Do I Have a Rare Disease?

 There’s an adage known to every medical student, intern, resident, and practicing physician.

When you hear hoofbeats behind you, don't expect to see a zebra.

This quote has been paraphrased into several iterations, but the quote is credited to Dr. Thomas Woodward in 1940. Dr. Woodward, a wise medical professor and Nobel Prize nominee, offered his advice to medical interns.  The aphorism has timeless value.

The point is for physicians to exhaust common explanations when confronting medical clues before broadening the exercise to contemplate rare potential diagnoses.  It is more likely that hoofbeats are made by a common horse than a more exotic hoofed beast.  Get it?

Let me offer a brilliant quote.  Common things occur commonly.  If a patient sees his physician to evaluate a fever, it’s unlikely that the doctor will entertain malaria as a diagnostic consideration, even though fever is a hallmark of malaria.  Seasoned clinicians will widen their diagnostic view when persistent symptoms defy explanation.  While any diagnosis is possible, rare things occur rarely. 

Hoofbeats?  Must be those zebras again!

There are two circumstances when zebras are given undue consideration.  It is often invoked by young physicians, particularly during their training years.  These professionals haven’t accumulated sufficient experience to have a deep data bank of common medical maladies.  And since they are in teaching or academic programs, the patients they see may be skewed since many patients with unexplained symptoms are referred there to crack the puzzling cases.  So, a ‘rare’ diagnosis may occur more commonly in a medical training program.  Let’s assume an intern in the hospital is caring for 2 patients with joint pains attributed to lupus. If this intern evaluates a patient with knee pain, she might wonder if this patient also has lupus instead of common arthritis, based on her experience.

The other scenario where zebras are considered is by patients themselves offering their physicians diagnostic advice.  Every doctor has had multiple instances of patients suggesting very rare diagnoses responsible for their unexplained illnesses. Try googling for illnesses responsible for fatigue, abdominal pain, nausea, dizziness, sleep disturbances and various rashes and prepare yourself for an avalanche of diagnostic possibilities.  Naturally, a patient with unsolved symptoms might feel that he has found a novel path to pursue. Doctor, how do you know I don't have a parasite?  This has been a recurrent inquiry from my patients over the years.

I don’t have a rigid template guiding me when to let the zebras in.  This process needs to be customized to the individual patient.  Medicine is an art. Physicians, like everyone, are flawed members of the human species.  When a patient harbors a rare condition, it may take several physicians sometime before their diagnostic arrows hit the target. In the meantime, let common sense reign.  If you hear a horn blaring outside, is it more likely to be a car or a musician playing the trumpet?