A new study by European researchers suggests that long haul flights do not generally pose a high risk of thrombosis among healthy travellers, but there are some groups that could be at higher risk that should take precautions.

The study is published this week in the open access journal PLoS Medicine and is the work of Frits Rosendaal and colleagues from Leiden University Medical Center, Academic Medical Center Amsterdam, and Nestlé Medical Services.

The authors wrote that since 1951 venous thrombosis has been linked to air travel but despite numerous studies none has assessed the absolute risk of venous thrombosis after long haul travel.

Knowing what the absolute risk of getting a thrombosis is after a long haul flight helps people decide what measures to take to prevent them and if they are worthwhile.

In the first study to assess the absolute risk of venous thrombosis after air travel, Resendaal and colleagues surveyed 8,755 employees working for international companies about their long haul travel journeys between 1 January 2000 and 31 December 2005 and the incidence of thromboses (both deep vein and pulmonary).

The authors defined a long haul flight as one lasting at least 4 hours, and exposure was defined as lasting for 8 weeks after a flight.

Altogether the data covered 38,910 person-years, of which 6,872 comprised exposure to long haul flights.

During the follow up, results showed that: 53 thromboses occurred, 22 within 8 weeks of a long haul flight.
The incidence rate was 3.2 per 1,000 people for those exposed to long haul travel compared to 1.0 per 1,000 for those who were not.
This is equivalent to a risk of one event for every 4,656 long haul flights.
More frequent flying and longer flights increased the risk.
Risks were higher for people under 30 years of age, women on oral contraceptives, particularly short and particularly tall people, and those who were overweight.
the rate of thromboses was highest in the first two weeks after travel, and gradually went down to baseline after 8 weeks. The researchers said that because they studied a working population with an average age of 40 years, the absolute risk of venous thrombosis in the population at large is likely to be greater than suggested here.

Nevertheless, their conclusion was that:

"The results of our study do not justify the use of potentially dangerous prophylaxis such as anticoagulant therapy for all long-haul air travellers, since this may do more harm than good."

"However, for some subgroups of people with a highly increased risk, the risk-benefit ratio may favour the use of prophylactic measures," they added.

A deep venous thrombosis (DVT) is a blood clot that forms inside a vein deep inside the body (a vein takes blood to the heart, as opposed to an artery that takes blood from the heart).

DVT usually occurs in the deep veins of the leg, and the symptoms include pain, swelling, and redness in the leg.

The condition is usually treated with heparin or warfarin, anticoagulants that stop the blood clot getting larger.

If not treated, a piece of the blood clot (embolus) can break off and travel into the lungs where it causes a potentially fatal condition known as pulmonary embolism (PE).

DVT and PE are quite rare, except people with an inherited blood clotting disorder, women on oral contraceptives and those who have had some types of surgery, may be at higher risk.

Increased risk of DVT and PE after long haul flights is said to be due to the enforced lack of movement and sitting still for long periods during the flight, which slows down blood flow.

"The Absolute Risk of Venous Thrombosis after Air Travel: A Cohort Study of 8,755 Employees of International Organisations."
Saskia Kuipers, Suzanne C. Cannegieter, Saskia Middeldorp, Luc Robyn, Harry R. Büller, and Frits R. Rosendaal.
PLoS Medicine Vol. 4, No. 9, e290 doi:10.1371/journal.pmed.0040290.

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: Catharine Paddock

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