Swollen lower limb—1: General assessment and deep vein thrombosis

The most common cause of leg swelling is oedema, but expansion of all or part of a limb may be due to an increase in any tissue component (muscle, fat, blood, etc). A correct diagnosis requires consideration of whether the swelling is acute or chronic, symmetrical or asymmetrical, localised or generalised, and congenital or acquired. Chronic swelling, particularly if asymmetrical, is usually a sign of chronic oedema arising from venous or lymphatic disease, whereas symmetrical lower limb swelling suggests a systemic or more central cause of oedema, such as heart failure or nephrotic syndrome. Oedema develops when the rate of capillary filtration (lymph formation) exceeds lymphatic drainage, either because of increased capillary filtration, inadequate lymphatic flow, or both. Extracellular fluid volume is controlled prinicpally by the lymphatic system, which normally compensates for increases in capillary filtration. Most oedemas arise because filtration overwhelms the lymph drainage system. Increased capillary filtration may occur because of raised venous pressure, hypoalbuminaemia, or increased capillary permeability due to local inflammation. The two main causes of a swollen lower limb are deep vein thrombosis and lymphoedema (a failure of the lymph drainage system). This article concentrates on deep vein thrombosis and next week's article on lymphoedema.