Is Everyone at Risk for Venous Thrombosis on Long-Distance Air Flights?

Abstracts & Commentary

Synopsis: Although there is an increased risk for venous thromboembolic events after long-haul airplane flights, the relative risk is in the lower range of the established transient risk factors and the highest risk applies only to individuals with pre-existing permanent risk factors (ie, age older than 45 years and/or abnormally elevated body mass index).

Sources: Schwarz T, et al. Arch Intern Med. 2003;163: 2759-2764; Perez-Rodriguez E, et al. Arch Intern Med. 2003;163:2766-2770.

During the past decade, at least 200 cases of air-travel related venous thrombosis have been reported in the literature1 and almost certainly, thousands more cases have occurred but have not been reported. In fact, the term "economy class syndrome" was coined in 1977 to suggest that the restricted seat space in economy sections of aircraft was a major contributor to this syndrome.2 Other physiological mechanisms such as decreased air pressure and release of nitric oxide in airplane cabins associated with dehydration (due to the consumption of alcohol and caffeine) along with venous stasis were also considered to be contributory factors to this potentially very serious travel problem. The World Health Organization declared that a link probably existed between air travel and deep venous thrombosis (DVT), that it mainly affects passengers with additional risk factors, but that scientific data were insufficient and that further studies were warranted.3

Two interesting articles published in the December 2003 issue of the Archives of Internal Medicine address the problems of isolated calf muscle venous thrombosis (ICMVT) with and without complicating pulmonary emboli. Schwarz and associates reported the results of their study of 964 flight passengers who intended to take a long-haul flight and compared them with 1213 nontraveling control subjects. Ultrasonographic studies were performed 1 week before the outgoing flight, and a second examination was performed within 48 hours after the return flight. Examinations were performed in the same

way in the control group. ICMVT was found in 20 passengers and 10 controls, whereas 7 passengers and 2 control subjects presented with DVT. Symptomatic pulmonary embolism occurred in only one passenger with DVT. All of the subjects had normal findings on baseline ultrasonography. The second article by Perez-Rodriguez and colleagues was a retrospective review of pulmonary thromboembolism (PTE) occurring among international travelers at the Madrid-Barajas airport for a 6-year period of time. They concluded that air travel is a risk factor for PTE and that the incidence increases with the duration of the air travel, but because the incidence is quite low, social alarm is not justified.

Comment by Harold L. Karpman, MD

Air flights longer than 8 hours in duration double the risk of ICMVT; however, in the well-controlled study performed by Schwarz et al, all the passengers with ICMVT or DVT had at least one risk factor (ie, older than 45 years of age or elevated body mass index, which, incidentally, was present in 21 of the 27 passengers with venous thrombotic events).4 The results in this study also suggested that the incidence of ICMVT may be used as a surrogate marker of DVT associated with air travel. Schwarz et al had also previously demonstrated that short-term low-molecular-weight heparin in patients with ICMVT prevents thrombus progression.4

There were at least 3 limitations to this study: 1) The results may not apply to the general population; 2) The baseline characteristics of passengers and controls did not match in each category; and 3) For ethical reasons, it was recommended to all passengers that they not consume alcoholic beverages and that they perform stretching exercises during their flights. These 3 limitations may be significant and may have had important effects upon the final results; however, this is the first controlled study evaluating DVT after long-haul flights and the conclusions of Schwarz et al are important. They concluded that their results supported the World Health Organization statement that, although there exists an increased risk for venous thromboembolic events after long-haul flights, the relative risk is in the lower range of the established transient risk factors and that the highest risk applies only to individuals with pre-existing permanent risk factors. In addition, although stretching exercises and adequate hydration for all passengers is clearly recommended, prophylactic anticoagulant therapy and/or compression stockings should not be recommended at this time, but that their use should be carefully investigated in future, well-designed, randomized studies in large, at-risk groups.

Dr. Karpman, Clinical Professor of Medicine, UCLA School of Medicine, is Associate Editor of Internal Medicine Alert.

References

1. Kesteven P, et al. Thorax. 2000;55(Suppl 1):32-35.

2. Symington IS, et al. Br J Dis Chest. 1977;71:138-140.

3. World Health Organization. Consultation on air travel and venous thromboembolism. Geneva, Switzerland: World Health Organization; March 12-13, 2001.

4. Schwarz T, et al. Blood Coag/Fibrinolysis. 2001;12:597-599.