Thrombosis

Thrombosis is the clotting of blood within a blood vessel. It occurs when a blood clot blocks a vein or an artery, obstructing or stopping the flow of blood.

Thrombosis can occur anywhere in the body's bloodstream. There are two main types of thrombosis:

• venous thrombosis, which is a blood clot that develops in a vein, and

• arterial thrombosis, which is a blood clot that develops in an artery.

One of the most common types of venous thrombosis is deep vein thrombosis (DVT), which is a blood clot in one of the deep veins of the body.

Arterial thrombosis often occurs in arteries that supply the heart, resulting in a heart attack. It can also occur in the arteries of the brain, causing a stroke.

Sometimes a blood clot (or part of one) can come away from its original site and travel through the bloodstream. If this occurs, the clot can become lodged in another part of the body. This is known as an embolism. A blood clot that lodges in one of the lungs is called a pulmonary embolism.

In classical terms, thrombosis is caused by abnormalities in one or more of the following:

• The composition of the blood (hypercoagulability);

• Quality of the vessel wall (endothelial cell injury);

• Nature of the blood flow (stasis, turbulence).

The formation of a thrombus is usually caused by Virchow's triad. The pathogenesis includes an injury to the vessel's wall by trauma, infection, or turbulent flow at bifurcations; by the slowing or stagnation of blood flow past the point of injury (which may occur after long periods of sedentary behaviour, for example, sitting on a long airplane flight); by a blood state of hypercoagulability (caused, for example, by genetic deficiencies or autoimmune disorders).

Thrombosis is a widespread condition and a leading cause of death in the UK. Every year, one in every 1,000 people in the UK is affected by venous thrombosis.

Anyone can be affected by thrombosis, although it usually develops in older people and becomes more common with age. As well as age, there are a number of other factors that make developing thrombosis more likely, including family history, inactivity and obesity.

Thrombosis often has few or no symptoms, and is sometimes referred to as a silent condition. Therefore, it is important to be aware of the signs and risk factors of thrombosis. There are several things that you can do to help prevent the condition occurring.

Thrombosis and embolism can be partially prevented with anticoagulants in those deemed at risk. Generally, a risk-benefit analysis is required, as all anticoagulants lead to a small increase in the risk of major bleeding. In arterial fibrillation, for instance, the risk of stroke (calculated on the basis of additional risk factors, such as advanced age and high blood pressure) needs to outweigh the small but known risk of major bleeding associated with the use of warfarin.