Causes

DVT can be caused by a narrowed or blocked vein which allows the blood to clot. This can be brought on by an injury to the vein (such as a sharp blow to the leg), or following surgery or radiation therapy. Anything that makes it harder for blood to return from the legs to the heart can increase the chance of deep vein thrombosis. When a person sits with legs cramped in a confined space for long periods, blood flow slows, making blood clots more likely. Blood clots can form in the legs if you sit for long hours in a cramped position, such as on a plane, train or bus.

Some people have a higher risk of blood clots: those with a history of heart failure, obesity, chronic leg vein trouble, recent bone fractures, or cancer; women on oestrogen medication, including oral contraceptives. In addition, they can occur during pregnancy as an increased tendency for the blood to form clots occurs naturally so as to prevent excessive bleeding during child-birth. DVT can also occur as the result of severe infection and liver disease Smoking increases the risk among each of these groups.

DVT is more common in the elderly, but healthy young people can develop it as well. Some people are not born with vital blood thinning substances and are therefore always more susceptible to blood clots.