The exact cause of restless legs syndrome is unkown.
Xiang Gao of Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts, and colleagues, write about their findings in the 12 June online issue of Neurology.
In a statement Gao says their study "highlights the importance of recognizing this common but underdiagnosed disease", which affects 5 to 10% of adults around the US.
Restless Legs Syndrome
Exactly what causes restless legs syndrome is somewhat of a mystery. As it seems to run in families, there could be a genetic factor, according to the National Institute of Neurological Disorders and Stroke (NINDS).
For example, in 2007, two independent groups of scientists discovered genes linked to restless legs syndrome. One of the groups also found a link between iron deficiency and one of the genes.
In a more recent study, researchers investigating the biology behind restless leg syndrome identified factors behind disrupted sleep, cardiovascular dysfunction and pain in the protein profile of people with the condition.
The syndrome has also been linked to kidney disease, the nerve disorder peripheral neuropathy, and other medical conditions. Alcohol and sleep deprivation may worsen or trigger symptoms in some people, and the condition can also occur with certain drugs and during pregnancy.
There is no specific test for restless legs syndrome. To diagnose it, doctors use a checklist of criteria, such as: is there an overwhelming urge to move the affected limb(s), do symptoms get worse at night, are they triggered by rest or sleep, do they go away with movement?
Prospective Study Followed Nearly 18,500 Men for 8 Years
The evaluation found 3.7% (690) of participants met the criteria for restless legs syndrome when the study started.
Over the eight years of follow-up, the researchers monitored the participants and collected health information about major chronic diseases every two years.
During the follow-up, 2,765 of the participants died. Among them were 25% (171) of those with RLS, compared with only 15% (2,594) of those without RLS.
When they analyzed the link, the researchers found men with restless legs syndrome had a 39% higher risk of death compared to men without it.
Even when they took into account factors that could influence the link, such as lifestyle, body mass index, medical complaints, and sleep disorders, the higher risk only dropped to 30%.
And when they excluded participants with major conditions such as high blood pressure, and diseases such as cancer and heart disease, the link between restless legs syndrome and early death rose to 92%.
Increased Risk of Early Death Not Tied to Usual Risk Factors
Gao, who is at the Channing division of network medicine at Brigham and Women's Hospital, and also with Harvard School of Public Health, says they did not find that the increased risk was tied to usual known risk factors like being older, being overweight, having insufficient sleep, being a smoker, having an unhealthy diet or not doing enough exercise.
"The increased mortality in RLS was more frequently associated with respiratory disease, endocrine disease, nutritional/metabolic disease and immunological disorders," he adds.
As their study was a prospective study (that is one that follows a group of people over a period of time and monitors what happens to them), the results do not prove if restless legs syndrome actually causes early death.
Gao says we need more studies to find out exactly why and how restless legs syndrome may cause risk of dying early.