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‘Traveller’s Thrombosis’ – Does It Exist and Are We Ready to Give Prophylaxis?
simultaneously in an individual; each of these factors affects the probability
Table 1: Absolute and Relative Risks After Long-haul Flights for
of disease. Hence, the term ‘traveller’s thrombosis’ is as inappropriate as the Whole Study Population as Well as Stratified by Sex, Age
‘cholesterol myocardial infarction’ or ‘smoker’s stroke’ would be.
Category, Oral Contraceptive Use, Height and Body Mass Index
Preventing Venous Thrombosis After Air Travel
Category (n) Air Travel Flights (n) Case/Number ## RR (CI95)
of Travellers ###
There are no studies concerning the prevention of thrombosis after air travel.
All (8,755) No 1$
There are guidelines, sometimes referred to as ‘common sense advice’, that
Yes 102,429 4,656 3.2 (1.8–5.6)
include avoidance of alcohol, liberal intake of non-alcoholic beverages and
Men (4,915) No 1$
regular exercise of the legs. It is not very likely that dehydration plays a role Yes 76,461 5,882 2.7 (1.2–6.0)
in the development of thrombosis, but it is plausible that regular movement
Women (3,819) No 1$
of the legs will be beneficial. Exercise is obviously also without a risk of side
Yes 25,780 2,864 3.3 (1.5–7.5)
effects, but this is not the case for other suggested prophylactic measures,
<30 years (1,392) No 1$
Yes 8,014 2,671 7.7 (1.6–38.4)
such as elastic stockings, aspirin and low-molecular-weight heparin. A
30–50 years (6,017)No 1$
recent survey of physicians travelling to Australia showed that many used
Yes 73,624 4,908 3.7 (1.8–7.5)
aspirin before the flight.
13
The effect of aspirin on thrombosis occurrence is
>50 years (1,345) No 1$
minimal, but it does increase the risk of major haemorrhage. Elastic Yes 20,791 5,198 1.4 (0.4–4.6)
stockings prevent oedema and have been shown in other risk situations to OC$$ No No 1$
decrease thrombotic risk. It is, from a biological viewpoint, unlikely that
Yes 18,085 4,938 2.2 (0.6–8.1)
stockings have much effect in the absence of leg muscle movement. Several
OC$$ Yes No 1$
Yes 7,695 1,808 3.6 (0.8–14.9)
studies focusing on asymptomatic clots detected by ultrasound observed a
<165cm No 1$
decrease in the number of such clots occurring in those wearing elastic
Yes 14,250 2,036 9.8 (3.1–30.9)
stockings, but in one study grade I elastic stockings caused symptomatic
165–185cm No 1$
superficial thrombosis in 3% of patients.
14
Therefore, elastic stockings are
Yes 69,095 6,281 1.9 (0.9–3.9)
not without potential side effects. Elastic stockings should exert a pressure >185cm No 1$
that is graded from distal to proximal, and should be individually tailored. It
Yes 18,242 4,561 3.7 (0.8–16.9)
is highly implausible that the ‘one-size-fits-all’ socks that are sold over the
BMI <25 No 1$
counter at airports have any effect in preventing thrombosis.
Yes 51,958 7,423 1.9 (0.8–4.7)
BMI>25 No 1$
Low-molecular-weight heparin has proved to be highly effective in
Yes 49,509 3,301 4.9 (2.3–10.6)
many situations to prevent thrombosis, but also clearly increases the risk of
No = no exposure to air travel within 8 weeks; Yes = exposure to a flight of at least four
major haemorrhage. hours; ## RR = relative risk, adjusted for age and sex with 95% confidence interval (CI95);
$ = reference category; $$ = oral contraceptive use among women <50 years of age;
### = number of travellers (flights) among whom one case will occur.
In the absence of data on prophylaxis of thrombosis after air travel, we can
only try to extrapolate from existing data. The potential benefit is be one per 17,000 travellers, and for low-molecular-weight heparin one in
determined by the magnitude of the risk and the efficacy of the 3,500.
15
This implies that prophylaxis with stockings, aspirin or heparin in all
intervention, and can be expressed as the number needed to treat (NNT) and travellers is not indicated. These figures, particularly NNT, will be different for
number needed to harm (NNH). NNT is the number of individuals one has individuals with risk factors; however, given the substantial uncertainty
to treat to prevent one thrombotic event and NNH is the number one treats concerning the estimates of benefit and harm and the absence of data,
before causing a major haemorrhage. When prophylaxis is given caution is needed in prescribing any prophylaxis beyond exercise.
indiscriminately to all travellers and the treatment is completely efficacious,
4,656 travellers needed to be treated to prevent one thrombosis (NNT). Conclusion
Aspirin is far from perfectly efficacious, and reduces risk by, at most, 25%. Air travel increases the risk of thrombosis, with an average risk of
Hence, with aspirin one would need to treat 18,000 travellers to prevent one one per 4,500 travellers. Long flights, as well as several flights in quick
thrombotic event. Heparin reduces risk more efficiently and with a risk succession, lead to higher risks, as does the presence of risk factors such
reduction of 70%, and NNT for heparin is still nearly 7,000. The NNT for as obesity, being tall or short, use of oral contraceptives and
elastic stockings is unknown, and may lie somewhere between the region prothrombotic mutations. Current data, which are limited, do not favour
of 7,000 and 18,000 travellers wearing them in order to prevent one case prescription of prophylactic interventions such as aspirin, low-molecular-
of thrombosis. The benefits of these interventions do not outweigh the risks weight heparin or elastic stockings and prevention should be limited to
of either thrombosis inducement (stockings) or major haemorrhage encouraging exercise and discouraging behaviour that will restrict
(aspirin, heparin). Risks of major haemorrhage for short-term use of aspirin movement, such as excessive alcohol intake and the use of sleeping
and heparin are low, but may outweigh the equally small benefit. For medication. There is a need for studies into the efficacy and safety of
instance, for aspirin the risk of major haemorrhage in healthy users would preventative measures in high-risk individuals. ■
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of Travel (WRIGHT) project: Final Report of Phase I, 2007. 7. Beasley R, Raymond N, Hill S, et al., Eur Respir J, 2006;3(8).
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4. Simpson K, Lancet, 1940;2:744. 10. Bendz B, Rostrup M, Sevre K, et al., Lancet, 2000;356:1657–8. 2001;357:1485–9.
5. Louvel J, Four cases of phlebitis due to air travel, Arch Mal 11. Kuipers S, Cannegieter SC, Middeldorp S, et al., PLoS Med, 15. Rosendaal FR, J Thromb Haemost, 2006;4:2306–7.
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EUROPEAN HAEMATOLOGY 2007 51
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