Traveling for more than 4 hours by air, car, bus or train is associated with an increased risk of venous thrombosis. In a large study (the MEGA study) of nearly 2000 people with a first thrombosis in the Netherlands, Dr. Suzanne Cannegieter and colleagues from the Leiden University Medical Center looked at the risk factors for thrombosis compared with their partners, who did not have thrombosis.

The results, published in the international open-access medical journal PLoS Medicine, showed that 233 of the people with thrombosis had traveled for more than 4 h in the 8 weeks preceding the event. Although the overall risk of developing thrombosis is still low, traveling in general was found to increase the risk of venous thrombosis 2-fold. The risk was highest in the first week after traveling, and the overall risk of flying was largely similar to the risks of traveling by car, bus, or train.

In particular groups of people the risk was increased. For example, the risk was up to 8-fold in people who also had a specific mutation in one of the genes involved in clotting (factor V Leiden); almost 10-fold in those who had a body mass index of more than 30 kg/m2; 4-fold in those who were more than 1.90 m tall; and more than 20-fold in those who used oral contraceptives. For air travel these findings of risk in particular groups were more apparent than for other modes of travel, and in addition, people shorter than 1.60 m had an almost 5-fold risk of thrombosis after air travel. However, the numbers of people in each of these groups was small and hence the estimates of risk must be interpreted carefully.

The authors conclude that the risk of venous thrombosis is moderately increased for all these modes of travel, and that in particular groups of people the risk is highly increased. The study could not show the mechanism of the increased risk, although the association of thrombosis with all types of travel, not just air travel, suggests that immobility is a key factor. Other mechanisms, such as reduced oxygen levels triggering clotting, may be involved in the particularly increased risk seen in air travel in some groups.

For those who have an increased risk, such as oral contraceptive users and individuals with factor V Leiden, the authors say that preventative measure such as exercises may be warranted. However, the study's results apply only to people younger than 70 y of age and it is likely that other characteristics exist that also increase the risk. These characteristics are being investigated in an ongoing study -- the World Health Organization Research Initiative into the Global Hazards of Travel (WRIGHT).

Citation: Cannegieter SC, Doggen CJM, van Houwelingen JC, Rosendaal FR (2006) Travel-related venous thrombosis: Results from a large population-based case control study (MEGA study). PLoS Med 3(8): e307.

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