If clotting occurs in the surface veins, it is usually not dangerous and often heals on its own. If both the surface and deep veins are involved, it may be dangerous. Heart attacks and strokes are very often due to thrombosis. The morbidity and mortality associated with clot formation is staggering. There are an estimated 5 to 20 million cases of venous thrombosis in the United States each year.
Age is a risk factor in thrombosis and clotting commonly occurs in older adults.
Blood clots in people younger than 40 are generally due to genetic abnormalities. Some of the inherited risk factors are abnormalities of protein C, protein S, antithrombin, and resistance to activated protein C. Clinical situations which may increase the risk of thrombosis are surgery, pregnancy, hormonal therapy, and prolonged bed rest. Each factor probably has a different risk potential for thrombosis. Once a patient reaches a certain threshold, a clot develops. Many factors remain unknown and the interaction between factors has not been clearly defined.