Horses, Zebras, Ninjas

Ben Bryner -- I was reading this story the other day, in which a camp counselor was mistaken for a ninja, which then prompted a school lockdown. It reminded me of the old medical adage, "when you hear hoofbeats, think horses, not zebras." Briefly, if you’re in an area where horses are more common, when you hear hoofbeats outside, it's much more likely to be the sound of horses, not zebras. The idea is that when a patient presents with symptoms that are consistent with a common disease, but are also consistent with a much less common disease, you work under the assumption that it is the more common disease until you can confirm it. In other words, if you’re in New Jersey, a person dressed in a ninja getup is more likely to be a regular person who’s just into karate or dress-up than an actual ninja.

The saying is usually used to correct a student or resident’s differential diagnosis. When you’re on rounds and presenting a new patient with an unknown or not-quite-certain diagnosis, when you get to the end of your presentation, your attending will generally expect you to list the “horses” (the more common diseases) first and the “zebras” second. If you don’t, the attending may request that you do so by saying “Horses, not zebras,” or by the less-conventional technique of whinnying while slapping his or her legs to simulate hoofbeats.

So you should follow what I like to call the “Family Feud” strategy of presentations, based on the game show of the same name. (If you are wondering whether I think all of medical school can be reduced to elements of game shows, the answer is: No, only 80%.) The point of this show was to guess the most popular answers to open-ended questions, with one team trying to list off the top answers to build up points, and the other team waiting for their chance to pounce and steal the points by giving an answer the other team neglected. In a presentation, if you go through the most common possible diagnoses and then get down to the more obscure ones, it’s less likely that someone else on your team will steal the diagnosis you’re waiting to reveal, or that you’ll get interrupted before listing the most important diagnosis. Also, if you’re on a surgical rotation, you should look around after listing off more than two or three potential diagnoses, as your team has probably already moved on to the next patient.

This is not to say that you can ignore the zebras. You try to confirm the presence of a horse before moving on to investigate the presence of a zebra. And you do this mindful of the setting. If you are in feudal-era Japan and you see a shadowy masked figure running around outside, then “Ninja!” is a pretty good theory. By the same token, identical symptoms in a newborn, a teenager, and an adult may prompt very different diagnoses.

As long as you’re not in a true emergency situation, in which you have to try to rule out even uncommon diagnoses if they could cause death rapidly, taking the Family Feud approach to diagnosis has its advantages. Less money is wasted on low-yield tests and evidence-based medicine has a better chance of being followed. When you jump straight to the weird diagnoses, patients get scared, easy fixes get missed and everyone gets confused. Think of the kids in that school who are probably less likely to take a future lockdown seriously. They’ll laugh -- “What is it this time, a pirate in the cafeteria?” And then when ninjas really do attack, they won’t be ready.

   

And if you’re not ready for a ninja, you don’t have a chance.