Thrombosis Introduction Thrombosis means the clotting of blood within an artery or a vein. This is always abnormal and is often dangerous, as it can reduce or stop the flow of blood. When thrombosis affects important arteries, such as the coronary arteries (causing a heart attack) or the arteries supplying the brain with blood (causing a stroke), it is a major cause of death and serious illness. Thrombosis of arteries supplying the legs leads to pain on using the leg (or even when at rest if the thrombosis is severe). If the blood flow is cut off completely, the leg dies and becomes gangrenous. If this happens in the arteries to the intestines, gangrene of a segment of bowel follows. This is a surgical emergency. Thrombosis of the arteries to the kidneys can cause serious kidney damage. Thrombosis of deep leg veins causes swelling, redness and pain in the leg. A clot may break off and, carried with the flow of blood, lodge in the lungs. This is called a pulmonary embolism and is a common cause of sudden, unexpected death (see articles on Embolism and Pulmonary embolism). Thrombosis of superficial veins in the legs has no connection with deep-vein thrombosis and is not dangerous. It can be caused by skin infection or repeated blows to the skin over the veins. In intravenous drug abusers it can occur where a vein is regularly punctured with a needle. Prevention The risk of coronary thrombosis (leading to heart attacks) and thrombosis of brain arteries (causing strokes) can be reduced by reducing the risk factors for these conditions (see articles on heart attack and stroke) and by reducing atherosclerosis. Regular small doses of aspirin may help reduce the risks of arterial thrombosis, but there is no evidence so far that aspirin reduces the risk of venous thrombosis. The risk of deep-vein thrombosis can be reduced by avoiding becoming very overweight, staying active in general, and avoiding prolonged periods of immobility with the legs down. This is particularly important on long journeys by any form of transport, when leg exercises and getting up and moving around regularly can help keep blood flowing through veins. This is the subject of much research at present in the wake of media publicity about traveller’s thrombosis in airline passengers.