Pathogenesis of Venous Thromboembolism: When the Cup Runneth Over

A comprehensive understanding of the pathogenesis of venous thrombosis is essential for identifying patients at increased risk and who may therefore benefit from more aggressive preventive and therapeutic measures. As for other pathologies, the pathogenesis of venous thromboembolism is multifactorial. All risk factors, either congenital or acquired, are relatively "innocent" when considered alone. However, when an individual is unlucky enough to inherit one or more abnormality, compounded in many cases by environmental hazards, that person may be propelled over a threshold that precipitates the development of thrombosis. An appropriate analogy is that where "the last drop makes the cup run over." A reinterpretation of the traditional Virchow's triad (abnormal vessel wall, abnormal blood flow, and abnormal blood constituents) was provided by Eberhard Mammen throughout his research, and this has contributed greatly to the understanding of the pathogenesis of this serious disorder. Mammen postulated immobility as the leading event, because it reduced blood flow as a result of decreased muscle contraction. The subsequent "stasis of flow" led to accumulation of blood within the intramuscular sinuses, especially of the calf, triggering hypercoagulability due to local accumulation of activated clotting factors and coagulation activation products and the simultaneous consumption of blood coagulation inhibitors. On Mammen's "hit list" nearly 20 years ago were included (among inherited abnormalities) decreased protein C, protein S, antithrombin III, plasminogen, and tissue plasminogen activator, and increased plasminogen activator inhibitor-1, whereas (among acquired predisposing conditions) surgery, trauma, previous thromboembolism, prolonged immobility and paralysis, malignancy, congestive heart failure, obesity, advanced age, pregnancy and puerperium, varicose veins, and oral contraceptives were also identified. Some two decades later, the situation has perhaps not changed so much, although studies continue to expand our knowledge of this topic, clarifying the relative contribution of each single risk factor in the pathogenesis of venous thrombosis.