Thrombosis

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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.