When You Hear Hoof-Beats, Should You Look For Zebras?

Every medical student has heard the old adage (which, by the way, I can’t find in any anthology of famous quotations): “When you hear hoof-beats, think of horses, not zebras. ” What this means is that common medical disorders are more frequently encountered than rare ones. For example, if a patient comes in with a “runny nose,” a physician should think of an upper respiratory illness (i.e, a common cold) before considering the possibility of a cerebrospinal fluid leak. Unfortunately, many medical educators often spend much more time focusing on those zebras than the horses during medical school and residency training.

Our medical school has an excellent “problem-based learning approach” in which medical students take case history examinations at the end of each “learning block.” A recent examination was about a patient with gastrointestinal symptoms who turned out to have a very rare disorder (Whipple’s disease, if you are interested). I was a grader for the examination and enjoyed  one student’s discussion of his differential diagnosis in which he wrote something to the effect that “the diagnosis must be Whipple’s disease since it’s so rare and the type of diagnosis that might turn up on a problem-solving examination.” How right the student was. Even if he becomes a gastroenterologist, it is unlikely that he will ever encounter a case of Whipple’s disease.

How to tell a horse from a zebra

I’m not saying that it is always so easy to make diagnoses and that we should not consider uncommon disorders as part of differential diagnoses. Rather, that physicians should, if it’s not an immediate life or death situation, first focus on the most likely reasons for the patient’s signs and symptoms than on all of the possibilities, no matter how remote the likelihood. Such a “staged” approach can save both an incredible amount of money and patient discomfort, or even risk of complications from this or that diagnostic procedure or empiric therapy. More and more I am finding young physicians “throwing the kitchen sink at” a set of patient signs and symptoms rather than first thinking things through and maybe looking to see if there are any horses in the pasture.

In my next entry, we’ll come back to this question of common versus rare disorders as it relates to the diagnosis of congenital hypothyroidism.

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