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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.

Symptoms The symptoms of thrombosis depend on the effects of restriction of the blood flow through the vessel it affects. Causes In general, thrombosis seldom occurs in a healthy artery, because the smooth inner lining prevents the clot from forming. Injury to a blood vessel, or any disease process affecting the smoothness of the inner lining, can start the process of thrombosis. The commonest cause of thrombosis in arteries is atherosclerosis (see article on atherosclerosis). Atherosclerosis causes strokes and heart attacks and is now the number one killer of the Western world. Atherosclerosis causes rough, raised plaques (which contain cholesterol) on the inner lining of arteries. These eventually tear, and the thrombosis starts to build up where the plaques are torn. Even when arteries are normal, a clotting tendency can result from hormonal or biochemical changes in the blood. The tendency to thrombosis in arteries may be greater during pregnancy, in women using oral contraceptives, in people with cancer that has affected blood vessels, and in people whose blood is thicker than normal (those with polycythaemia, which means there are too many cells in the blood). Thrombosis in veins is encouraged by local pressure, inflammation (thrombophlebitis) and stagnation of blood flow through inactivity.

Diagnosis Thrombosis is diagnosed by its effects on a particular artery or vein. Special forms of X-ray or scanning may assist in diagnosis. Treatment Clotbusting drugs (fibrinolytic therapy) can be useful in treating some forms of thrombosis. There is good evidence that the combination of fibrinolytic drugs such as streptokinase with aspirin can significantly reduce the death rate from coronary thrombosis.

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 travellers thrombosis in airline passengers. Find out how many calories you can burn doing the activities you enjoy Work out your body mass index number here

Glossary   Blood Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.

Heart The heart is a muscular organ that pumps blood around the body.

Brain The brain controls thought, memory and emotion. It sends messages to the body controlling movement, speech and senses.

Lung Lungs are a pair of organs in the chest that control breathing. They remove carbon dioxide from the blood and replace it with oxygen.

Cholesterol Cholesterol is a fatty substance made by the body that lives in blood and tissue. It is used to make bile acid, hormones and vitamin D.

Blood vessel Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessels are veins, arteries and capillaries.

Artery Arteries are blood vessels that carry blood from the heart to the rest of the body.

Vein Veins are blood vessels that carry blood from the rest of the body back to the heart.

Kidney Kidneys are a pair of bean-shaped organs located at the back of the abdomen, which remove waste and extra fluid from the blood and pass them out of the body. 

X-ray An x-ray is a painless way of producing pictures of inside the body using radiation.

Embolism An embolism is the sudden blockage of a blood vessel, usually by a blood clot or air bubble.

Intravenous Intravenous (IV) means the injection of blood, drugs or fluids into the bloodstream through a vein.

Pain Pain is an unpleasant physical or emotional feeling that your body produces as a warning sign that it has been damaged.

Inflammation Inflammation is the body's response to infection, irritation or injury, which causes redness, swelling, pain and sometimes a feeling of heat in the affected area.

Heart attack A heart attack happens when there is a blockage in one of the arteries in the heart.

Dose Dose is a measured quantity of a medicine to be taken at any one time, such as a specified amount of medication.

© Queen's Printer and Controller of HMSO, 2005

Crown copyright material is reproduced with the permission of the controller of HMSO and the Queens Printer for Scotland.



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