A study published Monday strengthens the evidence that long-distance travel can lead to potentially fatal blood clots in some people — showing that the risk grows in tandem with the length of the trip.
In an analysis of 14 previous studies, researchers found that, in general, travel was associated with a nearly three-fold increase in the risk of venous thromboembolism (VTE) — blood clots that form in the veins, often in the legs.
If such a clot dislodges and travels to the lungs, it can cause a potentially fatal condition called pulmonary embolism.
Several high-profile deaths have brought attention to the risk of VTE among travelers, particularly those on long-haul flights. Experts think a combination of factors — including dehydration and hours of sitting in cramped conditions — explains why some people develop blood clots.
However, not all studies have found a clear link between travel and VTE. To look at the discrepancy, the researchers who conducted the current review, published in the Annals of Internal Medicine, analyzed 14 studies involving more than 4,000 cases of VTE.
Some of the studies compared VTE patients with a "control" group of people who had been referred for possible VTE symptoms, but were found to not have a clot — a comparison that carries the risk of bias because the control group likely has a higher-than-average risk of blood clots.
In other studies, the control group consisted of healthy people from the general population — which are more likely to capture the true VTE risk associated with travel, explained lead researcher Dr. Divay Chandra of the Harvard School of Public Health in Boston.
When the researchers looked only at those studies, they found that travelers had a nearly three-fold higher risk of blood clots than non-travelers.
What's more, the risk climbed along with the duration of the trip — rising 18 percent for every two hours of any type of travel, and by 26 percent for every two hours of air travel.
But while the current findings confirm the travel-VTE link, Chandra told Reuters Health, "there is no reason for panic" because the absolute risk to any one traveler is still low.
Still, Chandra said, "people who travel long distances should be aware of the risk of blood clots and learn to recognize the symptoms."
Symptoms of a blood clot in the leg include pain, warmth, swelling and redness in the limb. If the clot travels to the lungs, it may cause sudden shortness of breath, chest pain or a cough that produces blood.
To help reduce the risk of VTE, experts generally recommend that long-distance travelers periodically move around and stretch their legs, and drink plenty of water to stay hydrated.
Certain people are at increased risk of blood clots, Chandra noted — including cancer patients, people who have recently had major surgery such as a joint replacement, and women on birth control pills. They may want to talk with their doctors about any precautions they should take when traveling, he said.