Formation of a blood clot within a deep-lying vein
- More common over the age of 40
- Slightly more common in females
- Sometimes runs in families
- Prolonged immobility and excess weight are risk factors
Deep vein thrombosis is a condition in which a blood clot forms in a large vein in a muscle, usually in the leg or pelvic region. It affects an estimated 1 in 1,000 people in the UK each year, many of them over 40.
The formation of a clot in a deep vein is usually not dangerous in itself. However, there is a risk that a fragment of the clot may break off and travel through the heart in the circulation. If the fragment lodges in a vessel supplying the lungs, a potentially fatal blockage called a pulmonary embolism occurs.
What are the causes?
Deep vein thrombosis is usually caused by a combination of slow blood flow through a vein, an increase in the natural tendency of the blood to clot, and damage to the wall of the vein.
Several factors may slow blood flow and thus increase the risk of deep vein thrombosis. Long periods of immobility, such as those experienced during long flights or car journeys or while bedridden, especially during an illness or after surgery, are a common cause of slow blood flow. Other causes include compression of a vein by the fetus during pregnancy or by a tumour. Leg injury may also slow blood flow and cause a clot to form in the deep veins of the leg.
The blood may clot more easily as a result of an injury, surgery, pregnancy, or taking a combined oral contraceptive (see Contraception) or hormone replacement therapy. Some people have an inherited tendency for the blood to clot too readily (see Hypercoagulability).
What are the symptoms?
If a blood clot has formed in a deep-lying vein, it may produce symptoms that include the following:
Pain or tenderness in the leg.
Swelling of the lower leg or thigh.
Enlarged veins beneath the skin.
Pulmonary embolism occurs in a small proportion of cases of deep vein thrombosis. The symptoms that occur with pulmonary embolism usually include shortness of breath and chest pain that is made worse with breathing. If a large proportion of the blood supply to the lungs is blocked, the condition is potentially life-threatening.
In some cases, thrombosis causes permanent damage to the vein and varicose veins may appear later.
What might be done?
Your doctor may need to carry out various tests urgently to confirm the diagnosis of deep vein thrombosis because the symptoms are often similar to those of other conditions, such as cellulitis. You may have Doppler ultrasound scanning to measure blood flow through the veins and sometimes a venogram, in which dye is injected into a vein and then X-rays are taken to reveal blood clots (see Contrast X-rays). MRI may also be used. A sample of your blood may be taken and analysed to assess how easily it clots.
Thrombolytic drugs may be prescribed to dissolve the blood clot in the vein and reduce the risk of a pulmonary embolism. You may also be given injections of anticoagulants to prevent further clots. Although treatment can take place in hospital, you may be able to administer anticoagulant drugs yourself at home. Rarely, surgery is required to remove the clot.
After you have had the initial treatment, your doctor will prescribe drugs to reduce the risk that the condition will recur (see Drugs that prevent blood clotting).
Can it be prevented?
Certain surgical procedures carry a risk of deep vein thrombosis, and your susceptibility to developing the condition will be assessed beforehand. If there is a high risk, low doses of short-acting anticoagulant drugs will probably be given before and after surgery to prevent the blood from clotting. Your doctor may also advise you to wear special elastic stockings for a few days after the operation to help maintain blood flow in the veins of the leg. Women who use a combined oral contraceptive or hormone replacement therapy may be advised to stop for a period of around 4 weeks before surgery.
The risk of deep vein thrombosis can be reduced by avoiding long periods of inactivity. If you are confined to bed, you should regularly stretch your legs and flex your ankles. Your doctor may also recommend that you wear compression stockings. During a flight, walk around at least once an hour, and, during a long drive, stop regularly to stretch your legs. It is also important to keep well hydrated.
What is the prognosis?
Usually, if deep vein thrombosis is diagnosed in its early stages, treatment with thrombolytic drugs and anticoagulants is successful. However, if the affected vein is permanently damaged, persistent swelling of the leg or varicose veins may develop and there is a risk that deep vein thrombosis may recur.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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