Sieving a Functional Unit
“The student begins with the patient, continues with the patient, and ends with the patient, using books and lectures as tools, as means to an end.”
– Sir William Osler –
To form a differential diagnosis of oedema, we now simply take the components of our functional unit and sieve them:
Obstruction to lymph drainage
Congenital: absence of lymph tissue
Infective: filariasis worms (block up the lymph)
Tumour: malignant invasion of the lymph spaces
Iatrogenic: surgical lymph removal.
2. Increased capillary permeability
Inflammation – local infections, allergic reactions.
EM: vitamin deficit, eg Vitamin C
Trauma: burns
DPT: toxins
Increased capillary hydrostatic pressure
Tumour – masses obstructing venous vessels
EM – excess fluid retention (eg heart failure, excessive renal retention,
mineralocorticoid excess)
Trauma / mechanical – prolonged inactivity / lower leg dependency
– deep venous thrombosis.
– excess body heat (causes vasodilation)
Reduced oncotic pressure
Inflammation – sepsis / catabolism
EM – failure to produce proteins (seen in malnutrition / malabsorption,
nephrotic syndrome or cirrhosis).
Trauma – Burns and woundsAnd that completes the clinical picture of oedema. Now that we have a differential diagnosis we can start to think about what clinical questions to ask, what to examine most carefully, and what tests to do on our patient.
You can make a functional unit of any problem you encounter – just put together all the anatomical and physiological constituents, breaking the problem down into easily digestible pieces. It is much more productive than simply memorising the five commonest causes! Do not worry if you could fit few causes of oedema into the functional unit by yourself – the system is the important thing. Remember again Osler’s advice: All students need is sound methods and a proper point of view: all other things will be added as experience grows. With every new case you see new ‘leaves’ will be added to your great ‘diagnostic tree’.
Functional units require you to return to first principles and basic medical science – so don’t be daunted if they seem too complex to begin with. If you take the time to thoroughly learn your basic science in this functional and practical context, you will be much more likely to remember what you have learnt.
Note how other sieves there are ‘nested’ inside this larger sieve of oedema – like cirrhosis, malabsorption, heart failure, and nephrotic syndrome. This is typical of a clinical medicine – analysing one problem often leads you to thinking about another. Just think how many causes for oedema we could generate if we exhaustively sieved all of these ‘nested sieves’ as well! Instead of doing this here, we just put them where they fit best.
Start to apply functional units in your everyday clinical thinking. Your approach to medicine will become much more logical and simple – and your patients will be grateful for your new-found diagnostic abilities. To ensure you have grasped the method the next section will take you through a number of functional units in action. Of course, we also still need to find a cause from our differential diagnosis that suits our particular patient.William Harvey (1587-1657)
One of the greatest discoveries in science was the discovery of the circulation of the blood by William Harvey.
Harvey was born in England to a wealthy Kentish family, and after graduating from Cambridge, went to study at Padua University in Italy, where Vesalius had made his discoveries. There he was tutored in the tradition of Vesalius and became interested in the circulation.On returning to England, Harvey showed that the blood circulates around the body due to the pumping of the heart. Before this time it was believed that the blood simply fluxed to and from the peripheries due to pulsation of the vessels. He was heavily influenced by the ‘experimental philosophy’ of investigation, and proved all of his ideas with very careful measurement and reasoning. For this reason he is held to be the founding father of physiology. His work was criticised by the journalists of the day, to whom he responded: “The crowd of foolish scribblers is scarcely less than the swarms of flies in the height of summer, and threatens with their crude and flimsy productions to stifle us as with smoke.” Harvey was also the first to suggest that humans reproduce by fusion of egg and sperm – two centuries before a human egg was actually observed.
For his work he was charged with being physician to princes and kings of England, and was once engaged to “accompany the young Duke of Lenox in his travels beyond the Seas”. A proud man, it is said that in his younger days he always carried a dagger, and when enraged (which was often) would not be afraid to draw it.