In a high-paced society, even two-hour flights can seem “long-haul.” This year, researchers flew for as much as 12 hours to attend the XXth Congress of the International Society of Thrombosis Hemostasis (ISTH) held August 6-12, 2005 in Sydney, Australia. It is perhaps fitting that of the multitudes of studies presented, several concerned the association of air travel as a risk factor for deep venous thrombosis (DVT).
Symington and Stack, who reported an apparent increase in DVT in air travelers compared to non-travelers, first described “Economy Class Syndrome” or “Traveler’s Thrombosis” in 1977. The incidence was attributed to cramped seating in the coach cabin. Prolonged periods of inactivity caused by space limitations may slow circulation and produce edema (leg swelling). In addition, bent knees compress the popliteal vein (the deep vein behind the knee), creating a potential site for clot formation over time. Low oxygen, low humidity (dry air), and low cabin pressure at high elevations have a dehydrating effect that concentrate the blood, making it sluggish. This effect is worsened when passengers consume alcohol, or do not adequately replenish fluids lost by dehydration. The body’s natural clot busters typically counter the tendency to clot in these situations; however, in the presence of known hypercoagulable risk factors such as the Factor V Leiden and the Prothrombin G20210 mutations (as well as obesity, smoking, and oral contraceptives) the risk of DVT could be further increased. At this year’s ISTH meeting, several studies attempted to scientifically document a passenger’s general tendency to clot after air travel.
Studies from Ospedale La Carità and University Hospital of Berne (Switzerland) found that changes in clotting enzymes during long periods (6 hours) of sitting increase levels of certain anticoagulants (Tissue Factor Pathway Inhibitor and Antithrombin). This physiologic response resulted in several other adjustments to the clotting cascade that ultimately balanced the effect by increasing Factor V and VIII levels. Thus, the net effect of immobility itself did not necessarily increase a clotting tendency.
Another group led by Anja Schreijer at Academic Medical Center and Leiden University Medical Center (Netherlands) investigated the issue further by comparing thrombin levels in air travelers versus immobile non-flying individuals who watched movies for 8 hours. Thrombin is the product of preliminary reactions in the clotting cascade and activates fibrinogen to form fibrin (the meshwork of a clot). They found a 223% rise in levels due to traveling compared to 46% rise due to immobility. This suggests that a mechanism other than immobility caused the travelers to be at an increased thrombotic risk.
The Dutch research group also headed the WRIGHT Study, which retrospectively surveyed the occurrence of DVT amongst employees of three international companies for four weeks and after a flight of at least four hours. Compared to non-traveling employees, the frequent flyers were found to be 3.65 times more likely to develop a DVT. The risk of developing a clot on a flight was found to be 1 in 5944 flights.
If more than one flight was taken in the four-week window, the risk of clotting was slightly elevated.
AirHealth.org, a non-profit organization dedicated solely to the topic of travel-associated thrombosis, reports that 3-5% of air travelers will develop clots. Despite continued research, case reports, and a physiological basis for increased risk, not all experts agree that air travel is a direct cause of thrombosis due to a lack of large-scale population studies. Such studies are difficult to evaluate, in that the extended period between flight time and presentation of DVT or PE symptoms induces variability.
In the absence of large-scale studies, some airlines feel there is no scientific basis to warrant thrombosis prevention on flights. Ironically, most airline inflight magazines include extensive instructions on how to avoid DVT during flight. However, the apparent increase in risk has prompted some afflicted travelers to site the Warsaw Convention of 1929, which holds airlines liable for damages when passengers are injured by an accident, as grounds to file negligence claims against the carriers. The debate is whether a blood clot is a preventable event, or an individualized reaction to normal flight operations.
The first cases presented in United States district courts began in Texas (Reynolds vs. American Airlines, 2002) and California (Miller and Wylie vs. Continental Airlines, 2003). Since then, hundreds of cases have been filed domestically and internationally. To date, inconclusive medical evidence has resulted in most airlines to disclaim responsibility. Such attention in both the medical and legal arenas emphasizes the importance of the latest medical studies, as they continue to investigate a cause-and-effect relationship between long-haul air travel and thrombosis.
Until it is clarified whether airline travel is or is not a risk factor for thrombosis it appears appropriate for travelers to engage in some DVT prophylaxis exercises, be well hydrated, and avoid alcoholic beverages during flights.
First Published Fall of 2005 – Posted October 26, 2008
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