Minor injury linked to blood clot

“Minor injuries can cause blood clot,” reads the headline in The Daily Telegraph . The report it refers to says that the risk of clots following major injuries has always been recognised, but new findings show that minor leg injuries which include “ankle sprains, torn muscles and other minor injuries” can lead to a “higher risk of developing blood clots”.

The newspaper report is based on a Dutch study which compared people who had blood clots with those who didn’t, looking at their history of minor injuries in the three months before the diagnosis. However, this study excluded people who had surgery; those admitted to hospital; and those who had prolonged bed-rest, been put in plaster cast, or who had cancer. All of these things are known to be associated with an increased risk of deep vein thrombosis (DVT).

The reports should not be interpreted to mean that people with minor injuries have now been found to be at greater risk than the groups known to be at high-risk of DVT. This study has not compared the two risks; further studies are needed to clarify this issue.

Where did the story come from?

Karlijn van Stralen and colleagues from Leiden University Medical Center, in Leiden, Holland carried out this research. The study was funded by the Netherlands Heart Foundation, the Dutch Cancer Foundation and the Netherlands Organisation for Scientific Research. It was published in the peer-reviewed medical journal Archives of Internal Medicine .

What kind of scientific study was this?

The study was a case-control study comparing people aged between 18 and 70 who had deep vein thrombosis (DVT) of the leg or a pulmonary embolism (PE) with people who didn’t have these conditions, between March 1999 and August 2004.

The researchers included patients from six clinics who had a definite or probable diagnosis (established through hospital records or family physicians) of PE or DVT. Partners of the patients were invited to form one control group that, as shared lifestyle factors would mean that they would in some way be matched to the cases. Additionally, another control group was made up of people contacted through randomly generated telephone numbers. In total, 2,471 patients and 3,534 controls were included in analyses.

All participants were sent a questionnaire that asked about injuries, surgical procedures, plaster casts, other immobilisations, family history of thrombosis, height and weight and sporting activities that had taken place in the year before the study started. They were also asked about their most recent minor injury. Those reporting an injury in the three months before the diagnosis of DVT or PE (or, for the control group, the completion of questionnaire) were included in the analysis.

Using the questionnaire, the researchers were able to exclude people who reported undergoing surgery, having a plaster cast, hospitalisation or extended home bed rest in the year before the study started. Partners of such patients were also excluded from the control group. Blood or DNA (through mouth swabs) was collected from some people in the sample, to look for hereditary blood conditions that are known to carry an increased risk of blood clots. The researchers compared the groups to investigate whether having a minor injury increased the risk of a diagnosis of DVT or PE.

What were the results of the study?

Researchers found that 289/2,471 (11.7%) of patients had a minor injury in the three months before the study started compared with 154/3534 (4.4%) of the control group. This means that people with a blood clot were nearly three times more likely to have had a minor injury. They found that this association was stronger if the injury was in the month before the diagnosis; and that when they considered injuries per week in the 10 weeks before the diagnosis, there was no difference in risk between groups.

There was no great difference between men and women in the likelihood of minor injuries in both groups. Injury to the leg was associated more strongly with DVT than injuries to other body parts. The researchers established that there was a strong link with family history, and adding this risk factor to injury increased risk of thrombosis. In the subgroup of people from whom DNA or blood was available, the researchers established that people with a particular mutation that affects blood clotting (V Leiden mutation) plus an injury, were at 50 times greater risk of thrombosis than people with no mutation and no injury. However, the number of people in this study was small.

What interpretations did the researchers draw from these results?

The researchers concluded that minor injuries “that do not require surgery, a plaster cast, or extended bed rest were associated with a three-fold greater relative risk of venous thrombosis”. This increase in risk was not affected when the researchers adjusted for sex, age, sport activities and body mass index.

What does the NHS Knowledge Service make of this study?

This case-control study has some weaknesses associated with these types of studies:

• Patients and controls may “recall” their history of minor injuries in a different way. People who have an injury and are then soon (within 10 weeks in this study) diagnosed with thrombosis may be more likely to remember the injury. This may influenece the comparison between people with and without thrombosis. The researchers do discuss this and believe it is unlikely. However, it is impossible to know for sure whether such a bias had any effect on the results.
• Overall, there were only a small number of minor injuries in the entire sample. In total, 4.4% of controls and 11.7% of patients had experienced one in the three months before the study started. 
• The researchers adjusted for other known risk factors for thrombosis, including sport and BMI. However, other factors which they didn’t take into account may have explained some of the differences. These could include aeroplane travel, smoking, oral contraceptive use and pregnancy.

The fact that minor injuries to the leg, particularly partial ruptures of muscles or ligaments, may be associated with increased risk for thrombosis is a plausible finding. As the authors discuss, this may be due to factors such as damage to the blood vessel or blood stasis in the vein, or some degree of immobilisation or restricted use of the leg following injury (only those defined as “prolonged bed rest” or “immobilisation by plaster cast” were excluded from this study). Likewise, the finding that people who had certain hereditary blood conditions, such as Factor V Leiden, were at much greater risk, agreees with previous findings.

The report may lead to the interpretation that people with minor injuries have a higher risk than those previously known to be at risk, that is, those with major injuries, recent surgery, prolonged bed-rest, or cancer. This study has excluded these groups, and therefore it does not compare the risks of DVT or PE in those with minor injuries with the risk in these other people, which may be higher.

Sir Muir Gray adds...

Effective treatment of minor injuries is important. Rest, elevation and gentle pressure, perhaps with ice if available (frozen peas are useful), to minimise the initial injury and speed recovery.