Air travel may increase the risk of blood clots, especially long flights

December 08, 2003

CHICAGO - Two articles in the December 8/22 issue of The Archives of Internal Medicine, one of the JAMA/Archives journals, report an increased risk of blood clots associated with air travel.

Venous thromboembolism (VTE, or formation of blood clots in the veins) result from an interaction between genetic and environmental factors, according to information in the article. Genetic factors can include a lack of certain proteins in the blood that help prevent or break up clots, and environmental factors include the use of oral contraceptives, recent surgery, cancer, pregnancy and prolonged immobilization. Studies have suggested that air travel may be a risk factor in VTE.

Ida Martinelli, M.D., Ph.D., of the University of Milano, Italy, and colleagues investigated whether individuals with certain genetic predispositions for developing VTE and those taking oral contraceptives are more likely to develop VTE during flights than those without these risk factors.

Thomas Schwarz, M.D., of the University of Dresden Medical School, Germany studied the incidence of VTE associated with "long-haul" flights lasting eight hours or longer.

Dr. Martinelli and colleagues studied 210 individuals with VTE and 210 controls without VTE. DNA analysis and blood tests were performed on all participants to look for genetic factors and levels of blood components associated with an increased risk of VTE.

In the month preceding VTE (or for controls, during the previous month), air travel was reported by 31 patients (15 percent) and 16 controls (8 percent). The researchers found that 102 (49 percent) of patients who had VTE had factors that increased their risk for VTE compared with 26 (12 percent) of controls. Oral contraceptives were used by 48 patients with VTE, and by 19 controls.

The researchers found that the risk of VTE in patients with genetic or other risk factors for VTE and who had traveled by air in the past month was 16 times higher compared to patients without these risk factors. Women who used oral contraceptives and traveled by air in the past month were 13 times as likely to develop VTE.

"Air travel is a mild risk factor for venous thromboembolism, doubling the risk of the disease," write the researchers.

Dr. Schwarz and colleagues studied 964 passengers returning from long-haul flights (longer than eight hours) and 1,213 control participants who did not fly. Venous thrombosis was documented in 27 passengers (2.8 percent) and 12 controls (1.0 percent); of these, 20 passengers (2.1 percent) and 10 controls (0.8 percent) had isolated calf muscle venous thrombosis.

The researchers found that "Long-haul flights of eight hours and longer double the risk for isolated calf muscle venous thrombosis. This translates into an increased risk for deep venous thrombosis as well. In our study, flight-associated thrombosis occurred exclusively in passengers with well-established risk factors for venous thrombosis," the authors write.
(Arch Intern Med. 2003;163:2771-2774, 2003;163:2759-2764) Available post-embargo at Editor's Note: Dr. Martinelli's research was supported by a grant from the Ministry of University and Scientific and Technologic Research, Rome, Italy. Dr. Schwarz's research was funded by Aventis, Bad Soden, Germany, Sanofi-synthelabo, Berlin, Germany, Novartis, Nurnberg, Germany and Barmer Health Insurance, Sachsen, Germany.

For more information, contact JAMA/Archives Media Relations at 312/464-JAMA (5262) or e-mail .

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