(CNN) - Varicose veins may be an early warning sign of potentially deadly blood clots, suggests a study published Tuesday in the journal JAMA.
Enlarged and gnarled varicose veins and deep venous thrombosis, a clot that forms in the deep veins of the body, are strongly associated, the Taiwanese researchers found.
Though they looked at the health records of more than 425,000 adults, the researchers say that even more work is needed to understand whether this relationship is one in which varicose veins directly cause blood clots or whether the two conditions simply have a similar origin.
"The most common question from a varicose vein patient in the vein clinic is: 'Will varicose vein bring any health risk for me?' " said Dr. Shyueluen Chang, first author of the study and a phlebologist and dermatologist at Chang Gung Memorial Hospital in Taoyuan, Taiwan.
For this reason, Chang said, learning about potential relationships between "varicose veins and health-threatening diseases is important."
Varicose veins, usually caused by pregnancy or the effects of age weakening the blood vessels, are common.
In the United States, nearly a quarter (23%) of adults have the condition, which doctors rarely associate with serious health risks.
By contrast, other vascular conditions and diseases -- such as deep venous thrombosis, pulmonary embolism and peripheral artery disease -- are thought to be serious and risky. Pulmonary embolism is a blood clot that occurs in the arteries in the lungs, and peripheral artery disease narrows the arteries leading to the legs, stomach, arms and head. Both can have serious health consequences that may become deadly.
"Not much is known about varicose veins and the risk of other vascular diseases," Chang said. So he and his colleagues decided to investigate.
They began with the thought "that varicose veins might be causal," he said. Chang hypothesized that since patients with varicose veins have increased inflammation, the inflammatory processes may cause abnormal blood clotting.
For the study, Chang and his colleagues used claims data from Taiwan's National Health Insurance program for a 14-year time span beginning January 1, 2001. The researchers focused on two groups of patients: One group had varicose veins, and the other did not.
Overall, they compared 212,984 patients with varicose veins and 212,984 patients without varicose veins. Both groups contained similar proportions of women -- nearly 70% -- and were the same average age -- about 55 years old.
The varicose veins group had higher incidence rates for deep vein thrombosis, the researchers found: 10,360 cases versus just 1,980. This translates to 6.55 per 1,000 person-years versus 1.23 per 1,000 person-years. One person-year is a year lived by each participant for the duration of the study.
Varicose vein patients, then, "had about a 5 times greater risk of developing deep vein thrombosis than the other group," said Chang.
The varicose veins group experienced 793 cases of pulmonary embolism versus just 451 cases in the other group. The researchers also discovered 16,615 cases of peripheral artery disease among the varicose-vein group, compared with 9,709 cases for the adults without gnarled veins.
Compared with the other adults, then, the varicose vein patients had slightly less than twice the risk of developing either condition.
Chang and his co-authors concluded that among adults diagnosed with varicose veins, there was a "significantly increased risk of incident deep vein thrombosis," though the findings for pulmonary embolism and peripheral artery disease are "less clear," based on their statistical analysis.
The authors also noted weaknesses in their work. They could not factor out confounding behaviors, such as smoking, which can lead to blood clots. Also, some varicose veins patients may have been misclassified into the group without the condition, as not all women notify their doctors when the veins appear.
Analyzing the results through the lens of sex, varicose veins were associated with an "increased risk of deep vein thrombosis in both women and men," Chang said, yet "this association is stronger in men than in women." The team did not analyze ethnic differences since Taiwan's population is almost entirely Asian, he said.
"We believe that varicose veins are not merely a cosmetic or symptomatic concern," Chang concluded. Based on the strong relationship between varicose veins and an increased risk of more serious diseases, "patients with varicose veins deserve careful monitoring and early evaluation" for possible health consequences, he said.
Other researchers say the study results may be "robust," but they're not unfamiliar.
Inflammation is a 'key player'
Dr. Gregory Piazza, cardiovascular medicine specialist at Brigham and Women's Hospital, said the study has a "number of strengths," including the large number of people studied. Piazza, who was not involved in the research, explained that the researchers "undertook several additional statistical techniques" to weaken possible confounding factors that might bias the results, and "these steps help to build a case for the study observations being robust."
The study results are not entirely new, Piazza said, since several studies have demonstrated an association between varicose veins and blood clots. Yet "fewer studies have established an association between varicose veins and peripheral arterial disease," he said.
Inflammation has become "increasingly recognized as a key player in the development of vascular disease, including deep venous thrombosis, pulmonary embolism, stroke and heart attack," Piazza said.
Whether varicose veins cause these other conditions or share the same cause "remains the focus of ongoing research studies," he said.
Dr. Andrei L. Kindzelski, program director at the Translational Blood Science and Resources Branch at the National Heart, Lung and Blood Institute, also said the findings "are not new or unexpected."
"Varicose veins have been included in all major risk assessment tools" for obstructed blood vessels based on the "known association between the two conditions," Kindzelski said. The major weakness is that the study is not a "true" population study.
"True healthy people are not included in this study at all," Kindzelski said, since the researchers drew data from medical records using codes. The effect, then, of "confounding diseases" is "unknown," he added.
Though the study confirms that associations between varicose veins and blood clots exist, it does not help to estimate the strength of this association, Kindzelski said. To better understand the "intricate" genesis of these diseases, "more research is needed."
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