LONDON — Amid growing concern over whether cramped airline seating on long-haul flights causes deadly blood clots in the legs, medical experts say the real culprit is passengers sitting still for too long.
Reports of passengers dying from clots right after a long flight have mounted in recent months, prompting Australia's two biggest airlines to announce plans this week to print warnings on tickets. An Australian law firm has prepared compensation claims against six airlines for about 1,000 people who claim to have suffered deep vein thrombosis while on flights.
The problem has misleadingly been dubbed "economy class syndrome" because some people thought it was linked to the cramped seating in coach class. But fatal clots have also occurred in passengers in business and first classes, and in people who sit for long periods in buses, cars, theater seats and at desks.
Experts say nobody knows what the true incidence of deep vein clots on airplanes is or how long people have to stay still to get them.
Dr. Robert McBane, a blood clotting expert at the Mayo Clinic, believes many cases can be blamed on thrombophilia. While hemophilia describes a hereditary disorder in which the blood has difficulty clotting, thrombophilia describes the opposite problem--a genetic propensity for the blood to clot too easily.
Between 3% and 5% of people have that problem, McBane said.
"When these vulnerable people are put in the correct situation, like surgery, a car accident, coma, or maybe they have an extra-long flight or something, they are more prone to forming a blood clot," he said.
"If you look at the number of people traveling on airplanes every day and the number of people who develop a blood clot in that situation, it's pretty darn low," McBane said. "What about driving a truck? I'm not convinced it's about sitting in an airplane or even sitting at a desk so much as an underlying predisposition."
"You don't know who's vulnerable," McBane added. "We may all be forming blood clots all the time. There's something different about somebody who goes on to develop a clinical clot and someone whose clot is transient."
Some experts say one reason such clots seem to be on the rise is that greater numbers of high-risk people are flying because improved health care allows them to lead more active lives.
Deep vein thrombosis--in which a blood clot forms in a deep vein in the calves--is common, affecting about one in 1,000 people, McBane said. It is the fourth-leading cause of death in the United States.
Signs include aching legs, pins and needles, and problems bearing weight on the legs. If the clot moves to the lungs, chest pain is often a sign. The clot can kill if it breaks off from the leg vein and travels to the lungs.
In 1856, German researcher Rudolf Virchow, the father of blood clotting science, established three conditions involved in forming blood clots--lack of circulation, injury to a vein and increased coagulation, or the changing of liquid blood into a more jelly-like substance.
Scientists have since discovered more, including about a dozen inherited risk factors. Until 1995 the majority of those were deficiencies of proteins that inhibit clot formation. More recently, they have discovered there can also be an excess of pro-clotting chemicals and that such abnormalities are quite common.
Experts don't know exactly why blood clots start to form in the deep veins of the legs when circulation slows down, McBane said.
Risk factors that increase the likelihood include having had one before, having relatives who have suffered one, and having recent surgery, pregnancy, cancer or hormone therapy.
The most important thing to do to avoid a clot is to move around, experts say.
"It's always advisable to get out of your seat, have a walk every once in a while when you are sitting for a long time. It's common sense," said Dr. Valentin Fuster, director of the Cardiovascular Institute at Mount Sinai Medical School in New York and former president of the American Heart Assn.
"The advice of many of us is to walk around every two hours or so."
At least shove your feet under the seat in front to straighten the legs as much as possible and flex, other experts said.
It's not clear whether taking an aspirin before a flight does any good. While aspirin works well as a clot preventer in the arteries around the heart, it is not as well proven in the veins, where clots form slightly differently, Fuster said.
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Deep Vein Thrombosis
Sitting too long in one position can cause blood clots in the legs, a condition known as deep vein thrombosis. It can be serious if the clot ultimately blocks blood flow in the lungs.