Deep venous thrombosis
What is it?
Deep venous thrombosis is a condition where a blood clot forms within the deep vein system. The principal veins affected are those in the lower abdomen, groin, inner thigh and the calf muscles. The acronym DVT is often used in medical literature and it is also sometimes referred to as peripheral thrombosis. It needs to be distinguished from an arterial thrombosis, which is a clot occurring in the arteries.
The condition is more common in people over the age of sixty and affects approximately 2 people in every 1,000.
The principal complication of DVT is pulmonary embolism.
What causes it?
The risk of acquiring DVT is increased if there is thickening of the blood or a reduction in the flow of blood. Certain groups of people are at greater risk of developing DVT. This includes those subjected to prolonged bedrest, sitting or immobilisation.
Typical examples of this would include people who have recently undergone surgery, especially orthopaedic surgery, gynaecological surgery or heart surgery. Such patients are usually given low dose anticlotting drugs during their hospital stay. Such people are also encouraged to get back on their feet as quickly as possible after surgery in order to reduce the risk of clotting. This is in marked contrast to the policy of several years ago when prolonged bed rest was the norm.
Cancer patients are also more at risk because they may develop increased thickening of the blood in association with their cancer. There is also an increased incidence of DVT in women using the contraceptive pill or hormone replacement therapy for the menopause. This risk is small but a previous history of DVT would be regarded as a contraindication to the use of these medications.
Concern has also been expressed regarding the impact of long-haul air travel and the development of DVT. This phenomenon has been called "economy class syndrome".
What are the symptoms?
The principal complaint is one of pain in the leg. The pain can be quite severe and the person may not be able to weight bear on the affected leg. Typically the symptoms are confined to one leg only. The leg is usually tender to the touch over the area where the thrombosis has occurred. There may also be associated oedema or swelling of the leg. The skin may also be discoloured and the extent of this discoloration depends on the severity of the thrombosis.
How is it diagnosed?
Most cases of DVT are diagnosed on the basis of the history of the event and the associated physical signs. Confirmatory medical evidence is obtained by various imaging techniques.
A Doppler ultrasound scan can illustrate reduced blood flow in the affected extremity. This procedure is similar to the scans performed during pregnancy except in this case the device is applied to the affected limb and not the abdomen.
There are more specialised techniques available in the radiology department including a special x ray called a venogram. This involves the injection of a special dye into the leg, which reveals the obstruction to blood flow caused by the clot. There are other sophisticated techniques for evaluating blood flow, which can be used if simpler diagnostic methods are inconclusive.
If it is suspected that there may be a clotting problem in the blood this can be evaluated through measuring the various clotting elements in the blood. This involves the taking of a small blood sample and subjecting it to analysis in the laboratory.
What is the treatment?
Many cases of DVT will resolve spontaneously but an initial period of hospital based treatment is usually required. The main priority in such treatment is to prevent a portion of the clot from breaking off and travelling on to cause damage in the lungs. This process is called pulmonary embolism and is the most serious complication of DVT.
Occasionally an embolus can travel to the heart or brain but this complication is less frequent than pulmonary embolism.
Pulmonary embolism is prevented with anticoagulants, such as heparin and warfarin.
Heparin is an injectable drug and warfarin is taken by mouth. Both drugs thin the blood and prolong clotting times and the duration of the clotting time is closely monitored in order to prevent excessive thinning of the blood which would increase the risk of haemorrhage.
A minority of cases will require medication to dissolve the clot and a further minority may require surgery to physically remove the clot.
There is some evidence that low dose aspirin may be helpful in reducing the risk of DVT but further research is required in order to demonstrate if it is as effective as prescription treatments.
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