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Deep vein thrombosis
Published by BUPA's Health Information Team
June 2003

A deep vein thrombosis (DVT) is a blood clot (thrombus) that develops in a deep vein, usually in the leg. This can happen if the vein is damaged or if the flow of blood slows down or stops. About one in 2,000 people in the UK develops a DVT each year.

Deep vein thrombosis can cause pain in the leg, and can lead to complications if it breaks off and travels in the bloodstream to the lungs.

Where do DVTs happen?
The are two types of veins in the legs: deep and superficial (see diagram, below). The deep veins pass through the centre of the leg, surrounded by the muscles. It’s here that DVTs most often develop. Less commonly DVTs occur in the deep veins of the arm or pelvis.

Who gets a DVT?
Certain factors make a DVT more likely to occur. They are more common in people aged over 40 and in people who are obese, or who have already had a DVT. Several inherited conditions make the blood more likely to clot than usual, increasing the risk. Other factors include:

prolonged bed rest, (immobility)
major injuries, or paralysis
surgery, especially if it lasts more than 30 minutes, or involves the leg joints or pelvis
cancer and its treatments, which can cause the blood to clot more easily
long-distance travel, because of prolonged immobility. It is unclear whether or not air travel is more risky than other long journeys - for example by car or coach
pregnancy and childbirth - related to hormone changes that make the blood clot more easily and because the fetus puts added pressure on the veins of the pelvis. There is also risk of injury to veins during delivery or a caesarean. The risk is at its highest just after childbirth
taking a contraceptive pill that contains oestrogen. Most modern pills contain a low dose, which increases the risk by an amount that is acceptable for most women
hormone replacement therapy (HRT). For many women, the benefits outweigh the increase in risk.
other circulation or heart problems


The main leg veins

Problems caused by a DVT
A DVT below the knee is unlikely to cause complications and may only need to be monitored. But when a clot forms in or above the knee, there is a risk that it will break away and travel up the vein to block a blood vessel in the lung. This is called a pulmonary embolism (PE). Depending on the size of the clot, it can be a life-threatening condition. But with appropriate treatment, it is rare for a DVT to lead to a pulmonary embolism.

A DVT can damage the valves in the vein, so that instead of flowing upwards, the blood pools in the lower leg. This is called post-thrombotic syndrome, and can result in pain, swelling, discolouration and sores on the leg.