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While airline passengers wedged in cramped seats for long flights may have a greater risk of developing blood clots in their leg veins, adverse health effects as a result are unlikely, according to a study appearing in the May 12 issue of The Lancet, a British medical journal.

Deep vein thrombosis (DVT) is a condition of blood clots in veins. Our bodies routinely develop clots and then dissolve them. The potential for danger from DVT exists, however, as fragments of clots can break away from large vessels such as those in the leg and travel to smaller vessels in the lung causing a blockage. Such a blockage is called a pulmonary embolism and can be fatal.

In recent months, reports of airline passengers dying of complications from blood clots after long inter-continental flights have made headlines.

Prolonged air travel has been blamed for these deaths and survivors have, in fact, filed lawsuits against airlines. The question remains, however, as to what role, if any, flying played in the development of DVT.

John H. Scurr, FRCS, of Royal Free and University College Medical School in London, led the first randomized, controlled study looking at the association between long flights and DVT.

The study involved 200 participants over the age of 50 who took multiple flights of at least 8 hours within a 6-week period. Researchers used an ultrasound test on fliers’ leg veins to ensure that no participants had DVT before traveling. Within 48 hours of their return from flying, the participants were examined again for DVT. Researchers also used a sensitive blood test to look for evidence of recent clot formation to confirm the ultrasound results.

Of the 200 passengers, half were randomly selected to wear elastic compression stockings during their trips. Stockings may improve circulation and prevent clots during periods of inactivity. The remaining 100 passengers flew without stockings.

Scurr and colleagues found that 10 percent of the participants who did not wear stockings developed DVT, a surprisingly high proportion. All cases of DVT were symptomless. Two-thirds of the DVT cases resolved without treatment, and the remaining one-third were immediately given clot-busting prescription drugs — it is unknown whether these subjects’ DVT would have progressed to a more dangerous condition.

None of those who wore stockings developed DVT. A small number of stocking wearers (3 percent) did experience an irritation of varicose veins.

The authors conclude that wearing compression stockings during long-haul flights is associated with a reduced risk of DVT. Though not mentioned by the authors, the success of the stocking at preventing DVT may imply that one probable cause of the development of clots on a plane is the long period of stasis and inactivity.

Although 10 percent sounds like a frighteningly high proportion of people developing DVT from long, cramped flights, in an editorial accompanying the study Drs. Jack Hirsh and Martin O’Donnell provide reasons not to be overly concerned. They found that the study likely over-reports the number of cases of DVT. The ultrasound evaluation, they say, may have identified clots that did not actually exist (“false positive”) or were too small to be of consequence, since the sensitive blood test used to check for DVT did not confirm the presence of clots in many of these cases. Ultrasound technicians may also have been aware in some cases if a participant wore stockings, leading to a potential bias when subjectively identifying clots.