What is thrombosis?

Thrombosis: Formation, Types, and Clinical Implications

Thrombosis is the formation of blood clots (thrombi) composed of fibrin and blood cells that can occur in any part of the cardiovascular system, including veins, arteries, the heart, and the microcirculation, leading to vessel obstruction, embolism, or consumption of hemostatic elements. 1

Composition and Types of Thrombi

• Arterial thrombi form under conditions of high flow and are composed mainly of platelet aggregates bound together by thin fibrin strands. They typically develop at sites of disturbed flow or atherosclerotic plaque rupture, which exposes thrombogenic subendothelium to platelets and coagulation proteins. 1

• Venous thrombi form in areas of stasis and are predominantly composed of red blood cells with large amounts of interspersed fibrin and relatively few platelets. They commonly occur in the lower limbs and can lead to pulmonary embolism. 1

• Mixed platelet-fibrin thrombi form in regions of slow to moderate flow and contain a mixture of red cells, platelets, and fibrin. 1

• Microvascular thrombosis occurs in small vessels and is often a complication of disseminated intravascular coagulation or generalized platelet aggregation, leading to tissue ischemia, red cell fragmentation, and potential hemorrhage. 1

Pathophysiology of Thrombus Formation

• Arterial thrombosis is triggered by:

  • Rupture of atherosclerotic plaques exposing thrombogenic subendothelium 1
  • Disturbed blood flow in stenotic vessels 1
  • Activation of both blood coagulation and platelets 1

• Venous thrombosis is primarily caused by:

  • Blood stasis/turbulent flow 1
  • Activation of blood coagulation (more important than platelet activation) 1
  • Hypercoagulable states (inherited or acquired thrombophilia) 1

• Thrombus evolution follows a predictable pattern:

  • Initial formation at site of vessel injury or stasis 1
  • Incorporation of leukocytes attracted by chemotactic factors 1
  • Gradual replacement of platelets by fibrin 1
  • Eventually, fibrin clot digestion by fibrinolytic enzymes 1

Clinical Consequences of Thrombosis

• Venous thromboembolism accounts for over 300,000 hospital admissions annually in the US, with pulmonary embolism contributing to death in 12% of hospitalized patients. 1

• Arterial thrombosis is responsible for many acute manifestations of atherosclerosis, including:

  • Myocardial infarction
  • Unstable angina
  • Ischemic stroke
  • Peripheral arterial disease 1

• Complications of thrombosis occur through three main mechanisms:

  • Local obstruction of blood vessels
  • Distant embolism of thrombotic material
  • Consumption of hemostatic elements 1

Risk Factors for Thrombosis

• General risk factors include:

  • Advanced age (though thrombosis can occur at any age) 2
  • Surgery or trauma 3
  • Cancer 3
  • Immobility 2

• Specific risk factors for venous thrombosis in young adults:

  • Oral contraceptives (in women) 2
  • Pregnancy and postpartum period 2
  • Inherited thrombophilias (Factor V Leiden, protein C/S deficiency) 1, 3
  • Central venous catheters (especially in children) 1

• Specific risk factors for arterial thrombosis:

  • Atherosclerosis 1
  • Hypertension 1
  • Diabetes mellitus 1
  • Smoking 1

Diagnostic and Treatment Considerations

• Venous thromboembolism is effectively treated with anticoagulants, while platelet-suppressing drugs are less beneficial. 1

• Arterial thrombosis may be treated with both anticoagulants and antiplatelet drugs, as both pathways are important in its pathogenesis. 1

• Pediatric thrombosis, particularly in children with congenital heart disease, requires special consideration:

  • Central venous lines should be removed as soon as clinically possible 1
  • Anticoagulation may be reasonable for documented acute venous thrombi 1
  • Children with congenital heart disease have unique risk factors for thrombosis 1

Prevention Strategies

• Risk assessment should identify patients at high risk for thrombosis based on individual risk factors. 3

• Prophylactic anticoagulation is effective in preventing venous thromboembolism in high-risk settings. 1

• Early mobilization after surgery or during hospitalization reduces the risk of venous thrombosis. 2

• Addressing modifiable risk factors such as smoking cessation and blood pressure control can reduce arterial thrombosis risk. 1