When oedema is due to generalised fluid retention, its distribution is determined by gravity. It is usually observed in the legs, back of the thighs and the lumbosacral area in the semirecumbent patient. If the patient lies flat, it may involve the face and hands, as in children with acute glomerulonephritis. Regional rises in venous pressure also influence the distribution of oedema as exemplified by pulmonary oedema in left heart failure and by ascites in portal hypertension.
The cardinal sign of subcutaneous oedema is the pitting of the skin, made by applying firm pressure with the examiner's finger or thumb for a few seconds. The pitting may persist for several minutes until it is obliterated by the slow redistribution of the displaced fluid. However, pitting on pressure may not be demonstrable until body weight has increased by as much as 10-15%. Day-to-day alterations in weight usually provide the most reliable index of progress or response to treatment.
Myxoedema is due to infiltration of the tissues by a firm mucinous material. In contrast to oedema, it does not pit on pressure. Chronic lymphoedema may also fail to pit.
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