Deep vein thrombosis economy class syndrome

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Rim Halaby, M.D. [2]

Overview

Economy class syndrome is the occurrence of venous thromboembolism (VTE) among air travelers.^[1] VTE consists of deep vein thrombosis (DVT), pulmonary embolism (PE), or both. DVT occurs when a deep vein is partially or completely blocked by a blood clot, most commonly in the legs. The clot may break off and travel to the vessels in the lung, causing a life-threatening PE.

Historical Perspective

VTE associated with air travel was first described in the early 1950s.^[2]
The term was first coined in the late 1980s^[3] when it turned out that people who had traveled long distances by plane were at an increased risk for thrombosis, especially deep venous thrombosis and its main complication, pulmonary embolism. Although all these diseases had been recognised for a long time, the possibility of litigation against airline companies brought them into the limelight when this "syndrome" was reported.
The WHO Research Into Global Hazards of Travel (WRIGHT) project, which was set up on 21 June 2001, is a comprehensive research programme developed by the WRIGHT group under the auspices of the World Health Organization. The project consists of a series of research studies to fill the key information gaps in available knowledge on the suspected link between air travel and venous thrombosis. The studies which cover epidemiological, clinical and physiological areas, will provide key information on VTE and air travel. The objectives of the WRIGHT (WHO Research Into Global Hazards of Travel) project were to confirm that the risk of VTE is increased by air travel and to determine the magnitude of risk, the effect of other factors on the risk and to study the effect of preventive measures on risk.^[4]

Pathophysiology

The mechanism for thrombosis in travelers is probably due to a combination of immobilization, dehydration and per-existing underlying VTE risk factors. Patients with diseases that predispose them for thrombosis, such as antiphospholipid syndrome or cancer, are probably at a much greater risk. The highest risk groups include the elderly, pregnant women, those suffering serious medical conditions such as cancer and those with recent orthopedic surgery (legs or knees).

Risk Factors and Triggers

Travel Related Factors

Most information about blood clots and long-distance travel comes from information that has been gathered about air travel. However, anyone traveling more than four hours, whether by air, car, bus, or train, can be at risk for blood clots.^[5]^[6]

Air flight related factors that increase the risk of VTE are:
- Increased duration of the air flight^[7]^[8]
  - It has been reported that there is a 18% higher risk of VTE for every 2 hours increase in the duration of travel.^[9]
- Several flights within a short period of time^[7]

Preexisting VTE Risk Factors

The risk of VTE among travelers is higher among subjects who have risk factors for VTE. Please note that the word risk factors refers to those epidemiologic and genetic variables that expose someone to a higher risk of developing venous thrombosis, whereas the word triggers refer to those factors in the patients immediate history or environment that may have lead to the occurrence of the venous thrombosis. Shown below is a list of predisposing factors to VTE.^[10]^[11] The risk factors are classified as moderate or weak depending on how strongly they predispose for a VTE.

Moderate risk factors	Weak risk factors
❑ Chemotherapy ❑ Chronic heart failure ❑ Respiratory failure ❑ Hormone replacement therapy ❑ Cancer ❑ Oral contraceptive pills ❑ Stroke ❑ Pregnancy ❑ Postpartum ❑ Prior history of VTE ❑ Thrombophilia	❑ Advanced age ❑ Laparoscopic surgery ❑ Prepartum ❑ Obesity ❑ Varicose veins

Preexisting VTE Triggers

The risk of VTE among travelers is higher among subjects who have triggers for VTE. Please note that the word risk factors refers to those epidemiologic and genetic variables that expose someone to a higher risk of developing venous thrombosis, whereas the word triggers refer to those factors in the patients immediate history or environment that may have lead to the occurrence of the venous thrombosis. Shown below is a list of triggers of VTE.^[10]^[11] The triggers are classified as strong, moderate, or weak depending on how strongly they predispose for a VTE.

Strong triggers	Moderate triggers	Weak triggers
❑ Bone fracture (hip or leg) ❑ Hip replacement surgery ❑ Knee replacement surgery ❑ Major general surgery ❑ Significant trauma ❑ Spinal cord injury	❑ Athroscopic knee surgery ❑ Central venous lines ❑ Chemotherapy	❑ Bed rest for more than 3 days ❑ Prolonged car or air travel ❑ Laparoscopic surgery ❑ Prepartum

Epidemiology and Demographics

The annual incidence of VTE in the general population has been estimated at 0.1% but is higher in subpopulations with risk factors for VTE. The actual incidence of travel-related VTE is difficult to determine, since there is no consensus on the definition of travel-related VTE, particularly in regards to duration of travel and time window after travel. Estimates of travel-related VTE incidence vary because of differences between studies in duration of travel, measured outcome, time window after the flight, and the populations studied.^[12]

In general, the overall incidence of travel-related VTE is low. Two studies reported that the absolute risk of VTE for flights >4 hours is 1 in 4,656 flights and 1 in 6,000 flights. People who travel on long-distance flights are generally healthier and therefore are at lower risk for VTE than the general population. Five prospective studies that assessed the incidence of DVT among travelers at low to intermediate risk for VTE after travel >8 hours yielded an overall incidence of VTE of 0.5%, while the incidence of symptomatic VTE was 0.3%.^[12]

Numerous studies have examined the association between travel, particularly air travel, and VTE. However, these studies had differences in methods. Outcomes ranged from asymptomatic DVT to symptomatic DVT/PE to severe or fatal PE. Asymptomatic DVT is estimated to be 5- to 20-fold more common than symptomatic events. Definitions of long-distance travel ranged from flight duration >3 hours to >10 hours (most >4 hours). The time window until illness after the flight ranged from hours after landing to ≥8 weeks (most 4 weeks).^[12]

Published studies have yielded varying results; some studies found that long-distance travel increased the risk of VTE, and others either found no definitive evidence that it increased the risk of VTE or found that it increased the risk only if ≥1 additional risk factors were present. Most studies found long-distance air travel to be a weak risk factor for VTE, and most VTE occurred as asymptomatic DVT of uncertain clinical significance in passengers with additional preexisting risk factors.^[12]

Long-distance air travel may increase the risk of VTE by 2- to 4-fold. A similar increase in risk is also seen with other modes of travel, such as car, bus, or train, implying that the increase in risk is caused mainly by prolonged limited mobility rather than by the cabin environment. The risk is the same for economy-class and business-class travel. The risk increases with increasing travel duration and with preexisting risk factors. The risk decreases with time after air travel; most air travel–related VTE occurs within the first 1–2 weeks after the flight and returns to baseline by 8 weeks.^[12]

Symptoms