A deep vein thrombosis (DVT) is a blood clot that forms in the larger veins deep within the body. It usually affects the legs, but may form in the arms, or even in the abdomen.

It is differentiated from a superficial vein thrombosis, which forms in the superficial veins just beneath the skin., and which is much less potentially harmful.

A deep vein thrombosis (DVT) is dangerous, because the clot can dislodge, and travel in the circulation and lodge in the lung. This is known as a pulmonary embolus, and can be life-threatening, as the embolus (a clot travelling in the circulation), if it lodges in the lung, reduces the blood oxygenation, producing breathlessness.

The diagnosis is difficult to make on symptoms alone and is usually confirmed in only one out of three cases when it is suspected. A DVT does not always cause symptoms either.

The limb affected by a DVT becomes swollen, warm, and reddened. The superficial veins may become distended. The swelling is due to fluid that can be indented with a finger. A mild fever may occur, and the swollen limb is usually painful and tender.

Occasionally, chronic long term swelling of the limb may occur, particularly if a DVT is not treated, and this may lead to skin ulceration.

A DVT is more likely to occur in the following circumstances:

• Immobility ;including long distance air travel and prolonged bed rest.
• Post-surgery (particularly orthopaedic leg surgery)
• Patient has had a previous DVT
• Presence of cancer
• Obesity
• Pregnancy
• Oestrogen containing oral contraceptive pill(increase relative risk 3-4 times)
• Hormone replacement therapy(increases relative risk 3-4 times)
• Inherited conditions &endash abnormalities of clotting proteins that allow spontaneous clots to form.

1. Venogram - this is the most accurate test, but does involve an injection of dye into a vein.

2. Ultrasound - ultrasound will detect 95% of clots , and positive tests usually mean a clot is truly present (low false positive rate). It is usually the test of first choice. Occasionally a venogram is required if the ultrasound result does not give a clear result. Alternatively a second ultrasound can be performed after a short period if the first test is doubtful, or negative, but a strong clinical suspicion still persists.

3. Impedance plethysmography - a technology that measures the closing pressure of the veins. It is less accurate that ultrasound.

4. Iodine 125 fibrinogen scans - a scan that detects radioactive fibrinogen injected into the patient. The fibrinogen becomes attached to the clot, thereby allowing its visualisation with a gamma camera. It is less accurate that the above mentioned tests.

5. D-Dimer - a blood test that measures a breakdown substance of fibrinogen. If the D-Dimer test is negative, the patient is unlikely to have a DVT, but a positive tests is less useful as there are many causes of positive tests.

6. Full clotting studies are done to exclude hereditary clotting disorders which predispose people to clots forming.

The goal of treatment is to prevent the life-threatening pulmonary embolus. In the leg (which is the limb affected in most cases) treatment depends on whether the clot is confined to below the knee (distal DVT), or involves the veins above the knee (proximal DVT). This is because the distal DVT is very unlikely to travel to the lungs, and usually will only do so after the clot has spread to the upper leg.

Therefore a distal DVT can be left untreated, provided a follow up ultrasound is performed after a week, to ensure the clot has not grown in the vein to the upper leg vein. This occurs in about 20% of cases.

Because of this risk and the fact that the clot itself can cause later problems in the leg(post-thrombotic syndrome),most doctors will advise treating significant calf thrombosis.

Where the DVT involves the proximal vein, a risk of pulmonary embolus is high unless treatment is given.

Treatment of DVT

Treatment of DVT involves the administration of clot-thinning agents (anticoagulation).

Graded compression stockings can help reduce (by 60%) later problems with the veins of the legs (eg. post-phlebotic syndrome).

Anticoagulation treatment is usually commenced with heparin, which can be given in an intravenous drip continuously, but nowadays is more commonly given as twice daily subcutaneous injections (low molecular weight heparin). Certain patients may be suitable for home administration of subcutaneous heparin, but otherwise heparin is usually given in hospital.

An oral drug, warfarin or coumarin, is commenced shortly after the heparin. The dose of the warfarin needs to be individualised for each patient. Important changes in dose need to be made according to the results of the INR test, which monitors the blood response to the warfarin. It is important to remember that some other drugs can enhance or reduce the effects of warfarin.

When the warfarin has reached a satisfactory level on the INR, the heparin can be discontinued. The warfarin is continued for three to six months in the case of a first DVT, but in cases of recurrent DVT may need to be continued life long.

The major risk of anticoagulation is the risk of bleeding. This varies from patient to patient but is approximately 0.5% per year. Careful monitoring of the INR and caution with drug interactions should help reduce this risk.

For recurrent or severe (very large clots)cases,occasionally filters in the vena cava(main vein running to the heart) can be inserted.

Patients who have had a DVT need to be aware that they are at risk of recurrence. If symptoms recur, they should see their physician promptly.

Patients with risk factors facing surgery, particularly major surgery, should discuss ways of reducing their risk with their surgeon. Low dose heparin injections (risk reduction about 65%) have been found effective in a range of surgical patients, without unduly increasing the risk of bleeding.

General preventive measures include:

- exercising the legs regularly,
- maintaining a weight that's appropriate for your height (obesity is a rik factor for DVT),
- avoid sitting or lying in bed for long periods of time without moving the legs
- wear TED compression stockings
- taking low dose aspirin to "thin the blood"

Note: Taking the contraceptive pill increases your risk of DVT (especially if over the age of 35).

Preventive measures for travellers

- exercise the legs at least once every hour during long-distance travel
- don't cross your legs for long periods at a time
- keep well-hydrated by drinking normally
- avoid alcohol (to prevent dehydration)
- wear graduated compression stockings (TEDs)