LONDON - A blood clot in a person's vein nearly doubles the risk of heart attack or stroke within a year, a Danish study showed on Friday, providing strong evidence the three conditions are linked.
Blood clots in coronary arteries have long been known to be a common cause for heart attacks and strokes, but scientists had largely believed blood clots in veins were unrelated, said Henrik Sorensen, an epidemiologist at Denmark's Aarhus Hospital.
The Danish study showed the vein condition boosted the risk of either heart attack or stroke by about 90 percent within a year, compared with people without clots.
"We have shown the link between the diseases for the first time in a very large study," said Henrik Sorensen, who led the team. "The diseases had been regarded as totally different."
The relative risk compared with people without blood clots in veins also remained about 20 to 40 percent higher for at least 20 years, said the study, published in the journal Lancet.
They said they did not know why vein clots, heart attacks and strokes were linked, but obesity could be a key factor.
Vein blood clots happen when circulation is restricted in a deep vein -- often a leg. Deep vein thrombosis itself is not fatal but can kill if the clots move through the body to the lungs, causing a pulmonary embolism.
The Danish team said the fact that vein clots seem linked to strokes and heart attacks was surprising because the condition is very different from what has been considered the main cause of heart attacks, Sorensen said.
For instance, scientists have long known that heart attacks and strokes occur due to the effects of hardening artery walls. But blood clots do not cause veins to harden.
"The veins don't have the same process," Sorensen said. "Therefore it is surprising there is a link."
The team analysed data from Denmark's national medical databases over a 20-year period. They excluded patients with heart disease and then gauged the risk of heart attack and stroke in more than 25,000 patients with deep vein thrombosis.
Establishing a link between the conditions could help prevent heart attacks and stokes by getting more people on drugs aimed at reducing blood clots, Gordon Lowe, a researcher at Glasgow University, said in a commentary in Lancet.
The study is also important because many previous trials were far smaller and lacked statistical weight.
"Such studies have reported conflicting results, partly because of small event numbers and hence limited statistical power," he wrote.
