A large international study from Leiden attempts to answer some of the questions regarding venous thrombosis in air travelers and is available here in PLOS.This was a large (n=8755) follow-up study (over a five year period) involving employees from a number of companies.
First on all, short flights-less than 4 hours-seem to not be an issue while longer ones may be.
Some interesting observations were made.
Shorter travelers ( less than five five) and taller ones(over six one) seemed at greater risk and- perhaps surprisingly- so did younger folks (less than 30 years of age). So, it seemed best to not be too young, short or tall. Some speculations regarding vein compressions in the short and not enough room for the legs in the tall seemed plausible but why younger travelers. The investigators suggested that one of the many epidemiological traps for the unwary might be in play.This may be an example of the "attrition of the susceptibles". Individuals who are susceptible to a given event are likely to experience it soon after the start of exposure.The younger travelers likely had not been doing long haul flights for very long while the older employees had and those with clotting tendencies may have been deselected-i.e. no longer traveling or perhaps even taking prophylactic measures.
The relationship between risk and "extremes" of height had been made previously as had their other major observation that women on oral contraceptives had an increased thrombosis risk (Incidence rate ratio of 3.6). I had blogged previously about a 2006 JAMA article which presented some evidence that a major factor in the pathogenesis was blood flow alterations as opposed to coagulation system changes brought about by the flight.
The best advice still seems to be to get up and walk around as much as you can on long plane trips. What about the patient who has previously experienced a thrombotic event in temporal proximity to a long air flight and who is ready to travel again? Should they take low molecular weight heparin pre- flight? I think I would.