MEDICAL MATTERS: About five years ago, there was a degree of media excitement concerning a possible link between air travel and the development of leg clots or deep vein thrombosis (DVT).
The term "economy class syndrome" was coined; it accurately reflected concerns about sitting for long periods in cramped spaces but misleadingly suggested the problem was confined to those seated in cheaper seats at the back of the plane.
In fact, the first report suggesting a possible link was reported in the New England Journal of Medicine in 1954, when a physician who developed DVT following a 14-hour flight from Boston to Venezuela described his experience.
Following heightened public concern in 2000, the House of Lords Select Committee on Science and Technology produced a report, Air Travel and Health. It found that, based on evidence available at the time, it was impossible to quantify the risk of DVT during air travel. Calling for further quality research into the issue, it recommended that we stop using the term "economy class syndrome", as "Travellers thrombosis" or "flight-related DVT" would be more appropriate.
The World Health Organisation (WHO) also responded by setting up the WHO Research Into Global Hazards of Travel Initiative (Wright). Last month, the Lancet reported the results of an interesting study carried out under the initiative.
Dutch researchers chartered a Boeing 757 for a non-stop eight-hour flight to and from Schiphol airport, Amsterdam. On board were 71 volunteers who had blood tests taken before, during and after the flight.
Two weeks later the same volunteers, aged 20-39, were kept seated for eight hours while watching films; after a further 14-day interval, the subjects were asked to carry out eight hours of normal daily activities. Blood tests were also taken during these two controlled situations.
After the flight, average concentrations of thrombin/ anti-thrombin complex, a marker for clotting, increased by 30 per cent. Significantly, even though the volunteers were also sitting for a similar period at the cinema, the coagulation marker concentration decreased by 2 per cent in this group. And average concentrations dropped by 8 per cent after the subjects undertook normal activities.
The researchers had excluded those with known risk factors for DVT, including people with previous venous thrombosis, volunteers who had recent surgery, those with active cancer, women taking hormone replacement therapy and females who were either pregnant or had recently given birth.
Another interesting finding was that some people responded to flight conditions more markedly than others. Those with a genetic mutation (Factor V Leiden) associated with abnormal clotting and women taking the oral contraceptive pill had the greatest increase in clotting factor concentrations.
The well designed study indicates that when immobilisation is combined with flight-associated factors, such as low oxygen concentrations, blood coagulation is activated, making it more likely that a DVT could develop. And when you add these conditions to other factors, such as taking the pill or having a clotting factor mutation, the risk rises further.
Some 5 per cent of the population have the Factor V Leiden mutation, meaning that 15-20 people in a fully laden jumbo jet are at a significant risk of DVT.
"We conclude that the coagulation system is activated in some susceptible individuals after an eight-hour flight, indicating an additional mechanism to immobilisation for a travel-related thrombosis," the authors said.
However, it must be emphasised that most thromboses remain in the leg and are broken down naturally in the body, posing no danger to the air traveller. Symptomless DVT occurs in about 4 per cent of people after a long-haul flight, with the rate of symptomatic venous thrombosis estimated at one per 6,000 flights.
Only one per 2 million passengers die as a result of a flight- related DVT, Australian research has found. The danger occurs when a piece of clot breaks off and travels from the leg, through the heart, and lodges in the lung. A large clot can block one or more pulmonary arteries (pulmonary embolism), so that blood cannot travel from the right side of the heart to the lungs, quickly causing death.
If you are someone with the risk factors mentioned earlier, then you should consult your doctor before setting off on a long-haul flight. You may be a candidate for a single dose of low molecular weight heparin given shortly before travel which will help prevent DVT for up to 18-24 hours following the injection.
For the rest of us, common sense suggests we avoid taking alcohol or sedative drugs while flying, as they promote immobility. Exercising the muscles in our calves during flight should reduce the risk of blood pooling in our leg veins. And drinking lots of fluids will help avoid dehydration - itself a factor in slowing blood flow and increasing the risk of a clot developing in the deep veins of the leg.
Muiris Houston is pleased to hear from readers , but regrets he cannot answer individual queries.
