DVT affects approximately 900,000 Americans each year. The incidence rises dramatically with age, from 1 in 100,000 in childhood to 1 in 1,000 in those over 80 years old.

Virchow’s triad describes the three primary drivers of clot formation:

• Venous stasis – Impaired venous flow leads to blood pooling in the veins. Major risk factors are immobility, paralysis, and long distance travel.
• Endothelial injury – Damage to the vein intima exposes the procoagulant subendothelium. Major causes are surgery, trauma, and intravenous catheters.
• Hypercoagulability – Abnormal increase in clotting potential. Can be inherited or acquired from conditions like cancer, pregnancy, or oral contraceptives.

Other VTE risk factors include older age, obesity, smoking, inflammatory states, nephrotic syndrome, and certain medications like erythropoietin and tamoxifen.

In most cases of lower extremity DVT, these factors lead to thrombus formation within the deep calf veins. The clot may then propagate proximally up the leg. This can obstruct venous outflow, causing distal edema and venous hypertension.