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About thrombosis
Venous thrombosis
A blood clot within a vein is known as a venous thrombosis. The most common type of venous thrombosis is a deep vein thrombosis (DVT) in the leg.
If this happens, all the tissues drained by the vein can become swollen and painful due to the blood being unable to escape.
A major concern is that someone with venous thrombosis may develop a pulmonary embolism. When this happens, part of the clot (an embolus) may break off, travel up the body and through the heart to the lungs, blocking an artery.
This is a serious and potentially life-threatening condition and up to 1 in 10 people who suffer a pulmonary embolism will die if it is not treated.
The incidence of venous thrombosis
In the UK, up to one in every 1,000 people are
affected by venous thrombosis each year. Many
of those affected have pre-existing risk factors such as serious illness
or major surgery.
Around 30 per cent of people who have had a venous thrombosis develop further problems within the next 10 years, despite treatment. In nearly a third of cases, they develop post-phlebitic syndrome.
Venous thrombosis and pregnancy
The risk of venous thrombosis is significantly higher
for pregnant women. About 1 in every 1000 women develops thrombosis during
pregnancy, which is about 5 times greater than the risk for non-pregnant women
of the same age.
In the UK, venous thrombosis and pulmonary embolism are the main cause of death during pregnancy.
Download
our thrombosis and pregnancy fact sheet
(pdf format)
What causes venous thrombosis?
Blood is more prone to clot if one or more of
the following three factors occur:
- Changes in the blood vessel wall
- Changes in blood flow
- Changes in blood constituents
The most important risk factors are discussed in risk
factors for venous thrombosis
Deep vein thrombosis
A deep vein thrombosis (DVT) is a blood clot in the
deep veins, usually within the leg, although it can occur elsewhere in the
body. These deep veins go through the muscle and cannot be seen beneath the
skin.
A DVT can occur in the calf, behind the knee, in the thigh
or very high in the leg veins within the pelvis. Most DVT occur in the calf
veins, except during pregnancy, when they tend to lie within the thigh and
pelvis.
A DVT in the leg usually results from stasis or lack of movement, when the blood flow is restricted. This is more common in the legs that in other parts of the body.
Pulmonary embolism
When someone has a thrombosis such as a DVT in
the leg, they are at risk from a pulmonary embolism. When this occurs,
part of the clot breaks off (an embolus), travels up the leg, through
the right side of the heart and lodges in an artery in the lung (a pulmonary
artery).
The blocked artery restricts the blood supply to the lung, causing that part of the lung to die. It also affects the supply of oxygen to the lungs, resulting in breathlessness and pain.
Sometimes, over a period of time, many small particles may break off and cause multiple pulmonary emboli.
Post-phlebitic syndrome
Deep vein thrombosis may damage the valves in the deep veins, causing
long-term complications such as post-phlebitic syndrome.
Normally the valves in the deep veins prevent blood from travelling back down the leg. In post-phlebitic syndrome, damage to valves higher in the leg causes increased pressure in the veins of the lower calf and ankle.
Download
our fact sheet about venous thrombosis
(pdf format)
top of page
Diagram of a deep vein thrombosis in the calf with clot formation (shown
in red) along the length of a vein
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