Deep vein thrombosis, commonly referred to as DVT, is a disease of the circulatory system.
DVT occurs when a clot forms in one of the "deep veins" - often in the lower limbs - and this lodges, blocking the blood flow.
It mainly affects people who are unable to exercise properly such as those who have just had an operation, or those confined to a plane, car or bus.
Sometimes there are no symptoms at all. But there can be pain or tenderness in the leg or sudden swelling. There may also be pain on flexing the foot; chest pains or a cough.
Anyone suffering these symptoms after a period of inactivity should contact their doctor.
DVTs themselves are not life-threatening, but can cause serious complications such as a pulmonary embolism, which happens in about a quarter of DVT cases.
This is where a piece of the clot breaks away and travels through the body into the lung and affects breathing.
Without treatment this can kill up to one in ten people.
A less serious complication is post-thrombotic syndrome, which causes damage to the valves along the length of the vein, causing pain, swelling and ulceration; some people even need to have their limbs amputated.
Although it is not possible to know whether a particular traveller will get a DVT, some people do have higher risk factors. These are:
- Frequent flyers
- women taking hormone replacement therapy (HRT)
- people with a family history of DVT
- cancer sufferers
- obese travellers
- people who are paralysed or immobile.
Women taking the contraceptive pill are three or four times more likely to suffer from a DVT than those who do not. Women who are pregnant and those who have recently given birth are also at a higher risk.
Keeping as mobile as possible during a journey is vital to avoid DVTs.
Even in the cramped conditions of an aircraft it is possible to exercise - foot exercises such as rotating the ankles and wiggling the toes keep the circulation moving as do special foot cushions.
Air passengers can also buy specially designed flight stockings to protect themselves, but experts say these must be properly fitted.
Travel doctors also advise that passengers keep well hydrated, if only to ensure frequent toilet visits, which will necessitate the patient getting up and moving about.