thrombosis n : the formation or presence of a thrombus (a clot of coagulated blood attached at the site of its formation) in a blood vessel

English

Noun

1. The formation of thrombi in the blood vessels of a living organism, causing obstruction of the circulation.

Translations

Derived terms

• thrombotic
• thrombus
• deep vein thrombosis

Thrombosis is the formation of a clot or thrombus inside a blood vessel, obstructing the flow of blood through the circulatory system. Thromboembolism is a general term describing both thrombosis and its main complication which is embolisation.

Causes

In classical terms, thrombosis is caused by abnormalities in one or more of the following (Virchow's triad):

• The composition of the blood (hypercoagulability)
• Quality of the vessel wall (endothelial cell injury)
• Nature of the blood flow (hemostasis)

The formation of a thrombus is usually caused by Virchow's triad. To elaborate, the pathogenesis includes: an injury to the vessel's wall (such as 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 behavior - for example, sitting on a long airplane flight); by a blood state of hypercoagulability (caused for example, by genetic deficiencies or autoimmune disorders).

High altitude has also been known to induce thrombosis http://dx.doi.org/10.1371/journal.pmed.0040290http://www.mounteverest.net/news.php?news=16349. Occasionally, abnormalities in coagulation are to blame. Intravascular coagulation follows, forming a structureless mass of red blood cells, leukocytes, and fibrin.

Classification

There are two distinct forms of thrombosis:

Venous thrombosis

• Deep venous thrombosis (with or without pulmonary embolism; together classified as venous thromboembolism/VTE)
• Portal vein thrombosis
• Renal vein thrombosis
• Hepatic vein thrombosis (Budd-Chiari syndrome)
• Paget-Schroetter disease (upper extremity vein)
• Cerebral venous sinus thrombosis
• Thoracic outlet syndrome (the cause of most Subclavian Vein Thrombosis unrelated to trauma)

Arterial thrombosis

• Stroke (either thrombotic or embolic)
• Myocardial infarction (usually coronary thrombosis due to rupture of an atherosclerotic plaque)
• Thoracic outlet syndrome (may precipitate arterial thrombosis as well as venous)

Embolisation

If a bacterial infection is present at the site of thrombosis, the thrombus may break down, spreading particles of infected material throughout the circulatory system (pyemia, septic embolus) and setting up metastatic abscesses wherever they come to rest. Without an infection, the thrombus may become detached and enter circulation as an embolus, finally lodging in and completely obstructing a blood vessel (an infarction). The effects of an infarction depend on where it occurs.

Most thrombi, however, become organized into fibrous tissue, and the thrombosed vessel is gradually recanalized.

Prevention

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 atrial 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.

In people admitted to hospital, thrombosis is a major cause for complications and occasionally death. In the UK, for instance, the Parliamentary Health Select Committee heard in 2005 that the annual rate of death due to hospital-acquired thrombosis was 25,000. In patients admitted for surgery, graded compression stockings are widely used, and in severe illness, prolonged immobility and in all orthopedic surgery, professional guidelines recommend low molecular weight heparin administration, mechanical calf compression or (if all else is contraindicated and the patient has recently suffered deep vein thrombosis) the insertion of a vena cava filter. In patients with medical rather than surgical illness, LMWH too is known to prevent thrombosis, and in the United Kingdom the Chief Medical Officer has issued guidance to the effect that preventative measures should be used in medical patients, in anticipation of formal guidelines.

See also

• Anticoagulants

References

External links

• North American Thrombosis Forum - US organisation dedicated to clotting diseases of the circulatory system.

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