Minor Leg Injuries Also Pose Clot Risk

By Charles Bankhead
LEIDEN, The Netherlands, Jan. 14 -- Sprains, bruises, and other minor leg injuries triple the risk of venous thrombosis within three months, investigators here found.
Among almost 2,500 patients with venous thromboembolism, 11.7% had a recent history of minor leg injuries compared with 4.4% of a matched control group without VTE, Frits R. Rosendaal, M.D., Ph.D., of the University of Leiden, and colleagues reported in the Jan. 14 issue of Archives of Internal Medicine.
Risk of venous thrombosis was greatest in the first four weeks after injury, they said.
"The risk of venous thrombosis was threefold increased, resulting in a population-attributable fraction of 7.9%," the researchers said.
Venous thrombosis affects up to three individuals per 1,000 annually, they noted. Known risk factors include surgery, immobility, and prothrombotic genetic variants. Major injuries are known to increase the risk, but the association with minor injuries had not been studied carefully, they said.
So the researchers prospectively evaluated causes of venous thromboembolism in the Multiple Environmental and Genetic Assessment (MEGA) of risk factors for venous thrombosis study.
From March 1999 through August 2004, investigators at six anticoagulation clinics enrolled patients with a first episode of venous thromboembolism, including deep vein thrombosis in the leg and pulmonary embolism.
The study included 2,471 patients and 3,534 matched controls ages 18 to 70. Information for the study was obtained from patient self-reports, hospital records, and primary care physicians. The data collected included the most recent injury before the embolism and specifically addressed minor injuries.
The investigators determined that 289 (11.7%) of the patients and 154 (4.4%) of the controls had sustained minor injuries in the preceding three months. After adjustment for age and sex, the between-group difference translated into an odds ratio of 3.1 for recent minor injury in the venous thromboembolism patients.
The difference was attributable entirely to minor injuries in the leg (OR: 5.1, 95% CI: 3.9 to 6.7). Minor injuries in other parts of the body had no association with thromboembolism.
Factor V Leiden carriers with a minor leg injuries had a 50-fold increased risk of thromboembolism compared with noncarriers (OR: 49.7, 95% CI: 6.8 to 362.7).
Because clots were not associated with injuries in areas of the body other than the leg, the researchers ruled out systemic prothrombotic reaction as an explanation for the increased risk.
They also excluded patients requiring extended immobilization or bed rest. However, they acknowledged that even minor injuries might lead to reduced mobility short of bed rest and thus predispose a patient to venous thromboembolism.
Edematous vein obstruction might result in venous stasis, the authors allowed. They also suggested that minor injury might damage a blood vessel wall, thereby increasing the risk of clot formation.
The investigators wrote that they doubted recall bias for minor injuries among patients more than controls because patients reported leg injuries when they had a pulmonary embolism and not just venous thrombosis, but they could not exclude recall bias entirely.
The findings have implications for primary care physicians, the authors noted.
"Many individuals with minor injuries will have contacted the general practitioner first," they noted. "Therefore, there may be an important task for general practitioners to identify subjects who are at a high risk of developing venous thrombosis and, subsequently, to provide prophylactic measures."
The study was supported by the Netherlands Heart Foundation, the Dutch Cancer Foundation, and the Netherlands Organization for Scientific Research.
The authors reported no potential conflicts.
Primary source: Archives of Internal MedicineSource reference:Van Stralen JK, et al "Minor injuries as a risk factor for venous thrombosis" Arch Intern Med 2008; 168: 21-26.

Absolute Risk of Clots Higher for Frequent Flyers Who Take Long Trips

LEIDEN, The Netherlands, Sept. 25 -- The risk of symptomatic venous thrombosis for healthy individuals is low after an airflight of four or more hours, but increases with more and longer flights, researchers reported. In a survey of almost 9,000 business travelers, the researchers found that long flights tripled the risk of developing venous thrombosis, Suzanne C. Cannegieter, M.D., Ph.D., of Leiden University here, and colleagues, reported online in PLoS Medicine.
That rate amounted to a risk for one person out of more than 4,600 flights, they said.
However, the study included only healthy individuals with an average age of 40 and without previous deep-vein thromboses or pulmonary emboli, so the absolute risk might be higher in the general traveling population, the researchers cautioned.
Although the link between air travel and venous thrombosis was first noticed in the 1950s, the absolute risk of developing deep vein thromboses after a long flight has not been known, the researchers wrote.
Their findings emerged from a cohort study of 8,755 travelers who were followed from January 2000 through December 2005 for a total of 38,910 person-years, during 6,872 of which they were exposed to a long flight, defined as more than four hours in duration.
In the follow-up period, 53 thromboses occurred, of which 22 occurred within eight weeks of a long flight.
This led to an incidence rate of 3.2 per 1,000 person-years, as compared with 1.0 per 1,000 person-years for individuals not exposed to air travel (incidence rate ratio 3.2, 95% confidence interval 1.8-5.6).
This rate was equivalent to a risk of one event per 4,656 long-haul flights, the researcher reported, and was similar to the odds ratios found in most case-control studies.
The risk increased with exposure to more flights within a short time frame and with increasing duration of flights, the researchers said.
When someone was exposed to only one or two long flights, the rate per 1,000 person-years was 2.6, but it tripled after exposure to five or more long flights. With each extra flight, the risk increased 1.4-fold.
Expressed as risk per number of flights, the risk increased from 0.5 per 1,000 person years for flights under four hours to 5.91 per 1,000 person-years for flights lasting more than 16 hours.
The incidence was highest in the first two weeks after travel and gradually decreased to baseline after eight weeks, the researchers said.
The risk was particularly high for travelers younger than 30, women (especially those taking oral contraceptives), and individuals who were particularly short (pressure on the popliteal vein), tall (cramped position), or with a BMI over 25 kg/m2.
However, the researchers said, because of the small number of cases, some confidence intervals were wide, indicating considerable uncertainty for these estimates. Nevertheless, they said, the findings indicate a need for adjustable seating.
The findings for younger travelers, they said, might be explained by the fact that younger travelers were most likely to be frequent travelers, thus explaining their absolute higher risk.
Discussing the study's limitations, the researcher noted that results were based on responses to a web-based questionnaire and the response rate of only 32% suggests that those who'd had a venous thrombosis may have been more likely to complete the questionnaire than those who did not.
Also, they noted, these results cannot be generalized to an older, less-healthy population or to individuals with a history of thrombosis.
However, they cautioned, their findings do not justify the use of potentially dangerous prophylaxis, such as anticoagulant therapy, for all long-haul air travelers, since this may do more harm than good.
Nevertheless, for some subgroups of people with a highly increased risk, the risk-benefit ratio may favor the use of prophylactic measures. Large randomized trials are required to assess who would benefit most and from which prophylactic measures, the researchers concluded.
No financial conflicts were reported. This study was conducted as part of the WRIGHT project (World health Organization Research into Global Hazards of Travel). Additional source: PLoS MedicineSource reference: Kuipers S, et al "The Absolute Risk of Venous Thrombosis after Air Travel: A Cohort Study of 8,755