Zebra or Just Another Breed of Horse? A Medical Maxim Examined for Rare Disease Awareness Day

 Zebra, or an Unrecognized Horse?  A Medical Maxim Put to the Test
“If you hear hoofbeats, don’t think zebras.”
-Medical School Maxim
Many in the medical community have heard the commonly bantered about medical school maxim, “If you hear hoofbeats, don’t think zebras,”  dissuading newly minted doctors from seeking a diagnosis of a rare disease for patients with unusual symptoms not explained by routine, standardized tests.  The maxim never made much sense to me.  For one thing, as a metaphor it falls rather flat and as a maxim it lacks the resonance of truth.  Why wouldn’t someone think of zebras when hearing hoofbeats?  Why not wart hogs, cattle, antelopes, giraffes, or any such bovine or quadruped in possession of hooves?  Horses are not the only animal that make hoofbeat sounds when they run.
Finding the origins of truisms can often shed some light on how they came in to being and how they may have become popularized, generally accepted over time, and eventually distorted.  What I found about the zebras and hoofbeats medical maxim is that it apparently originated with  Dr. Theodore Woodward in the 1940's.  Dr. Woodward had an excellent reputation as a diagnostician and was a well-respected medical school professor.  Interestingly, the phrase about zebras and hoofbeats used today is a distortion of what Dr. Woodward actually said, that being “Don’t look for zebras on Green Street.”  At the very least this statement does make more sense as an aphorism because while it is true that a zebra makes hoofbeat sounds, it is not likely that a zebra would present itself on a street in downtown Baltimore.   How would this phrase become distorted over time, becoming misapplied in such a way as to be a potential source of under diagnosis and misdiagnosis of human disease?
Everyone has probably heard at one time or another, of  the whispering parlor game.  Several people are seated in a row and a sentence is whispered to the first person, who then whispers it to the next person, who then whispers it to his neighbor, and so on, until the last person in the row is reached.  This last person then stands up and says out loud what had been whispered to him.  The first person says the original statement out loud and everyone has a good laugh at how distorted the sentence became over several whisperings. 
Perhaps in the same way, a maxim repeated over several decades gradually loses its original intent as modifications and omissions distort it in to a fragmentary misinterpretation.  The playwright Henrik Gibson, in his play “An Enemy of the People” perhaps reflected this best through his character Dr. Stockman:
“A normally constituted truth lives, as a rule, seventeen or eighteen years, at the outside twenty, seldom longer, after which it presumably becomes a lie, a ghost.”
Maxims can become distorted with time and retelling.  What else may be at work to give a maxim a shelf life?  Time itself brings new discoveries in to being.  A new social paradigm can make earlier beliefs no longer apt.  In the case of the “hoof beats and zebra” maxim, medical knowledge about diseases once considered rare but are no longer found to be so erodes the analogy in an increasing number of cases.  One such example is interstitial cystitis, once thought to be a very rare disease of post menopausal women.  But decades of patient advocacy, networking and fund raising for research has changed medical understanding of the disease to include both men and women of all ages in the diagnosis, as well as a significantly higher prevalence in the general population than originally thought.  Such is the changing paradigm of a disease that loses its “zebra on Green Street” status.  In fact, I would hold that many, if not most, so called rare diseases are simply under-diagnosed.  Something rarely looked for is rarely found.
Did the “zebra on Green Street” analogy, then, ever have any use, and if so, what might that have been and why?  There is a use in medicine for training towards the  counter intuitive.  A surgeon must train to overcome the natural instinct that cutting in to a living human being’s flesh is a force of destruction.  An emergency room doctor who recoils with horror at an accident victim instead of setting immediately and dispassionately to work would not be of much use.
 Research suggests that the human mind intuitively recalls with greater acuity that which is unusual or extreme than those things that are ordinary and mundane.  Perhaps that is just one of the brain’s defense mechanisms - strange things impress more deeply because something that is out of order can be a threat worthy of instant recall.   By extension a medical student confronting a patient with a rare disease would remember that patient more vividly than the one who presents with a common cold.  The suggestion here is that the novice diagnostician must use trained reason to counteract the possibility that a rare disease memory will cause him to somehow “hallucinate” this disease recurring in a larger population than is the medically accepted norm.  And that is where the “zebra on Green Street” maxim comes in to play.  It was originally intended to protect a patient from unnecessary tests and possible inappropriate treatments through the mind’s error of over inclusion.
But what if seeing a disease in a larger population than is the medically accepted norm, is in fact the correct perception?  As medical knowledge increases, the trend seems to be precisely a discovery of higher than originally accepted prevalence of the so-called rare diseases.  Even discounting the probable actual higher incidence of individual rare diseases, it might do well to consider that if the current figure of approximately  6000 recognized rare diseases is correct,  odds are that a given population will have a distribution of these.  And diagnoses of exclusion are therefore somewhat suspect, for the sheer volume of possibilities makes a full exclusion rather unlikely.  Some of factors involved in impediments towards excluding rare diseases: www.ncbi.nlm.nih.gov/books/NBK56184/
Given what medical science knows today, rare diseases may indeed be much more common than the possibility of seeing a zebra on Green street.  Perhaps we need a new metaphor to describe them and a counter maxim as an antidote to the errors of exclusion promulgated by a maxim in need of retirement.  Maybe a person with a disease considered to be rare is more like an uncommon breed of horse, or even a not so uncommon breed of horse destined to live in an era in which only a limited number of horse breeds are recognized.  I sometimes think of myself not as a zebra in a world of horses, but as a Shetland Pony in a world that only sees the more common quarter horses, American painted horses, Arabian and Appaloosa.  Is there a possible aphorism as an antidote to the zebra on Green Street?  Something light hearted that is an equally apt metaphor for the error of under inclusion? I think so.  It might be something like, “If you hear hoofbeats, don’t assume that it can only be an American painted horse in your grandmother’s barn in Alabama.”