Deep venous thrombosis (DVT) occurs when blood in the deep veins of the legs (the veins within the muscles) clots suddenly. Contrary to popular belief, this often produces no pain and the swelling can be mild, so that many DVTs are diagnosed retrospectively. Approximately one third however will present with pain and swelling, typically in the calf of the affected leg.

DVTs can be diagnosed by routine ultrasound colour duplex scanning together with a blood test, although more complex scans are occasionally required. Treatment with blood thinning agents is currently provided on an outpatient basis, together with medical management of the swollen leg. More advanced cases occasionally require inpatient care.

Many people are aware of the secondary dangers of pulmonary emboli (clots breaking off into the circulation and travelling to the lungs), although this complication is mercifully rare. A far more common outcome following DVTs is the long term damage that it may cause to the venous function in the leg, if certain vital venous valves are damaged. The development of the post-phlebitic limb is insidious, but leads to pigmentation and thickening of the tissues in the gaiter area of the leg, recurrent attacks of inflammation and eventual ulceration. Early treatment directed at the affected leg will prevent this, although management and supervision may need to be extended for up to a year or more.

Much publicity has been given to the possible association between air travel and DVTs although these risks are relatively small. Greater risks for the development of DVTs include surgery particularly to the abdomen and pelvis, some cancers and various abnormalities in the blood. Treatment of DVTs should therefore be followed by investigation and if necessary treatment of any underlying causes in order to prevent further occurrences.