Statistics from Altmetric.com
The guidelines1 drawn up on prevention of venous thromboembolism and long haul flights should be welcomed, though seen in the light of incomplete evidence. We would like to draw attention to several points.
Scurr et al2 looked prospectively at a group aged over 50 years undergoing long haul travel from which anyone with recognised risk factors was excluded, putting such individuals into a low risk group for venous thromboembolism. Twelve of 116 people who were randomised to the “no stocking” group were diagnosed with symptomless calf deep vein thrombosis after their long haul travel. None of the group randomised to wear compression stockings developed deep vein thrombosis. Even if we take this surprising finding at face value, we are left with the question: what is the clinical significance of asymptomatic calf deep vein thrombosis in a low risk population? It is far from clear in such a low risk group that we need to treat such events, which weakens any guideline aimed at prevention. Unlike a postoperative patient, the traveller is mobile before and after flight as well as potentially during it.
Furthermore, considering the uncertainty of the evidence, it would be wise to advise passengers of potential side effects from our recommendation. Superficial thrombophlebitis occurred in four individuals (3%) wearing compression stockings in the study by Scurr et al, indicating some morbidity with their use.
As far as we are aware, no study has looked at the preventative effect of long socks or support tights recommended for low risk passengers. It is also a shame that no study has looked at the effect of lifestyle measures such as mobility and hydration or, indeed, compared these to intervention with stockings or socks. Until further evidence emerges, can we promote the use of support tights or long socks in low risk passengers compared with the non-invasive measures of mobility and hydration? If we are to recommend intervention, it should reflect Scurr's evidence and be compression stockings with warnings for thrombophlebitis.
In the passenger with a moderate to high risk of venous thromboembolism, the recommendations are for compression stockings and aspirin or anticoagulation. This is based on extrapolation from studies of postoperative patients,3 but it is not clear that passengers fall into the same category. While it may be prudent to make unsubstantiated guidelines for high risk individuals, are we really going to recommend pre-flight aspirin and use of compression stockings for every individual on hromone replacement therapy and the oral contraceptive pill? What will be the morbidity associated with aspirin use?
While we await further studies to answer our questions, we agree that all at risk patients should be strongly recommended to take lifestyle measures. This information should be dispersed by airlines and public health agencies. There is no evidence for the use of knee socks in any group, and this recommendation should be dropped. Patients with low to moderate risk factors should be advised that compression stockings have reduced venous thromboembolism in other situations, but that superficial thrombophlebitis can occur as a side effect. Any further intervention with aspirin or low molecular weight heparin can be offered to moderate to high risk individuals only on the basis that direct evidence is lacking and side effects are possible.
Finally, when making the recommendations, patients should be warned that case studies found an increased risk of thromboembolism in long distance travel, not just long haul flights.4
We appreciate the interest expressed by Drs Campbell and Rayner in the BTS fitness to fly guidelines and welcome their valuable comments. We would like to clarify the issues they raised.
Firstly, we agree that the clinical significance of asymptomatic calf deep vein thrombosis in a low risk population is as yet unclear, and for this reason we avoided didactic advice while awaiting further evidence. Our recommendations were that physicians may wish to recommend support stockings or non-elasticated long socks in patients at increased risk of venous thromboembolism. The physician's decision will depend on individual circumstances, including patient preference.
Secondly, Drs Campbell and Rayner raise the issue of superficial thrombophlebitis which developed in 3% of passengers who wore below knee elastic compression stockings. The significance of this result, as indicated by the confidence intervals, is unclear, and our recommendations did not include such stockings. Rather, we suggested the possible use of non-elasticated long socks which are less likely to compress varicose veins in the knee region.
Thirdly, we agree that further studies are required to examine the effects of lifestyle measures such as mobility and hydration, and our guidelines recommended further research into this area. With regard to the possible morbidity associated with a single tablet of low dose aspirin, we consider that this is likely to be very small.
Finally, regarding passengers on oral contraception, we have made it clear that the risk is not equal with all forms of contraception, and physicians and passengers will have to make their own decisions in the light of available evidence and individual circumstances.
The process of producing these guidelines has highlighted the fact that there are a considerable number of unknowns regarding flying with respiratory disease, and more research is clearly required.