November 17, 2023
Think Horses, Not Zebras
January 25, 2021
By Kristopher R. Maday, MS, PA-C, DFAAPA
During the fall semesters at my PA program, we have three cohorts at one time. It is a fun time for me as an educator because I get to teach to both the first- and second-year students. I get to see the excited look of new PA students starting their journey towards becoming a PA and I get to see the progression of my second-year year students and see how far they have come over the preceding year. But it is also challenging because I have to constantly adjust my own expectations for each group because of their experience levels. The first-year student is thirsty for knowledge and, like a dry sponge, wants to absorb everything you give them. They are focused on the “what” because they don’t have the experience or depth of knowledge to fully understand the “why.” The second-year student, prior to starting clinicals, is the same sponge, but is now fully saturated and heavy with the volume of information they have absorbed over the preceding three semesters. They now struggle with the “why” so they can make the cerebral and cognitive connections between the bits of information that are now deposited in their brain so they can rapidly, and accurately, recall them when they are caring for patients. This is where my struggle as an educator begins.
As many of your teachers and clinical mentors have undoubtedly told you at some point in your training, there is a difference between teaching what is in the textbook or on the boards, and teaching how to practice medicine in the real world. I think this is one of the reasons the phrase (which I’m not the biggest fan of, by the way) “those who can, do; those who can’t, teach” has persisted. It is easier to regurgitate the words in a textbook and tell students what “should” happen or “should” be done than it is to apply this material in a way that students can start to see the importance of learning it. Often I get questions from my second-year students about how to approach undifferentiated patients and why I think of certain differential diagnoses and leave others out. Most times I can explain it well enough that I can see the aha moment in their eyes; other times I have to fall back on “just because” or “my gut says so.” This happened recently during a talk I gave on acid-base balance during our first week in the simulation lab. I brought up the difference between Occam’s Razor and Hickam’s Dictum as it applies to diagnosing and I got a bunch of blank stares back. These principles I feel are absolutely paramount to understand for beginning students of medicine because it helps explain the “gut feeling” or “just because” answers to complicated cases.
Occam’s Razor is a heuristic principle from the 14th century that helps clinicians in developing differential diagnoses and sift through the information in order to come to a diagnostic conclusion. It is also known as the Law of Parsimony and simply states that the most likely hypothesis is the simplest one with the fewest assumptions. In medicine, we use this to try to explain all the signs and symptoms a patient has into one diagnosis or condition. The idea of the “razor” comes from “shaving” away possibilities to get to the simplest explanation.
Remember the show House, with Hugh Laurie? Dr. House would take seemingly unrelated symptoms and explain them all with one diagnosis. This is ultimate goal, right? Make everything fit in a nice, neat little package and fix it. As an educator, I often need to do this for new learners so they can begin to make connections and see patterns. They need to be able to take the information and package it in a coherent H&P, or oral presentation, to make it all fit into a cohesive story. While convenient, and – at times – accurate, PA students should not adhere to Occam’s Razor exclusively. This can make rare diagnoses more possible regardless of incidence, prevalence, and other patient variables because one disease is a simpler explanation than several. This is where Hickam’s Dictum comes in.
Dr. John Hickam was a physician and faculty member at Duke University in the 1950s and he asserted that diagnostic parsimony has limits in its application to the practice of medicine. Based off just a chief complaint, clinicians start to make a list of differential diagnoses before they even lay eyes or hands on the patient. As the flow of information starts to come in from the history and physical exam, there is a constant reshuffling of this list, along with testing new hypotheses with further questions, physical exam maneuvers, or ancillary studies. It is a fluid and continuous process. The core principle of Hickam’s Dictum is that no disease should ever be excluded from a differential based solely on its inability to fit into Occam’s Razor. More simply put, Dr. Hickam said: “Patients can have as many diseases as they damn well please.” Think about this: It is statistically more likely that a patient has several common diseases at the same time than one rare condition (zebra) to explain all their symptoms. The classic example of this is Saint’s Triad of hiatal hernia, cholelithiasis, and diverticular disease. These three conditions have no pathophysiological relationship and therefore completely blunt Occam’s Razor. Enter Sutton’s Law.
You have all heard of Sutton’s Law even if you have no idea what it is. Willie Sutton was the bank robber who famously said, in response to why he robbed banks: “Because that’s where the money is.” This same idea has been applied to medicine in the classic phrase “if you hear hoof beats, think horses, not zebras.” This phrase has been attributed to Dr. Theodore Woodward of the University of Maryland School of Medicine in the 1940s. His aphorism was directed towards his new interns because horses are more common in Maryland than zebras. When testing diagnostic possibilities, choose the most likely first as it is expected to be the most useful. If you chase one zebra in a field of horses, you just end up tired and empty handed. Cull the herd of horses first to make the zebra stand out.
So why is this so hard for new learners? Rare, exciting diseases are more easily remembered than the mundane, and thus are judged to be more probable. I bet there are conditions you still remember to this day that you will never see in clinical practice, but you can recall it just because it is interesting. Think about the conferences you have gone to and how many lectures are devoted to interesting, rare clinical cases. We are fascinated by these cases and do not want to miss out on the opportunity to make a once-in-a-career diagnosis.
Here is an example: 23-year-old male, currently in PA school, has a four-week history of intermittent headaches, palpitations, and diaphoresis while he studies to take his PANCE. I bet the students reading this vignette immediately thought pheochromocytoma and would order 24-hour urine metanephrines and catacholamines. The experienced PA thought: tension headache and anxiety. NSAIDs and anxiolytic. My experience has taught me that it is more likely this patient has tension headaches and anxiety (because he is studying for a life-altering test!) than he has an adrenal tumor with an incidence rate of 0.8 per 100,000 person-years. My job as an educator is to teach my students about both pheochromocytoma and tension headaches. Then, give them the tools and skills to be a regular ranch hand first, and a zebra hunter second.