Evidence
Alert

Long-haul flyers could reduce risk of leg blood clots with compression stockings

Airline passengers could reduce their risk of getting a deep vein thrombosis by wearing compression stockings, even if they are at low risk.

Deep vein thrombosis (DVT) is a clot in the deep leg veins, and immobility is a risk factor. The chance of air travel causing DVT in healthy people is low, and many DVT’s resolve without causing any symptoms, but the consequences can be severe, including potentially fatal clots traveling to the lungs (pulmonary embolism).

The review pooled results of nine trials comparing people who wore below-knee compression stockings with those who didn’t on flights lasting at least four hours. One in 1,000 healthy people wearing compression stockings developed a symptomless clot compared with 10 per 1,000 who didn’t wear them. In people with additional risk factors, stockings reduced risk from 30 cases per 1,000 down to 3 per 1,000.

All passengers have the option of buying compression stockings at a pharmacy. They should be fitted for the individual. Keeping moving and hydrated can also reduce risk.

 

Why was this study needed?

Every year about one person in 1,000 gets a DVT from any cause. The treatment costs the NHS about £604 million in total. Lengthy flights can increase the risk of DVT as passengers move less and blood flow is reduced. The World Health Organization estimated that about one in every 6,000 healthy people develop symptoms of a blood clot after flights lasting more than four hours, but more may be asymptomatic.

Compression stockings improve blood flow in the veins of the leg, preventing pooling which can lead to clots. Previous systematic review evidence shows that people who wear compression stockings in hospital are less likely to get DVT.

The increasing number of people taking long-haul flights makes this potential health risk an important issue. This Cochrane review examined evidence whether compression stockings can prevent DVT in people flying for at least four hours.

 

What did this study do?

The review identified 11 randomised controlled trials in 2,906 adults of different ages and risk of getting DVT.

Nine trials’ results (2,821 participants) were pooled in meta-analysis. They compared people who wore below-knee compression stockings on both legs with those who didn’t wear any, on flights lasting at least four hours. Other trials assessed full leg stockings, and one assessed stockings worn on alternate legs in two flights. Eight trials included people judged to be at low or medium risk. Anyone with past clot, clotting disorders, limited mobility, cancer, varicose veins, taller than 190 cm and heavier than 90 kg or on medication (for heart disease, diabetes or hypertension) was excluded from these trials. Two trials included high-risk people with one of these conditions.

The main outcome was DVT, with or without symptoms, diagnosed within 48 hours of the flight, using before and after ultrasound testing mainly (Doppler). Other outcomes included complications such as pulmonary embolism.

Participants couldn’t be blinded to wearing stockings, and self-reported some outcomes, which may introduce bias. However, the studies were high quality which gives confidence in the results.

 

What did it find?

  • In the nine pooled trials (2,637 people), 1 in 1,000 low-risk passengers who wore compression stockings developed a symptomless DVT compared with 10 in 1,000 chance for those who didn’t wear stockings. This equated to a 90% risk reduction (odds ratio [OR] 0.10, 95% confidence interval [CI] 0.04 to 0.25).
  • High-risk passengers who wore compression stockings had a 3 in 1,000 chance of developing a symptomless DVT compared with 30 in 1,000 for those who didn’t wear stockings.
  • None of the participants in nine trials developed a symptomatic DVT. Neither were there any serious events such as pulmonary embolism or death, so it was not possible to estimate comparative risk of these outcomes.
  • There was low quality evidence from six trials (1,246 people) that stockings reduced risk of leg swelling (mean difference in oedema score: −4.72, 95% CI −4.91 to −4.52).

 

What does current guidance say on this issue?

NICE Clinical Knowledge Summaries recommend that people who have moderate or high risk of developing a DVT should wear compression stockings during long-distance flights. It advises people at low risk to stay mobile and avoid dehydration.

SIGN’s guideline on prevention and management of venous thromboembolism says that the use of compression stockings during and after long-haul travel is not routinely recommended. Passengers are instead advised to remain as ambulant as is safely possible, before, during and after journeys. If a passenger chooses to use them, they should make sure they have the correct fit.

 

What are the implications?

Assuming that there is little risk of harm from stockings, airline passengers at low risk of DVT could benefit from wearing them during a long haul flight, just like those at high risk, even though the absolute risk in this group remains very low. The trials have not been able to assess the comparative or additional value of standard advice, like keeping hydrated and mobile, though many offered this advice as well.

It is unclear whether the findings may lead to a change in recommendations in future guidance updates.

Compressions stockings can be bought from pharmacies and fitted by a professional. This relatively cheap and simple intervention might appeal to low-risk travellers.

 

Citation and Funding

Clarke MJ, Broderick C, Hopewell S, et al. Compression stockings for preventing deep vein thrombosis in airline passengers. Cochrane Database Syst Rev. 2016:(9):CD004002.

No funding information was provided for this study.

 

Bibliography

CAA. Tips for a healthy flight. London: Civil Aviation Authority; 2016.

NHS Choices. Prevent DVT when you travel. London: Department of Health; 2016.

NICE. Cost impact and commissioning assessment: quality standard for venous thromboembolism (VTE) prevention. London: National Institute for Health and Care Excellence; 2010.

NICE Clinical Knowledge Summaries. DVT prevention for travellers. London: National Institute for Health and Care Excellence; 2013.

NICE Clinical Knowledge Summaries. Compression stockings. London: National Institute for Health and Care Excellence; 2012.

NICE Clinical Knowledge Summaries. Deep vein thrombosis. London: National Institute for Health and Care Excellence; 2012.

Sachdeva A, Dalton M, Amaragiri SV, Lees T. Graduated compression stockings for prevention of deep vein thrombosis. Cochrane Database Syst Rev. 2014;(12):CD001484.

SIGN. Prevention and management of venous thromboembolism. Guideline 22. Edinburgh: Scottish Intercollegiate Guidelines Network; 2010.

WHO. WHO research into global hazards of travel (WRIGHT) project. Geneva: The World Health Organization; 2007.

Produced by the University of Southampton and Bazian on behalf of NIHR through the NIHR Dissemination Centre

 

Definitions

Tests for asymptomatic DVT included Doppler ultrasound, which is used to find out how fast the blood is flowing through a blood vessel. One trial used D-dimer testing and fibrinogen tests only. These are blood tests, with a chance of false positive findings.

Risk for continuous journeys lasting more than 6 hours is classified as:

  • Low if the person has
    • No history of DVT or pulmonary embolism (PE), and
    • Not undergone surgery in the previous 4 weeks, and
    • No other risk factors to indicate moderate or high risk.
  • Moderate if the person
    • Has a previous history of DVT or PE. However, people with a recent DVT or PE who are on treatment are considered to be at low risk.
    • Has undergone surgery under general anaesthesia lasting more than 30 minutes in the previous 2 months but not in the last 4 weeks.
    • Is pregnant or postpartum.
    • Has clinically evident cardiac disease (such as recent myocardial infarction or uncontrolled heart failure) or other major acute illness (such as pneumonia).
    • Is taking combined oral contraceptives or hormone replacement therapy.
    • Is obese (body mass index greater than 30 kg/m2).
    • Has varicose veins with phlebitis.
    • Has a family history of venous thromboembolism in a first degree relative.
    • Has polycythaemia.
    • Has a lower-limb fracture in plaster.
    • Note: The presence of multiple risk factors will further increase the risk of an individual developing travel-related DVT.
  • High if the person:
    • Has undergone surgery under general anaesthesia lasting more than 30 minutes in the previous 4 weeks.
    • Has known thrombophilia.
    • Has cancer — untreated or currently on treatment.

Commentaries

Expert commentary

Public concern about DVT and pulmonary embolism as a consequence of "economy class syndrome" is fully justified, but the risk is similar when travelling by air, coach or car as it relates to sitting in a confined space, particularly falling asleep. This review highlights how elastic stockings empty the calf veins, increasing blood flow velocity and preventing asymptomatic DVT during long haul air travel. What is not widely recognised is that these small asymptomatic DVTs progress during further flights within a short period to become symptomatic DVT with permanent consequences or even fatal pulmonary embolism. More than two flights in eight weeks increase the risk of DVT four times whereas more than four flights increase the risk eight times.

Charles McCollum, Professor of Surgery, University of Manchester