Diseases

The formation or presence of a blood clot in a blood vessel.

Organ or part of body involved: Vein or artery
Symptoms and indications: The following are the most common symptoms of thrombosis. However, each individual may experience symptoms differently. Symptoms may include:
increased blood clots in arteries and veins; pain isolated to one leg (usually the calf or medial thigh); swelling in the extremity; and varicose veins. The symptoms of thrombosis may resemble other blood disorders or medical problems.
Causes and risk factors: 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.
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.