What Causes Deep Vein Thrombosis in the Leg?

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  • Written By: H. Colledge
  • Edited By: Heather Bailey
  • Last Modified Date: 03 February 2020
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Deep vein thrombosis, sometimes known as deep venous thrombosis or DVT, is a potentially serious condition in which a clot of blood, or thrombus, develops inside a vein. Most often, this occurs in the leg inside what is called a deep vein, which passes through a muscle in the thigh or calf. Anything which slows the flow of blood in the circulation, such as long air or road journeys or periods of illness where a person is confined to bed, can increase the likelihood of a deep vein thrombosis in the leg. Deep vein thromboses are also at risk to form if the walls of blood vessels are damaged by conditions such as injury or infection. Sometimes factors such as smoking or taking certain drugs alter the composition of the blood and increase its tendency to clot, making a deep vein thrombosis in the leg more likely.


When a person is inactive for long periods, the muscles in the leg, which would normally contract and help to keep blood moving, remain idle. This can lead to slower blood flow inside the deep veins that run through the muscles. A deep vein thrombosis in the leg could then result, causing symptoms of swelling, pain, warmth and redness. The condition is usually treated using anticoagulant drugs and compression stockings. If left untreated, there is a risk that a piece of the clot could break off and travel to the lungs, resulting in an inability to breathe which could be fatal.

Inactivity is the most common cause of a deep vein thrombosis in the leg. This could be the result of an illness such as a broken leg or a condition requiring treatment in intensive care, where someone is unable to move for a length of time. Long journeys may require that the legs remain in an immobile and cramped state, and it is important to move about as much as possible when traveling. Sometimes a surgical operation could take several hours to perform, increasing the chance that a venous clot might form. Being pregnant or overweight increases the pressure on blood in the legs and raises the risk of DVT occurring, and a failing heart can also lead to blood pooling in the lower limbs.

The wall of a vein may be injured by factors such as inflammation, and there is then a higher chance of clots developing at the damage site. Inflammation could be the result of cancer or other diseases, an infection, exposure to chemicals, or drugs such as those used in chemotherapy. Vein walls may also be injured by medical procedures, illegal drug use, or by a previously existing clot. Smoking, old age, hormone treatments and some medical conditions can increase the tendency for a clot, occasionally leading to deep vein thrombosis in the leg.



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