This condition is always associated with the retention of sodium and occurs when more than 3 litres of interstitial fluid collects.
The principle site of collection of fluid varies with respect to the underlying pathology. Periorbital oedema is seen more often in renal failure, ascites occurs in cirrhosis, and pulmonary and ankle oedema may occur in cardiac failure.
The causes of generalised oedema are, in general, the result of either an increased venous pressure (eg right heart failure), or a decrease in intravascular oncotic pressure (hypoalbuminaemia - test the urine of patients with bilateral swollen legs for albumin).
Anasarca is a term used to describe gross oedema.