Development of venous thrombosis is promoted by (1) venous blood stasis, (2) injury to the intimal layer of the venous vasculature, and (3) abnormalities in coagulation and/or fibrinolysis.
What is the natural history of venous thrombosis?
Resolution of fresh thrombi occurs by fibrinolysis and organization. Fibrinolysis results in actual clot dissolution. Organization reestablishes venous blood flow by reendothelializing and incorporating into the venous wall residual clot not dissolved by fibrinolysis. In the absence of new clot formation, the two processes generally are complete in 7-10 days.
Can patients with deep venous thrombosis be accurately diagnosed clinically?
No. The clinical diagnosis of deep venous thrombosis is neither sensitive nor specific. Less than 50% of patients with confirmed deep venous thrombosis present with classic symptoms of pain, erythema, and edema. Similarly, radiologic tests confirm the diagnosis in only 50% of patients who present with a high clinical suspicion of deep venous thrombosis.
How is the diagnosis of lower extremity deep venous thrombosis confirmed?
The test of choice depends on the likely location of the deep venous thrombosis. Contrast venography remains the gold standard but is associated with a higher incidence of adverse effects, primarily phlebitis. Radiolabelled fibrinogen scanning is highly sensitive for deep venous thromboses in the calf and lower thigh but loses sensitivity above midthigh, because accumulated radiofibrinogen in the large pelvic blood pool interferes with scanning. In contrast, impedance plethysmography is sensitive above but not below the knee. Similarly, Doppler/ultrasound has a sensitivity and specificity of 90-95% for proximal clots located cephalad of the popliteal vein. Its accuracy for vein thromboses in the calf is not well defined.
When should prophylaxis of deep venous thromboses be considered?
Two factors must be weighed in deciding to initiate prophylaxis of deep venous thrombosis: the degree of risk for thrombosis (see question 1 and 2) and the risk of prophylaxis. The risk factors for deep venous thrombosis are cumulative. The primary risk of pharmacologic prophylaxis is hemorrhage, which is generally uncommon if no coagulation defects or lesions with bleeding potential exist.