Pathogenesis of venous thrombosis.

This brief review attempts to describe the present understanding of the pathogenesis of venous thrombosis in general with special reference to venous thromboembolism in spinal cord injury patients with paralysis. The component parts of Virchow's triad are examined. Most venous thrombi seem to originate in regions of slow blood flow, ie, the large venous sinuses of the calf and thigh or in valve cusp pockets. Decreased blood flow or even stasis due to lack of the pumping action of the large muscle packages in paralyzed patients is undoubtedly one of the major factors. As blood pools, activation products of the coagulation system accumulate locally leading potentially to local hypercoagulability. Activation products of clotting and fibrinolysis can induce endothelial damage which in turn leads to further activation of the hemostasis system. Endothelial damage may also result from distension of the vessel walls by the pooling blood. Blood flow is further decreased by hyperviscosity due to elevated fibrinogen levels and dehydration. Some spinal cord injury patients may sustain direct trauma to the legs; others may encounter vessel wall damage by the immobilized limbs. Shortly after injury, certain changes develop in the clotting system, especially increases in components of the von Willebrand factor macromolecular complex and increased platelet aggregability which could further contribute to hypercoagulability. Recently, an inhibition of the fibrinolytic system was suggested which also could add to a prothrombotic state. All of these interrelated processes clearly explain the high risk of venous thromboembolism in spinal cord injury patients with paralysis which has been clearly demonstrated by many investigators. It is hoped that intense thrombosis prophylaxis will reduce the incidence of this potentially devastating complication.