Abstract
BackgroundRecent studies have indicated an increased risk of venous thrombosis after air travel. Nevertheless, questions on the magnitude of risk, the underlying mechanism, and modifying factors remain unanswered.Methods and FindingsWe studied the effect of various modes and duration of travel on the risk of venous thrombosis in a large ongoing case-control study on risk factors for venous thrombosis in an unselected population (MEGA study). We also assessed the combined effect of travel and prothrombotic mutations, body mass index, height, and oral contraceptive use.Since March 1999, consecutive patients younger than 70 y with a first venous thrombosis have been invited to participate in the study, with their partners serving as matched control individuals. Information has been collected on acquired and genetic risk factors for venous thrombosis. Of 1,906 patients, 233 had traveled for more than 4 h in the 8 wk preceding the event. Traveling in general was found to increase the risk of venous thrombosis 2-fold (odds ratio [OR] 2.1; 95% confidence interval [CI] 1.5–3.0). The risk of flying was similar to the risks of traveling by car, bus, or train. The risk was highest in the first week after traveling. Travel by car, bus, or train led to a high relative risk of thrombosis in individuals with factor V Leiden (OR 8.1; 95% CI 2.7–24.7), in those who had a body mass index of more than 30 kg/m2 (OR 9.9; 95% CI 3.6–27.6), in those who were more than 1.90 m tall (OR 4.7; 95% CI 1.4–15.4), and in those who used oral contraceptives (estimated OR > 20). For air travel these synergistic findings were more apparent, while people shorter than 1.60 m had an increased risk of thrombosis after air travel (OR 4.9; 95% CI 0.9–25.6) as well.ConclusionsThe risk of venous thrombosis after travel is moderately increased for all modes of travel. Subgroups exist in which the risk is highly increased.
Highlights
Interest in the role of air travel in the pathogenesis of venous thrombosis has heightened in the past 5 y [1,2,3,4,5]
We studied the effect of various modes and duration of travel on the risk of venous thrombosis in a large ongoing case-control study on risk factors for venous thrombosis in an unselected population (MEGA study)
The risk of venous thrombosis after travel is moderately increased for all modes of travel
Summary
Interest in the role of air travel in the pathogenesis of venous thrombosis has heightened in the past 5 y [1,2,3,4,5]. In terms of absolute risk, two studies found similar results: one performed in New Zealand found a frequency of 1% of venous thrombosis in 878 individuals who had traveled by air for at least 10 h [2], and a German study found venous thrombotic events in 2.8% of 964 individuals who had traveled for more than 8 h in an airplane, as compared to 1% in 1,213 controls [4]. The events in both studies were mostly asymptomatic. Certain other factors increased the risk of a blood clot even more, such as having a particular mutation (known as factor V Leiden) in a gene involved in blood clotting, having a body mass index of more than 30 kg/m2 (over 30 kg/m2 is defined as being obese), being more than 1.90 meters tall, and using oral
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