In Part one I explained what a DVT is and how it happens. In Part two I will look at the factors that make some people more likely to get a DVT than others and what can be done about it. In simple terms, a clot can form in the deep veins for three basic reasons:
- The blood is too sticky
- The blood isn’t flowing fast enough
- There is something wrong with the wall of the vein that makes the blood clot.
These three features are known to doctors as ‘Virchow’s triad’ – named after an obscure 19th century German doctor.
Some common problems in each of these areas that make some people likely to get DVT’s:
Blood too sticky:
- Genetic problems with the blood are known to make DVT’s more likely – the best known one is called V Leiden syndrome. It is called ‘Leiden’ because it was discovered in the Dutch city of Leiden and V (after the Roman numeral for 5) because it is a genetic error with clotting factor 5, which makes the blood more sticky than usual. Up to 30% of DVT patients have a form of this inherited abnormality. Mild cases have an 8 times elevated risk. Severe ones up to 100 times. If you have a strong family history of DVT, then this may be the reason.
- Drug problems – some drugs increase the risks of DVT by making the blood more sticky than usual – the commonest ones are the oral contraceptive pill, hormone replacement therapy or other steroid medications.
- Cancer – patients with active cancer are at greater risk of DVT. Some cancers are more problematic than others. This arises because some cancers secrete proteins into the blood stream that make the blood excessively sticky.
- Smoking – smoking releases lots of nasty chemicals into the blood that make it a lot more sticky than it should be. They also damage the blood vessel walls as well, raising the DVT risk a lot.
Blood not flowing fast enough:
- Immobility – classically after an operation or injury. The blood is pushed up the leg by the action of the calf muscle. This only works properly if we are walking around. Prolonged periods of immobility result in stagnant pools of blood forming in the veins and this makes clotting more likely.
- Obesity – Overweight people don’t move around enough. Also certain chemical changes in the blood make DVT more sticky. Together these double the risk of DVT compared to patients of a normal weight.
- Long Haul flight – The risk of DVT after a flight is actually quite low. Most patients who get a DVT already have an increased risk factor.
Something wrong with the vein wall:
- Previous DVT – A previous DVT always damages the wall of the vein and makes it ‘rough’ rather than smooth. This makes it much more likely that another DVT will form on the roughened bit of vein
- Having varicose veins – it was often thought that varicose veins did not increase the risk of DVT, but a recently published large study from Taiwan, suggests that there is in fact a significantly increased risk.
- Having recent superficial phlebitis – these are clots in varicose veins, so not DVT’s themselves, but they make a DVT more likely by triggering off the clotting process.
In this piece I have covered the major risk factors for DVT. In the next post I will discuss how a DVT presents from the patients point of view and how doctors diagnose a DVT using clinical examination, blood tests and medical imaging.