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NEW YORK (Reuters Health) - Restless legs syndrome tends to cluster in families, results of a Canadian study confirm.Dr. Guy Rouleau of the University of Montreal Hospital and his colleagues found sons and daughters of people with the bothersome syndrome were at nearly double the risk of having it themselves, while the risk for brothers and sisters of restless legs syndrome patients was nearly four-fold greater.Restless legs syndrome causes unpleasant sensations in the legs when a person is at rest, triggering an uncontrollable urge to move the legs to get relief. The exact cause of the condition is unknown. While restless legs syndrome has long been understood to run in families, there have been no large-scale systematic family studies, the investigators note in the latest issue of the Archives of Neurology. The risk for relatives of people with restless legs syndrome remains unclear. To help fill the research gap, Rouleau and his colleagues studied 671 individuals diagnosed with restless legs syndrome in Quebec, Canada. This included 192 people who were assessed at a special sleep clinic and 479 affected family members who responded to a structured diagnostic questionnaire. Rouleau and his team determined that for a person with restless legs syndrome, there is a 77 percent chance of another person in the family having the disease. The son or daughter of a person with the condition is nearly twice as likely as someone in the general population to have restless legs syndrome too. (For the current study, the researchers estimate that 10 percent of the general population has the condition, meaning the risk for someone with a parent who has the condition is about 20 percent.) Siblings of restless legs syndrome patients are nearly four times as likely to have the condition, the study found. The study also found that women are more likely to have "familial" restless legs syndrome, while pregnancy, arthritis, and iron deficiency further increases women's risk. According to Rouleau, iron deficiency seems to be a key problem behind restless legs syndrome, which helps explain its association with pregnancy (when many women experience anemia) and kidney disease, which can also cause iron deficiency. "If you have somebody who has restless legs, the first thing you do is measure their iron," he explained. If that person is anemic, he added, giving them iron supplements usually helps ease the condition. Researchers also suspect that an imbalance in the movement-regulating brain chemical dopamine plays a role in restless legs syndrome. Drugs that increase dopamine activity -- such as Requip and Mirapex -- are sometimes used to treat the condition. While doctors first described restless legs syndrome in the 1950s, "it's said to be the most common disease your doctor doesn't know about," Rouleau told Reuters Health. Critics have raised concerns that restless legs syndrome is a case of "disease mongering," which means that companies making drugs to treat the condition are trying to convince people with only mild symptoms that they need medication. But Rouleau disagrees. "I think it's the exact opposite. I think that by far it's most likely that people have the disease and they either don't realize that there's something that can be done about it, or they go see the doctor and the doctor doesn't know what it is."Rouleau also noted that restless legs syndrome is very common among French-Canadians, up to 15 percent of whom have the condition; it's also common among people of European or Middle Eastern background, but rare among blacks and Asians.