Towards an understanding of oedema

doi:10.1136/bmj.318.7198.1610

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Charles J Diskin (HNDT512@auburn.campuscw.net), clinical associate professor,
Thomas J Stokes, clinical associate professor,
Linda M Dansby, clinical associate professor,
Thomas B Carter, clinical associate professor,
Lautrec Radcliff, clinical associate professor,
Selby G Thomas, associate professor

Hypertension, Nephrology, Dialysis, and Transplantation Clinic, School of Pharmacy, Auburn University, Auburn, AL 36849, USA

Correspondence to: Dr Diskin

Accepted 20 January 1999

Sometime during the reign of Tiberius (25-50 AD) Celsus wrote:

A chronic malady may develop in patients who collect water under their skin. The Greeks call this hydrops. There are three species:

Sometimes the belly is tense. The Greeks call this tympanites.
Sometimes the body is rendered uneven by swellings arising here and there and all over. The Greeks call this hyposarka.
Sometimes, the water is all drawn within and is called ascites.1

Oedema has been recognised almost since the earliest recordings of medical history. Even the ancient Egyptians seem to have had a hieroglyphic for “water under the skin.” In the description of case number four in the Edwin Smith papyrus the hieroglyphic appears to be similar to the one used for water that floods from the Nile.2 This ancient familiarity, however, has not yet led to a complete understanding of oedema, and sometimes its treatment remains imperfect. None the less, an improved understanding of the pathophysiology and biophysics of oedema will allow most doctors to develop a more rational approach to treating it.

Summary points

The causes of oedema are not well understood
Three main variables are associated with the formation of oedema: oncotic (colloid osmotic) pressure, hydrostatic pressure, and membrane permeability
Differential diagnosis should be based on an understanding of which of the variables has been altered
Osmotic pressure is a measure of the entropy of the solution
Entropy can be altered without changing the number of molecules in a protein solution if the arrangement of those molecules is changed

Fig 1

Child with periorbital oedema

Methods

This article is based mostly on our own research and clinical experience.

Pathophysiology

When Bright first described the retention of water and waste by diseased kidneys3 he was deliberately vague about whether he was describing one disease or two since he could not …