Venous Thrombosis I: Introduction

A thrombus is an aggregate of platelets attached to the vein wall with a tail-like appendage containing white and red blood cells and fibrin. It forms when blood clots within a vein, partially or completely blocking blood flow. As the thrombus enlarges, it accumulates more blood cells and fibrin, potentially obstructing the vein's lumen.

Superficial vein thrombosis occurs in superficial veins like the saphenous vein, while deep vein thrombosis occurs in deep veins, often the femoral or iliac veins. Though less severe, superficial vein thrombosis can lead to complications if untreated. Deep vein thrombosis poses greater clinical risk due to its location and potential for serious complications.

Virchow's triad includes venous stasis, blood hypercoagulability, and endothelial damage. Venous stasis occurs when venous valves are dysfunctional or extremity muscles are inactive. Hypercoagulability raises clotting risk in conditions like polycythemia. Endothelial damage from catheterization, trauma, chemotherapy, or sepsis stimulates platelet activation and initiates thrombus formation.

Venous stasis disrupts normal blood flow when venous valves are dysfunctional or extremity muscles are inactive. This condition is common in obesity, atrial fibrillation, prolonged immobility, and extended surgical procedures. Sluggish blood flow increases contact time between blood components and vessel walls, promoting platelet aggregation and thrombus development.

Endothelial damage results from direct injuries like catheterization, trauma, or burns, or indirect causes such as chemotherapy, diabetes, or sepsis. Damaged endothelium stimulates platelet activation and initiates the coagulation cascade, increasing thrombus development risk. This injury exposes underlying tissue, triggering the body's clotting mechanisms.

Hypercoagulability results from polycythemia, severe anemia, various cancers, nephrotic syndrome, high homocysteine levels, and deficiencies in protein C, protein S, or antithrombin. Sepsis, certain medications like corticosteroids and estrogens, and smoking combined with oral contraceptives also increase risk. These conditions alter blood composition or clotting factor levels, promoting thrombus formation.

If a thrombus does not become firmly adherent to the vein wall, it can detach and travel through venous circulation as an embolus. Turbulent blood flow significantly contributes to embolization. The embolus can travel to the heart and lodge in the pulmonary circulation, causing a pulmonary embolism. Understanding this progression is essential for recognizing thrombosis clinical manifestations and diagnostic studies.