Deep vein thrombrosis and flying: check your genes not your class

Black Africans are at no risk of deep vein thrombosis, the so-called Economy Class Syndrome blamed for life-threatening blood clots in long-distance airplane passengers.

According to Professor Barry Jacobson of South Africa's University of the Witwatersrand, genetics is to blame rather than cramped conditions in jumbo jets. People of white and Indian descent may carry a genetic predisposition to this type of blood clotting. Black Africans are at absolutely no risk.

The good news, announced in Johannesburg in South Africa on July 4, was that the incidence of Economy Class Syndrome is far less - perhaps 1% of passengers - than earlier estimated.

The largest study ever undertaken on the subject, using passengers on the London to Johannesburg route flown by South African Airways, proved this - at a cost of three million rands. A full report is in the South African Medical Journal.


The bad news is that recommended remedies to prevent blood clots on airplanes are either ineffectual or downright wrong. Drinking lots of fluids made no difference. Drinking alcohol made no difference (to blood clots, that is; Professor Jacobson noted that avoiding alcohol and upping one's fluid intake on flights remained a good idea for other reasons!) And taking aspirin or disprin actually exacerbated the problem.

"Aspirin is good for clots in arteries. Aspirin is not good for clots in veins," he said. Another piece of bad news: moving to business class will not protect a high-risk patient. Both business and economy class passengers suffered equally from deep vein thrombosis.

Professor Jacobson now hopes to set up another study to work on what works to prevent clots. At present, high-risk passengers - those with a history of immediate family members dying of blood clots, or suffering from blood clots themselves - may be injected with an anti-coagulant shortly before takeoff. And low to medium-risk passengers may buy special pressure stockings and socks to wear during the flight.

Even then, there is a problem. Persons with narrowing of the arteries or varicose veins in the back of their knee, or people with oddly-shaped legs, may need expert advice before using the socks, available from Scholl. Jacobson said of the 434 passengers that had had a duplex scan, none had ultrasonic evidence of the venous thrombosis. At least 899 passengers were recruited for the study called BEST (Business class vs Economy class Syndrome as a cause of Thrombosis).


Tests were carried out on volunteers - 180 from business class and 719 from economy class - on the Johannesburg-London route of SA Airways before boarding and on disembarkation. The report said nine passengers tested at departure had elevated D-dimer levels and these volunteers were excluded from further study. D-dimer is a blood component that breaks down blood clots when they form due to bleeding, such as menstruation, bruising, pregnancy or recent operations.

"Our results show, encouragingly, that the incidence of deep vein thrombosis or clotting is much lower than previously reported in other research or in anecdotal evidence," Jacobson said.

Previous studies reported that as many as 10% of passengers travelling without prophylaxis for long distances on airlines may develop a venous thrombosis. Jacobson said: "There is, however, no evidence that the economy class travellers are at increased risk of this condition."

Jacobson said the study found that as many as 10% of passengers had abnormally elevated level of D-dimers in their blood suggesting that they had potentially developed small unrecognisable clots. Of major importance was that they were not restricted to economy class, and occurred just as often in business class, he said. At least 74 of the 899 volunteers had a raised D-dimer on arrival. Some 22 of the 180 business class passengers developed elevated D-dimers as compared to 52 out of 719 economy class passengers.

Jacobson said prophylactic measures currently advised to prevent the development of DVT while on long haul flights, such as onboard exercises recommended by the individual airlines, or changes in normal fluid intake, had no bearing on the development of raised D-dimers.

One of the factors that did correlate was the presence of Factor V Leiden mutation - a common hereditary factor known to increase the risk of thrombosis. He cautioned against screening people for his factor, saying passengers who did not have the gene might be lulled into a false sense of security, but might still be at risk of developing DVT. In addition, screening is expensive - about R1000.

The study found that the use of aspirin raised D-dimers. "Passengers should be actively discouraged to use aspirin like Disprin to prevent venous thrombosis." Jacobson said airlines and the medical profession should rethink the way they addressed the issue to prevent injury and death. Even though the percentage of people developing DVT was small, perhaps as low as one percent or less of all long-haul passengers, the sheer volume of passenger number was sufficient to present a problem to airlines, the medical profession and the medical insurance industry. 

Jacobson said the next stage of the research would be to assess better ways of prophylactic intervention to reduce the chances of long-haul passengers from developing the DVT. "We urgently need data to supply evidence-based adviceto low risk passengers on how best to prevent aviation related thrombosis."

He said a number of airlines had been dogged by lawsuits from people who suffered DVT. 


July 2003