new study has shown, lending weight to the theory that the absence of strong sunlight may be a factor.

The north of Scotland has been known for some time to have a high prevalence of MS, but the first study of its kind in 40 years has found that the rates in the Orkney Islands, Aberdeen and Shetland are not only very high but have increased since the 1980s.

The prevalence of the condition in Orkney is far higher than has been recorded anywhere else. By 1983/4, when the last study was done, both Orkney and the Shetlands had a rate of 190 cases per 100,000 inhabitants. Even then, say the authors of the new study from the University of Edinburgh, this was extremely high, given that 30 cases per 100,000 was considered a high-risk area.

But their research shows that the rate of probable or definite MS in Orkney is now 402 per 100,000 inhabitants. One woman in 170 in the islands suffers from the degenerative disease – it is more common in women than in men. Shetland has a rate of 295 per 100,000 and the city of Aberdeen has 229 cases per 100,000. The highest reported rates worldwide were previously 350 per 100,000 in Alberta and Nova Scotia in Canada.

Dr Jim Wilson, of the University of Edinburgh's centre for population health sciences and one of the authors of the study published in the Journal of Neurology, Neurosurgery and Psychiatry, said the answer was at least partly genetic. "Our study shows that Orkney has the highest prevalence rate of MS recorded worldwide. These findings may reflect improved diagnostic methods, improved survival or rising incidence. We are trying to work out why it is so high, but it is at least partly to do with genes."

The figures will also be welcomed by those who believe the high prevalence of MS in northern climes is linked to an absence of strong sunlight, which is needed to make vitamin D in the body. Some scientists and campaigners have lobbied public health authorities for mass dosing of vitamin D in Scotland.

Dr Wilson said this was an ongoing topic of research. "We have 2,300 people in Orkney who are having their vitamin D measured. We will certainly get the answer," he said. "It is probably important but it is not the only factor."

The study found the prevalence in Orkney was considerably higher than in the Shetlands, which are further north, but there could be other reasons for this, such as the amount of time people were willing to spend outdoors.

"I think a number of things will have to go wrong, including perhaps low vitamin D and having a genetic susceptibility and potentially other unknown factors," he said.