Deep vein thrombosis. The lower left leg of a patient affected by deep vein thrombosis. It is caused by thrombi (blood clots) found within deep- lying veins. It is generally triggered by a combination of slow-moving blood flow and some condition which increases the natural tendency of blood to clot. It causes symptoms such as pain, tenderness, swelling (oedema), redness (erythema) and sometimes ulceration of the skin in the area affected. A clot, or part of it, may break off and be carried to the lungs, where it may block an artery causing a serious condition known as pulmonary embolism. The patient is also affected by pre-existing varicose veins.