Exposure to high altitude: a risk factor for venous thromboembolism?

There are several genetic and acquired risk factors for venous thromboembolism. Exposure to high altitude (HA), either during air travel, ascension of mountains, or while engaging in sports activities, has been observed to result in a hypercoagulable state, thus predisposing to thromboembolic events. Although several previous studies have suggested that conditions present at HAs contribute to establish a prothrombotic milieu, published reports are contradictory and the exact underlying mechanism remains poorly understood. Results from HA studies also show that environmental conditions at HA such as hypoxia, dehydration, hemoconcentration, low temperature, use of constrictive clothing as well as enforced stasis due to severe weather, would support the occurrence of thrombotic disorders. The three leading factors of Virchow triad, that is, venous stasis, hypercoagulability, and vessel-wall injury, all appear to be present at HA. In synthesis, the large list of environmental variables suggests that a single cause of HA-induced thromboembolic disorders (TED) may not exist, so that this peculiar phenomenon should be seen as a complex or multifactorial trait. Further investigation is needed to understand the risk of TED at HA as well as the possible underlying mechanisms.