As a medical student, I used to enjoy the Fox show House M.D.—or at least, the first 20 minutes of the hourlong episodes. Each week, the cynical genius Dr. Gregory House would take on one new case, each seemingly more bizarre than the last. Early in the episode, House and his team would sit around a table kicking around the details of whatever mysterious ailment was afflicting their latest patient. They’d generated the so-called differential diagnosis, a list of possible conditions that should have included the real culprit. Their differential diagnosis was especially useful for a medical student because it was usually a reasonably accurate and inclusive list of the conditions that the patient ought to have had, were it not a fictional TV show. So, it was not only a good way to review, it was also inspiring to see these fantasy docs rattle off all the conditions that I was trying to wrap my mind around.
From there, the doctors would perform a series of tests in order to distinguish which among the candidate conditions was the likely culprit, with the goal of eventually arriving at the one definitive diagnosis. Of course, this was Hollywood and things were never so simple. The patient would always end up having some far-fetched condition that never made the original list, some collector’s item like Erdheim-Chester Disease, which most doctors, let alone hypochondriacs, will never hear of nor encounter. In medicine, we call these unusual conditions “zebras,” distinct from more common problems like heart attacks, strokes, and pneumonia, which we refer to as “horses.” From early on in medical school, students are taught that “when you hear hoofbeats, think horses, not zebras,” because most of the time, the more common condition will be the cause. Zebras, though, are widely overrepresented on shows like House, or just about any of the other well-known medical dramas (other than Scrubs, widely appreciated by doctors as the most accurate medical show ever, despite its categorization as a comedy). But back in the real world, abnormal clusters of symptoms are usually just horses in disguise—much to the chagrin of the medical student or intern who is looking to solve a medical riddle and become an instant legend in their hospital.
That’s why the new podcast DDx, (the abbreviation for differential diagnosis) is such a welcome addition to the interwebs. Hosted by an emergency physician and published by Figure 1, a social media platform and app developer for health care professionals, each 10-minute installment features a different physician telling his or her story about an unusual case. The twist is that the cases featured thus far have not been zebras, but what I would call “horses in disguise.” In these cases, the diagnoses were not particularly exotic, but they had details that would send many physicians down the wrong path, making the correct diagnosis far harder.
For example, in the first episode, a patient’s unusual electrocardiogram led her doctor to reassess what had initially seemed like a secondary symptom: a headache. That led to the diagnosis of a condition all emergency doctors are taught to consider in a patient who develops a sudden “thunderclap” headache: a subarachnoid hemorrhage of the brain. While that condition certainly would come to mind in a patient who leads with, “I have a horrendous headache that came out of nowhere, doc,” it would not come to mind in a patient whose initial complaint had been nausea and chest discomfort, as described here. So while this case is rare by virtue of its details (I can hear my colleagues grumbling already: “A subarachnoid bleed in a patient whose primary complaint was nausea? Come on!”), it gets at something that most public-facing medical nonfiction usually misses: Much of what doctors do is narrow the odds based on very finely honed (and often quite accurate) heuristic models. That’s wise because it permits us to avoid overtesting every patient for everything. What DDx highlights are situations in which something did not add up and the treating physician had the awareness to notice it and pursue alternative explanations that would have been inappropriate in 99 percent of cases featuring similar symptoms.
Getting to that 1 percent range can be tricky. The show’s second episode highlights the concept known as anchoring bias, which occurs when physicians “attach” ourselves to a diagnosis. In this case, a patient with sickle cell anemia visited the emergency room with lower abdominal pain. While both the patient and the treating physician chalked the patient’s pain up to a flare-up of the patient’s sickle cell disease, in fact the patient had a ruptured ectopic pregnancy, a true surgical emergency that occurs when a fetus grows in the fallopian tubes instead of the uterus, leading to massive bleeding. In a patient without a long history of sickle cell anemia flares frequently manifesting as lower abdominal pain, a ruptured ectopic would have been significantly easier to diagnose. But in this case, both the doctor and the patient were convinced (for quite a while) that the patient was suffering from an excruciating pain flare, a hallmark of sickle cell disease. That diagnostic inertia was nearly fatal, but in the end, it was avoided. On DDx, we get to hear from the doctor herself, describing exactly how the case played out.
Even leaving the zebras aside, diagnosing is one of the hardest parts of a doctor’s job—as each DDx episode underscores, even normal horses can approach from strange angles. But an even greater challenge often remains long after the case is over. Doctors who have made heroic saves because of an unusual observation—or conversely, failed to diagnose a life-threatening disease that did not “present normally”—can develop a type of hypervigilance that is not unlike post-traumatic stress disorder. After an exceptional case, good or bad, a doctor might be convinced that rare events are more common than they truly are. This also happens as we share our unusual cases with each other, as we do in the time-honored tradition called the “morbidity and mortality conference,” or M&M for short, a departmental meeting focused on assessing cases that did not go well. These conferences often feature some of the toughest cases we will ever face, and are framed as critical opportunities for learning. But the unfortunate adage that often punctuates M&M is the parting admonition to “maintain a high index of suspicion” for whatever zebra was just presented. That can be a harmful lesson: If we abide by it, we might end up inadvertently doing more harm, sending every patient with a mild headache for a CT scan, for example, which would expose many more people to unnecessary radiation than it might help save. Again, we see that diagnosis should make use of out-of-the-box thinking and sharp cognitive skills, but it should not be fueled by unreasonable paranoia.
It’s difficult enough to encourage the right amount of vigilance within the medical community. When we talk about the process of diagnosis in the public sphere, things become even more complicated. This is one of the problems of columns like the New York Times’ Diagnosis—while often excellent, some of the cases truly belong in museums devoted to medical curiosities. When they’re aired out in public, especially in a newspaper rather than a fictional TV show, these stories can serve as sustenance for pitiable neurotics who believe that they just happen to have whatever exceedingly rare disease they just read about. (We know this because “the worried well” occasionally show up in ERs armed with printouts from the internet demanding tests that are not even available for diseases they are exceedingly unlikely to have.)
That’s why the DDx podcast is so refreshing. It invites listeners from both outside of medical professions and from within them to consider the challenge that front-line providers of care face for the “undifferentiated” patient who could have anything or—as is commonly the case—nothing at all. It presents each outcome as equally interesting no matter how rare the disease. The cognitive challenge of detecting and treating dangerous conditions is at the core of the calling of the emergency medicine physician, at once the excitement and privilege of the job. But, as each episode of DDx has so far demonstrated, it’s more than merely applying knowledge (and, yes, keeping calm in the face of chaos). Emergency medicine is also about metacognition and the constant reassessment of even our most closely held assumptions, which give us the best opportunities to detect and treat horses, horses in disguise, and the occasional zebra.
DDx allows listeners the opportunity to get inside the thought processes of real doctors faced with plausible challenges. Unlike House and other similar shows, DDx recognizes that everyday people and everyday problems warrant attention too. It succeeds in telegraphing an important principle: You don’t need to be a zebra to gain our interest. The episodes accentuate our shared traits as a species without losing sight of the details that make each of us unique and fascinating. Incorporating all of that is one of medicine’s true challenges, and this show, like none I’ve ever encountered, provides a realistic and stimulating window into what it is we really do as physicians.
Disclaimer: The opinions expressed in this article are solely those of the author and do not reflect the views and opinions of Brigham and Women’s Hospital.
