Up TO one in 10 long-haul air travellers shows evidence of symptomless clotting in the legs, according to the latest research into "economy-class syndrome". The so-called syndrome, in which blood flow in leg veins is thought to be impaired by long periods of cramped air travel, has been associated with sudden death when a resulting clot travels to the lungs.
The study, in today's edition of the Lancet, also suggests that wearing elastic compression stockings during long flights reduces the incidence of deep venous thrombosis, which is the process of clot formation.
In the first study specifically designed to investigate deep venous thrombosis in air travel, Dr John Scurr and colleagues at the Lister Hospital in Chelsea and University College, London, recruited 200 passengers who were about to make air journeys of a minimum eight-hour duration. The participants were divided at random into two groups and half were given special compression stockings to wear during the flight.
The trial participants were all aged over 50 and underwent ultrasound testing to check for deep venous thrombosis before and after flying. Passengers arranged their own air travel in economy class, with no airline collaboration.
The results showed that 12 passengers who did not wear stockings developed a deep venous thrombosis. None of those wearing compression stockings did. All 12 with clots were symptomless; no leg swelling or pain was noted by them.
However, in an article accompanying the research, Canadian doctors criticise the trial on a number of grounds. They are concerned that the reported incidence of symptomless clotting is 40 times higher than previous estimates and propose that this high rate may be due to biased ultrasound assessment.
Dr Jack Hirsch and Dr Martin O'Donnell also question why venography - in which dye is injected into the legs - was not performed. The British authors decided that such testing would be unethical in patients who displayed no symptoms of deep venous thrombosis.
Dr Hirsch said: "If the results of the randomised trial are valid, they should be easy to reproduce and, if confirmed, they would clearly establish lengthy air travel as an important risk factor for thrombosis."
The World Health Organisation convened a meeting of scientists and airlines in March which agreed to carry out more rigorous trials into the syndrome.
While most clots will resolve spontaneously after the completion of travel, some will not. Further research is now required to establish the percentage of these symptomless clots which progress to give problems. In the meantime, all long-haul travellers should maintain good hydration, walk around the cabin and carry out in-seat leg exercises. Other measures, including wearing compression stockings, are appropriate for high-risk patients.