Venous thromboembolism (VTE) is a common and potentially lethal disease occuring at an incidence of 1 to 3per 1000, per year.22--27 Over 200,000 new cases occurring in the United States annually. It mainly manifests in the deep veins of the leg, but may occur at other sites, such as, the upper limbs, cerebral, intra-abdominal, liver, portal veins or retinal veins. Embolisation occurs when part(s) of the clot dislodge and are transported via the blood flow, usually through the heart to the pulmonary vasculature. VTE is more common with advancing age.

Pulmonary embolism patients face a high risk of death. Estimates of the case fatality rate, however, vary widely. Large natural history studies25,26,28 found 12 to 25% of all events of VTE fatal, while recent trials have found much lower figures, around 1 to 3% (5 to 10% for pulmonary embolism).28,29 Of these, 30% die within 30 days and one-fifth suffer sudden death due to pulmonary embolism. This wide range may be caused by the inclusion of thrombosis as a secondary cause of death in the studies with a high estimate. The Worcester study also showed that the case fatality rate was highly dependent on age, with a low mortality among those aged forty or less, at the time of thrombosis.25 The post-thrombotic syndrome (PTS) leads to chronic morbidity in a substantial number of patients.26 Despite improved prophylaxis, the incidence of venous thromboembolism has been relatively constant.

Independent risk factors for venous thromboembolism include increasing age, male gender, surgery, trauma, hospital or nursing home confinement, malignancy, neurological disease with limb paralysis, and central venous catheter/transvenous pacemaker, prior superficial vein thrombosis and varicose veins. Among women, risk factors include pregnancy, oestrogen-containing oral contraceptives, and hormone replacement therapy. About 30% of VTE surviving cases develop recurrent venous thromboembolism within 10 years.

Independent predictors for recurrence include increasing age, obesity, active cancer, and limb paralysis.30 About 28% of cases develop venous stasis syndrome within 20 years.

Only a reduction in the incidence of venous thromboembolism can reduce sudden death due to pulmonary embolism. The incidence of venous thromboembolism has been relatively constant since about 1980. Reduction of the incidence of venous thromboembolism will require better recognition of persons at risk, improved estimates of the magnitude of risk, and avoidance of risk exposure where possible. Compared with deep vein thrombosis alone, pulmonary embolism patients have reduced survival for up to 3 months after onset. Improved therapies for pulmonary embolism are needed, especially for patients with chronic heart or lung disease. Venous thromboembolism recurs frequently. While therapeutic oral anti-coagulation prevents recurrence, venous throm-boembolism sometimes begins to recur as soon as anti-coagulation is stopped. Therefore, venous thromboembolism should be viewed as a chronic disease with episodic recurrence. Recognition of venous thromboembolism as a multifactorial condition with genetic and genetic-environmental interaction has provided significant insights into the disease epidemiology and has offered the possibility of better identifying those at risk from VTE.

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