Deep Vein Thrombosis

What is Deep Vein Thrombosis (DVT)?

Deep Vein Thrombosis (DVT) is a condition in which a blood clot (thrombus) develops in a deep vein of the legs.

Clotting within blood vessels (thrombosis) is a common complication arising from major surgery, and is widely thought to be a side-effect of anaesthesia and enforced immobility.

DVT is not dangerous in itself. Studies show that up to 50 per cent of cases are completely asymptomatic. Around 30 per cent of post-operative DVTs have been shown to cause local complications, ranging from swelling, pain and tenderness in the affected leg, to skin deterioration, venous ulceration and physical disability, and a substantially greater risk of further DVT in future adverse circumstances.

However, some of the complications that can arise from DVT may be life-threatening. When a thrombus breaks away from the vessel wall and begins to flow with the blood (an embolus) it will block a blood vessel that it cannot pass through (an embolism).

This syndrome is called Venous Thrombo-Embolism (VTE) and its most serious manifestation is a blockage in the lungs - pulmonary embolism (PE) - giving rise to chest pain, breathing difficulties and, in the worst cases, death from respiratory failure. Around one per cent of post-operative DVTs result in pulmonary embolism.

Case studies produced over the past 50 years have increasingly suggested a link between air travel and DVT.


Background

The association of long haul flights with the occurrence of DVT is a relatively recent one, following extensive medical research precipitated by the deaths of a number of passengers on such flights.

Sixty years ago, the first research linking DVT to spatial confinement, immobility and constrained seating conditions was conducted by Professor Keith Simpson, who studied people spending long periods in air-raid shelters. He found that PE deaths were six times higher in people who had sat in hard-edged deck chairs than in those who had not.

Many subsequent studies also made this link between DVT, enforced immobility and unsuitable seating - particularly on aircraft. DVT has been especially linked to long-haul flights, because of longer periods of enforced immobility.

Indeed, a number of investigations from 1999 onwards found evidence of a close link between the length of flights and the incidence of DVT in passengers. A 2001 study published in the New England Journal of Medicine claimed that passengers on flights of over 3,100 miles were 150 times more likely to get a serious blood clot than those who flew on shorter flights. A 2003 New Zealand study published in The Lancet suggested that one per cent of all passengers on long-haul flights would suffer DVT.

A number of factors increase the risk of DVT: susceptibility is higher in those aged over 40; pregnancy ; former or current malignant disease; blood disorders leading to increased clotting tendency or inherited or acquired impairment of blood clotting mechanisms (believed to be present in 20 per cent of the population); some types of cardiovascular disease or insufficiency; personal or family history of DVT; recent major surgery or injury, especially to lower limbs or abdomen; oestrogen hormone therapy, including oral contraception; immobilisation for a day or more; and depletion of body fluids causing increased blood viscosity.

However, a case in 2004 of an otherwise healthy 14-year-old with DVT - as a result of sitting on his legs whilst playing on a games console - shows that DVT can and does occur, albeit rarely, outside these risk groups.

DVT can be treated effectively with anti-coagulants and blood thinning agents. The difficulty lies in detecting DVT before serious complications arise, as it takes weeks before many sufferers display any symptoms.


Controversies

The wide range of predisposing factors, and the lack of comprehensive studies comparing post-travel DVT incidence to its levels in the general public, make travel-related DVT a controversial issue. The delay in the appearance of symptoms makes it more complicated to determine the impact of air travel.

DVT has been called 'economy class syndrome' in the media for over 25 years. The term, which emphasises the contribution of smaller and less comfortable seats on cheap flights, is widely believed to be misleading because studies have shown that DVT is just as prevalent among business and first class passengers.

There have been claims of contributory negligence from air carriers to the death of passengers on their flights, along with widespread media coverage of lawsuits against major airlines. However, while it is accepted that long periods of inactivity are known causes, the contribution of other risk factors to the incidence of DVT in predisposed passengers remains unclear.


Statistics

90 to 95 per cent of DVT cases occur in already 'at-risk' groups

Recent research suggests a rate of asymptomatic DVT of up to 10 per cent in long-haul flyers

However, this figure is at least 40 times higher than estimates derived from three other case-control studies

Statistic 1: (Source: NHS Direct, 2004); Statistic 2: (Source: JH Scurr et al - 'Frequency and Prevention of Symptomless Deep Vein Thrombosis in Long-Haul Flights: A Randomised Trial', The Lancet (5), 2001); Statistic 3: (Source: Jack Hirsh and Martin J O'Donnell, 'Venous Thromboembolism after Long Flights: Are Airlines to Blame?', The Lancet (357), 2001)


Quotes

"It would be premature to legislate that airlines change the seating configuration or introduce other costly prophylactic procedures until there is more information on the extent of the problem and on the effectiveness of much simpler preventive measures."

Jack Hirsh and Martin J O'Donnell, 'Venous Thromboembolism after Long Flights: Are Airlines to Blame?', The Lancet (357), 2001

"[The risk of] developing deep-vein thrombosis is very small unless additional pre-existing risk factors for thromboembolism are present."

World Health Organisation, 'International Travel and Health', 2002