Travellers Thrombosis

There has been a great deal of interest in the possibility of a link between long-haul travel and venous thrombosis (VTE). This follows, in part, the death of a young woman at Heathrow following a trip from Australia. There have, however, been earlier high-profile cases: Richard Nixon suffered a deep vein thrombosis (DVT) on the presidential jet whilst visiting China (and this was given as the reason for his inability to give evidence at the Watergate inquiry); and Vice President Dan Quayle similarly experienced a deep venous thrombosis whilst visiting Europe.

History

The medical literature on this subject is relatively scanty but goes back a long way. During the first world war there were descriptions of soldiers, kept cramped in trenches for long periods, suffering venous thromboses. In 1940, Simpson famously described a high incidence of fatal pulmonary emboli in those who had spent the night on air-raid shelter deckchairs. The first report of DVT associated with flying came from a Boston surgeon who in 1954 described five cases of DVT following prolonged sitting (two flying; two in cars; and one at the theatre). The term 'economy class syndrome' (an extremely misleading name) stems from a 1988 Lancet paper in which three cases of DVT following trans-Atlantic flight were presented. One of these had in fact flown business class.

Current evidence

Only recently have rigorous scientific investigations (as opposed to case reports) been conducted in this field. Several studies have been published which examined the relative risk of this complication by comparing those presenting with a clinical DVT (i.e. one causing symptoms) after a long flight with various 'control' groups (groups representing the normal population). Depending on how the control groups were selected the results have shown a wide range of relative risks: from x6 (Rosendaal); x4 (Ferrari); to no risk at all (Kraaijenhagen). Two further studies examined travellers who became ill at the airport. The first, examining travellers who died at Heathrow airport, was able to demonstrate a highly significant difference between the numbers with pulmonary embolism who had recently disembarked from a long flight, compared to those who were waiting to start a flight. The second, also examining patients with pulmonary embolism, but at a Paris airport, was able to demonstrate that the longer the trip flown the higher the incidence of this complication. Both of these studies represent very strong evidence that the association between long-haul travel and VTE is real.

The first prospective study - in other words, examining travellers both before and after flight - was published by John Scurr in 2001. This showed that in travellers aged over 50 years (and therefore, in this respect, at relatively high risk), who undertook trips of over eight hours, there was an incidence of approximately 1 in 10 (10%) of DVT, although these clots were so small that the travellers were unaware of them (i.e. they were asymptomatic). This study also randomised subjects into either wearing below-knee surgical support stockings vs no prophylaxis (preventative treatment), and was able to demonstrate a highly beneficial effect in the support stockings group. Although there has been some criticism of the structure of this trial, these results showed a surprisingly high incidence of asymptomatic thromboses. Subsequent prospective studies, in a broader age range of travellers (and therefore at lower risk), have shown incidence figures of 1.4% (New Zealand study) and 2.8% (Belcaro). It is important to note that the majority of these cases were also asymptomatic, which, in other words, were thromboses of which the travellers would not have been aware. The significance of these thromboses is unknown.

Looking at all cases of DVT presenting for medical help in the north-east of England and asking each if they had travelled significant distances recently, the relatively low incidence of 1 per 30,000 of the general population annually was detected. Although it must be acknowledged that the population in the north-east of England do not frequently undertake long haul flights, these results suggested that only a small minority of the asymptomatic thromboses detected in the prospective studies translate to clinically significant DVTs.

Conclusion

In conclusion: the association between long distance travel and symptomatic venous thrombosis is almost certainly real, but weak. The association between travel and asymptomatic venous thrombosis appears to be stronger but of uncertain clinical significance. It is likely that this complication affects those who already carry risk factors for venous thrombosis but there is very little information in this area to select out such high risk travellers. Most authors agree that the major precipitating event in traveller's thrombosis is sludging of blood in the venous system of the legs due to seated immobility. Although other cabin factors are currently under investigation (low-pressure, low oxygen level) most efforts at preventing this complication have been aimed at avoiding blood sludging.

There is evidence to support the common sense view that the longer the trip the greater the risk of thrombosis. Unfortunately, it is not possible to define a minimum duration of travel which will always be safe. However, in terms of air travel, many specialists will advise the use of prophylaxis for trips lasting more than 2-3 hours. A question which has arisen from this research is: do other methods of transport carry a similar risk? There is no doubt that published case series of traveller's thrombosis have included DVTs following long journeys by car, bus and train (none by cruise ship, though).

It is obviously far easier to break journeys, or to get up and walk around, with these forms of transport. Nevertheless, those at high risk of thrombosis (see below) should take similar precautions to avoid seated immobility.

Treatment

Recommendations for those about to embark on a long distance trip must start with an assessment of risk. Table 1 shows a simple method of risk assessment, and those who fall into the very high risk category should seek medical advice prior to travel.

All travellers should be advised to undertake simple manoeuvres to prevent blood sludging in the legs, including exercises in the seat (many airlines describe these in the in-flight magazine); getting up and walking around where appropriate; and not getting stuck in a curled-up position for protracted periods. It has also been recommended that travellers should take plenty of fluids. Whilst there is no evidence that dehydration causes venous thrombosis, there is no doubt that the cabin air is extremely dry. So taking fluids will make the traveller more comfortable and will also encourage trips to the toilet which will exercise the legs (hopefully).

For those in the high-risk category, all of the above advice pertains but in addition travellers should wear grade II, below-knee surgical support stockings. This group should avoid sleeping tablets, or anything else, which might lead to cramped immobility for long periods whilst asleep in the aeroplane seat. Some authors have recommended aspirin in this situation although the frequency of side-effects and doubts as to its effect in preventing venous thrombosis suggests that aspirin cannot be recommended generally. Finally, some patients in the high-risk category will be prescribed low-molecular weight heparin to cover each leg of the trip (no pun intended). This is not completely without risk but appears to be reasonably safe administered at the DVT-prophylactic dose, by subcutaneous injection, immediately prior to departure. This should provide adequate protection for approximately 24 hours. For those patients already taking anticoagulants (e.g. warfarin) no further prophylaxis is required, provided the INR is in the therapeutic range prior to departure.

Risk Factors

Lesser Risks Major Risks Very High Risks

Prophylaxis

Lesser Risks Major Risks Very High Risks