Introducing the Sieve
Patients come to a doctor with a great variety of complaints, from painful ears to bleeding rears.  As doctors it is our task to clearly define the patient’s illness and work out what is causing it so that we can do something to help.  
 
Cause is the sine qua non of understanding, diagnosing and correctly treating disease.  It lies at the heart of successful medicine.  In the sieve we develop a systematic method for working out the causes of disease.
Over the next few pages we will introduce you to the sieve as we use it in medical practice.  From now on every time you diagnose anything, think through the sieve, and over time it will become you own.
 
One way to think of the sieve is as a tree with branches: it is a foundation for you to hang your leaves of knowledge on as you acquire them.  It does not matter that you cannot formulate an exhausting list of ear lumps at present – think of Osler’s advice: all a student needs is sound method and a proper point of view, all other things will be added as experience grows.
The first category in the sieve is congenital disease - diseases people are born with.  We divide them into two major types: developmental defects and inborn errors of metabolism.
The first category of congenital disease is developmental defects.   Most often these are structural problems caused by disturbances in the development of an organ (organogenesis).  In the case of a baby with a cleft palate this cause may be immediately obvious, but in certain congenital heart diseases for example, it may be a more difficult diagnosis to make, and only picked up much later in life.
 
Sometimes we encounter tumours present at birth that also result from defective developmental.  An example is a nephroblastoma; a tumour derived from and composed of embryonic renal elements.
The other important branch of congenital diseases involves inborn errors of metabolism (IEMs).  These defects, usually with a clear genetic basis, concern the basic metabolic building blocks of the human body.  Patients suffering from IEMs often seem normal at birth, but may fall ill days or years later.  Examples include cirrhosis of the liver caused by a disturbance in iron metabolism (haemochromatosis) or copper metabolism (Wilson’s Disease).
The next category a doctor must sometimes consider is the special case of transplacental diseases.  These occur when a disease-inciting factor is carried across the placenta to cause disease in the developing human, such as occurs in certain infections.  These diseases often announce themselves at or before birth.
If not born with a disease a person must have acquired it. If we study and classify diseases, as Sydenham did, we can see that there are really only a small number of ways that people can acquire diseases.   They are as follows:
 
Ask of any active executive or a leader in a profession the secret… and they will reply in one word, system; or as I shall call it, the Virtue of Method. “Pythion who lived near the temple of Earth suffered from twitching which began in the hands…” - Hippocrates (Epidemics Book III) -
One prominent proponent of classifying disease was the seventeenth century physician, Thomas Sydenham.  Sydenham pointed out that all diseases should be critically studied and categorised, so that diagnosis could be swiftly made and an appropriate remedy selected.  Using the analogy of botany, Sydenham suggested that among all diseases the physician must identify the genus and then search for characters that would distinguish the species.  Reducing many diseases to the same class, he considered, would greatly simplify thinking about medicine.
A patient may complain of a symptom of disease involving some part that has become red, hot and tender – if so, it is inflamed. If developing rapidly, this is acute inflammation. Slow (and more subtle) onset is seen in chronic inflammation. In either instance the inflammation may have been acquired by catching some invading organism. This is, of course, an infection and there are a number of possible culprits arranged in the sieve from small to large in size. Sometimes an inflammation can also develop quite some time after the body has overcome an infection. These are post-infective inflammations, of which rheumatic fever is a good example. Lastly, some part may be inflamed because the body is seemingly reacting simply to itself. These are non-infective inflammations, and autoimmune diseases are good examples.
Another common way for human bodies to acquire disease is by growing tumours.   These can affect all parts and systems of the body and therefore have a fascinating and surprising variety of presentations.  The successful clinician must be watchful for their presence and must not forget to think of them in order to find and treat them early.  
People can also acquire disease by wearing out. This method of developing disease can also affect every bodily system.
Diseases of metabolism affect the same sort of biochemical structures as the IEMs we met above.  However, they are acquired through the act of living, instead of bestowed on the patient at birth.
(Diseases caused by doctors).
Start using the sieve in medical practice – and you will never have to painstakingly memorise a list of causes again!
So how does the sieve work in practice?  Lets see how you might have responded on the earlier ward round, simply after meeting the sieve:
A little clumsy: Sydenham might comment that there are lots of ‘genera’ and few ‘species’. But, you will agree, a vast improvement on the blind stab at heart failure. We will aim for much better yet. The cause is always hidden somewhere in that tree, and so even using the broad categories, the differential diagnosis was useful. It did not take us right to the ‘leaf’ of the unusual condition called ‘chondrodermatitis nodularis helicus’ – but now a little about that condition has been added to our tree of knowledge ready for next time… if there is a next time for this condition! We inherit the sieve from an exceptionally rich and fascinating history of medical thinkers. In order to properly understand it, let us briefly explore each portion of the sieve in greater detail. We will also pay our respects to a selection of the greats of medical history and their discoveries along the way. Pay attention – they have much to teach us. You are advised, however, not to proceed until you have memorised the sieve - and memorised it so well that you will never forget it! The round approaches the next bed and everybody looks down at an elderly man. There is a knobbly crust obvious on the pinna of his right ear. The Professor prods and pokes at the crust. “Hmmm. Yes…yes. Rather interesting,” he mutters. Then you see a sly grin slowly spread across his face. You watch in dismay as he slowly turns his head and fixes his eyes on you. “Well? Student, what is it?” Oh no! You’ve never seen anything like it in your life! Your stomach immediately ties in a knot, you feel nauseous, your knees knock together, beads of sweat break out on your forehead. You’re forced to contract your sphincters. Your mind races… if only you could remember that lecture on painful knobbly crusts on the right ears of elderly men! And then……. **The sieve!** “I g-g-guess there’s a few possibilities…” you reply. “Is it a funny birth defect?” “No. And I’m sure he doesn’t think its very funny.” “Well, it could be a skin infection? “No.” Is it a skin tumour then?” “No.” “Well, is it a metabolic thingy, like a gouty tophus?” “No. And don’t say thingy.” “Did he injure it?” “No.” “Well, is it some sort of degenerative change, then?” The consultant chuckles. “Mmmm, sort of. Actually its chondrodermatitis nodularis helicus” Sir Thomas Addison (1795-1860). Addison’s father (a grocer) wanted his son to be a lawyer. Instead he entered medicine and became deeply interested in diseases of the skin. He was the first to describe Addison’s disease, a landmark in endocrinology. Addison was a brilliant lecturer and diagnostician and would tell his students that if he could not reach a diagnosis in a patient then he would think of all the possible explanations on his way to and from the hospital. His abilities to sift evidence and diagnose were unrivalled in his day, but he did not devote the same energies to alleviation or cure and so attracted few private patients. Addison suffered episodes of severe depression which he greatly feared. He eventually took his own life at the age of 65, shortly after retiring. Remembering is for dumbies - Understanding lasts forever!