A Thrombus
Thrombosis may be influenced by any of the three factors described in Virchows Triad:
- Damage to the endothelium (e.g. atherosclerosis) Phenomena of interruputed blood flow, eg stasis: risks include venous stasis, long surgical operations, prolonged immobility, and varicose veins.
- Alteration to the blood flow (e.g. hypodynamic states such as heart failure or shock)Phenomena associated with irritation of the vessel and its vicinity, eg endothelial or vessel wall injury: includes injury or trauma such as vessel piercings, damage arising from shear stress or hypertension, and subsequent contact with procoagulant surfaces, such as bacteria, shards of foreign materials, biomaterials of implants or medical devices, membranes of activated platelets, and membranes of monocytes in chronic inflammation.
- Change in the constituents of the blood (prothrombotic states e.g. underlying malignancy, haemoproliferative disorders, smoking )Phenomena of blood coagulation, eg hypercoagulability: risk factors such as hyperviscosity, coagulation factor V Leiden mutation, coagulation factor II G2021A mutation, deficiency of antithrombin III, protein C or S deficiency, nephrotic syndrome, changes after severe trauma or burn, cancer, late pregnancy and delivery, race, advanced age, cigarette smoking, hormonal contraceptives, and obesity.
The most common cause of thrombotic limb ischaemia is thrombosis of a vessel on a background of atherosclerosis; smoking increases this risk due to its prothrombotic effect on platelets.
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Less common causes of acute limb ischaemia are shown in the table.
Table
| Cause | Pathology | Signs to look for |
|---|---|---|
| Vasculitis | Inflammation of the arteries |
|
| Popliteal entrapment syndrome | popliteal artery is compressed by gastrocnemius during plantarflexion |
|
| Compartment syndrome | Swelling of tissues within fascial compartment (especially anterior compartment of leg) compresses artery |
|
| Iatrogenic | Injury of usually the common femoral or superficial femoral artery following catheterisation |
|
| Aortic dissection | The dissection flap may occlude the true lumen of a branch vessel causing end organ ischaemia |
|
| Graft occlusion | Thrombosis of graft especially if prosthetic rather than vein |
|
Popliteal aneurysms
Popliteal aneurysms tend to accumulate thrombus. As a consequence of their position behind the knee joint this can dislodge and embolise to the foot.
Alternatively the aneurysm may occlude due to thrombosis. Acute limb ischaemia due to a thrombosed popliteal aneurysm carries a 50% risk of amputation. Therefore, any patient with a suspected popliteal aneurysm and ischaemic symptoms (even if these are only mild, they may herald a more severe event) should be referred urgently to a vascular specialist.