Systematic review
Several databases were searched to 2001 for articles on the incidence of DVT and
thromboembolism. Studies reported all diagnosed patients in a defined general
population in a developed country. DVT diagnoses had to be confirmed by clinical
tests (like a scan) or a satisfactory validation study of the accuracy of the
diagnosis performed. The age range had to be specified, so incidence rates could be
calculated per 100,000 person years. For the purposes of the review, DVT cases had to
be new and not to be recurrent, combined with pulmonary embolism, and due to any
cause.
Results
Finally accepted were nine studies from a combined population of about 19 million
persons published since 1976. Most studies were conducted in Sweden or the USA.
Incidence rates adjusted to include only new DVTs, due to all causes, and for all
ages of the population are shown in Figure 1. Most studies clustered around an
incidence of 50 per 100,000. DVT occurred rarely below 20 years, but increased with
age (Figure 2), so that in over 70s the rate was 200 per 100,000. Incidence was about
the same in men and women.
Figure 1: Individual studies of DVT incidence in the general population
Figure 2: Age and DVT incidence
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The causes of DVT were attributed to cancer or previous hospital admission, for
about a quarter to a third of cases for each cause. About 40% of cases of DVT had
no known cause.
Risk factors for DVTs
A reasonably large case-control study in France [2] examined 636 patients
presenting with DVT and paired them by age and sex with a control group of
patients presenting with influenza or rhinopharyngeal syndrome. DVT had to be
documented by at least one objective test. Risk factors were classified as
intrinsic or permanent, and triggering or transient.
In 988 patients who had not undergone surgery or had a plaster cast on the lower
extremity during the preceding three weeks, a number of intrinsic and triggering
factors were associated with a higher risk of DVT (Table 1). Odds ratios of about
3 or above were found for a previous history of DVT or pulmonary embolism, venous
insufficiency and chronic heart failure, with pregnancy, violent effort or
muscular trauma, deterioration in general condition and being confined to bed or
armchair as the main triggers.
Table 1: Risk factors for DVTs
|
Risk factor
|
Case patients
(%)
|
Control patients
(%)
|
Odds ratio
|
Intrinsic factors
|
| History of DVT or embolism |
21 |
2.4 |
16 |
| Venous insufficiency |
70 |
41 |
4.5 |
| Chronic heart failure |
10 |
4.5 |
2.9 |
| BMI more than 30 |
15 |
7.0 |
2.4 |
| Standing more than 6 hours/day |
39 |
32 |
1.9 |
| More than 3 pregnancies |
17 |
9.8 |
1.7 |
Triggering factors
|
| Pregnancy |
2.4 |
0.3 |
11 |
| Violent effort or muscular trauma |
7.9 |
1.0 |
7.6 |
| Deterioration in general condition |
6.3 |
1.2 |
5.8 |
| Immobilisation |
8.0 |
2.0 |
5.6 |
| Long distance travel |
13 |
6.3 |
2.4 |
| Infectious disease |
19 |
13.0 |
2.0 |
Travel as a risk factor
Another case-control study from France examined this issue [3]. It used all
patients admitted for DVT or pulmonary embolism from 1992-1995, with control
patients those admitted for a an event other than these. Since it was a
cardiology department, these were mostly chest pain, hypertension and syncope.
There were 160 cases and controls.
Cases and controls were well matched for age (mean 66 years), but there were
more women in the cases (Table 2). More of the cases were obese (not defined in
this paper), had a history of venous thromboembolism, or had made a journey
lasting more than four hours in the preceding four weeks.
Table 2: Significant associations with DVT
|
Risk factor
|
Case patients
(%)
|
Control patients
(%)
|
Relative risk
(95% CI)
|
| Female |
48 |
34 |
1.4 (1.1 to 1.9) |
| History of DVT |
12 |
4.4 |
2.7 (1.2 to 6.3) |
| Obesity |
34 |
20 |
1.7 (1.2 to 2.5) |
| Recent travel |
24 |
7.5 |
3.3 (1.8 to 6.0) |
Of the 39 cases who had made a long journey, 28 made it by car, nine by plane
and two by train. The mean journey length was about six hours for each mode of
travel, and the average time between journey and occurrence of symptoms was 13
days, but with much variation. For 29 of the 39 cases there was no other
circumstance or disease to explain the event.
Flying and aspirin for DVT prevention
Is there any way to assess the benefit of taking aspirin to prevent DVT in a long
distance flight? One study has attempted to do that [4].
It took the risk of DVT as being about 20 per 100,000 travellers for one long
distance journey a year, based on a British Parliamentary Select Committee
estimate of the risk as being 0-40 per 100,000 travellers. Other literature
suggests this to be a reasonable figure, but the authors used a range of
estimates of 10-40 per 100,000 to represent different levels of risk, like age.
The potential benefit of aspirin was a risk reduction of 29% for 160 mg of
aspirin daily for 35 days from a 13,000 patient study of aspirin after hip
fractures.
Table 3 shows the results in terms of numbers of patients needed to be treated
with aspirin to prevent one of them having a travel-related DVT. Estimates range
from 8,600 at high risk, to 34,000 for low risk.
Table 3: Numbers needed to treat with aspirin to prevent one DVT, at
different levels of risk
|
Per 100,000 travellers or travellers treated with aspirin
|
| Estimated risk of travel-related
DVT |
10 |
20 |
30 |
40 |
| Number of travel-related DVTs prevented
by aspirin |
3 |
6 |
9 |
12 |
| Number of travel-related DVTs occurring
despite aspirin treatment |
7 |
14 |
21 |
28 |
| NNT with aspirin to prevent one
additional travel-related DVT |
34,000 |
17,000 |
11,000 |
8,600 |
Comment
DVT is uncommon, and especially uncommon in people without recent cancer or
previous hospital admission. With these conditions it is 50 per 100,000 per year,
and without them it is about 20 per 100,000 per year. Being older, having certain
medical conditions, and having a BMI over 30 increase the risk. Long journeys
also increase the risk. The risk is still about 1 in 5,000 in the general
population, and that sort of risk is attributed to long distance air travel. To
put it in perspective, the risk of being killed on the roads in the UK is about 1
in 17,000 each year.
Aspirin, or stockings, may help, but lots of people have to take aspirin for one
to be helped. We don't know what dose, and for how long, is effective. We don't
know how effective it would be, and we don't have a good handle on how potential
harms relate to potential benefits.
Despite a low risk to the individual, there are a lot of travellers by car, rail
and air. At peak levels, Heathrow airport handles about 250,000 travellers a day,
many on long flights. Even a low incidence of travel-related DVT should impact on
the overall incidence of DVT. But studying benefits of any intervention will not
be easy given the enormous size needed for any study. In the meantime the advice
we are given for air travel might be applied to other forms of travel or
behaviour.
References:
- FJ Fowkes et al. Incidence of diagnosed deep vein thrombosis in the general
population: systematic review. European Journal of Vascular and Endovascular
Surgery 2003 25: 1-5.
- M Samama et al. An epidemiologic study of risk factors for deep vein
thrombosis in medical outpatients. Archives of Internal Medicine 2000 160:
3415-3420.
- E Ferrari et al. Travel as a risk factor for venous thromboembolic disease:
a case-control study. Chest 1999 115: 440-444.
- YK Loke, S Derry. Air travel and venous thrombosis: how much help might an
aspirin be? Medscape General Medicine 2002 4 (3) (
www.medscape.com/viewarticle/441153
).
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