deep vein thrombosis

Deep vein thrombosis is a blood clot that forms in one of the large, deep veins, most often in the calf or thigh. Think of blood as a river that normally flows smoothly; when it slows down, the vessel wall is injured, or the blood becomes stickier than usual, a clot can build up like silt damming a sluggish stream.

These three predisposing conditions, sluggish flow, vessel injury, and increased clotting tendency, are known together as Virchow's triad, and they explain why long immobility, surgery, cancer, pregnancy, and certain inherited conditions all raise risk. A leg with deep vein thrombosis may swell, ache, feel warm, and look reddened, though many clots cause few or no symptoms.

The reason deep vein thrombosis matters far beyond the leg is that a piece of clot can detach and travel to the lungs, becoming a pulmonary embolism. For this reason deep vein thrombosis and pulmonary embolism are managed as one disease spectrum, and treatment with anticoagulation aims both to stop the existing clot from growing and to prevent a dangerous embolus.