Virchow's Triad — Stasis, Endothelial Injury and Hypercoagulability

Virchow's triad is the classic framework for why blood clots inside a living vessel. It groups every thrombotic risk factor into three buckets: (1) abnormal blood flow — stasis or turbulence, (2) endothelial injury (vessel-wall damage or dysfunction), and (3) hypercoagulability — an increased tendency of the blood itself to clot. The concept is credited to the Berlin pathologist Rudolf Virchow, whose work on venous thrombosis in 1856 identified these factors.

Thrombosis is pathological haemostasis: the same platelets, coagulation factors and fibrin that seal a cut form a clot where there is no bleeding to stop. A single factor may be enough (a ruptured coronary plaque), but most real patients have two or three at once — the post-operative orthopaedic patient is immobile (stasis), has had tissue and vessel trauma (injury), and is in a post-surgical hypercoagulable state.

It depends on the circulation. Robbins states that endothelial injury leading to platelet activation almost inevitably underlies thrombus formation in the heart and the arterial circulation, because high flow rates normally wash away clotting factors. That is why aspirin and other antiplatelet drugs are the backbone of coronary disease treatment.

In veins the picture flips: stasis is a major factor in venous thrombi, and hypercoagulability contributes infrequently to arterial or intracardiac thrombosis but is an important risk factor for venous thrombosis (Robbins). A young patient with recurrent DVT and no obvious trigger should make you think of an inherited thrombophilia; a smoker with an acute STEMI should make you think of plaque rupture.

Robbins divides hypercoagulable states into primary (genetic) and secondary (acquired) disorders. Inherited causes should be considered in young patients (under about 50 years) with venous thrombosis, even when an acquired trigger is also present, because a carrier's risk multiplies with pregnancy, bed rest or long flights.

Hyperhomocysteinaemia sits in both camps: it can be inherited (e.g. homocystinuria from cystathionine β-synthase deficiency) or acquired (folate/B₁₂ deficiency). If a question forces a choice between PNH and homocystinuria as 'acquired', choose PNH.

Both contain platelets, fibrin and trapped red cells — the proportions differ because of shear. Harrison explains that arterial thrombi are rich in platelets because of high shear in injured arteries and look white, whereas venous thrombi form under low shear, contain relatively few platelets and are mostly fibrin and trapped red cells, so they look red. StatPearls describes arterial thrombi as a platelet-rich 'white head' followed by a fibrin- and red-cell-rich 'red tail'.

Mural thrombi form in heart chambers or the aortic lumen. Abnormal myocardial contraction (arrhythmia, dilated cardiomyopathy, MI) or endocardial injury promotes cardiac mural thrombi; ulcerated plaques and aneurysms promote aortic ones. Robbins notes that about 80% of systemic emboli arise from intracardiac mural thrombi, two-thirds of them linked to left-ventricular infarcts.

This is a favourite pathology and forensic question. Lines of Zahn are grossly and microscopically visible laminations — pale bands of platelets and fibrin alternating with darker red-cell-rich layers. They form only in flowing blood, so they usually mark a thrombus formed before death.

Most vignettes describe a patient, not a limb of the triad. Translating the scenario into stasis, injury or hypercoagulability — and noting the mechanism — is what separates a guess from a sure answer.

Inherited and acquired factors add up. Robbins points out that factor V Leiden carriers are at particular risk when an acquired trigger — pregnancy, prolonged bed rest, a long flight — is layered on top, and that among patients with recurrent DVT the frequency of the Leiden mutation approaches 60%. This is why a first DVT in a young patient, or an unusual-site thrombosis, justifies a thrombophilia work-up (protein C, protein S, antithrombin, APC resistance/factor V Leiden, prothrombin variant and antiphospholipid antibodies).

Deep vein thrombosis of the leg is the textbook triad disease: the veins of the lower extremity account for about 90% of venous thromboses. Every major DVT risk factor maps to a limb — immobility and long travel (stasis), surgery and trauma (injury), cancer, pregnancy, oestrogens and thrombophilias (hypercoagulability). DVT is usually unilateral; clinical signs are unreliable, so objective testing (compression ultrasound) is needed, and a DVT may first declare itself as a pulmonary embolism.

The Wells criteria for DVT turn the history and examination into a pre-test probability. Each item scores 1 point, and one item subtracts 2:

In the three-tier version, ≤ 0 = low, 1–2 = moderate and ≥ 3 = high probability.

Rudolf Virchow's name appears across pathology, surgery and neuroanatomy. Match-the-following questions love to mix them up:

• Identify the limb — 'Atrial fibrillation contributes to thrombosis mainly by…' → stasis. 'Smoking…' → hypercoagulability and endothelial injury.
• Primary initiating factor of an arterial thrombus → endothelial injury (NEET PG 2020 pattern).
• Most common inherited thrombophilia → factor V Leiden (asked in INI-CET 2023 and 2024 and NEET PG 2018).
• Acquired vs inherited → PNH, antiphospholipid syndrome, cancer, OCPs are acquired (INI-CET 2024).
• Lines of Zahn → antemortem thrombus in flowing blood; postmortem clot is 'chicken fat and currant jelly', unattached.
• DVT statements → DVT is mostly unilateral; clinical assessment is unreliable; it can present first as PE (NEET PG 2019).
• NOT a risk factor questions → check each option against stasis, injury or hypercoagulability.