It’s a phrase that all doctors hear during their medical training: “If you hear hoofbeats, think ‘horses,’ not ‘zebras.'” We are taught that the most common diagnosis is the most likely diagnosis. But it’s not always correct. There’s something of a thrill for a doctor who makes a rare diagnosis—we love hunting zebras. We get excited over rare diseases—yes, even if it means a lifetime of suffering or even death to the patient we care for.
But we’re not bad people; this excitement is purely academic. And personally, I feel a slight twinge of guilt every time I make an interesting diagnosis that has bad implications for my patient. We don’t wish these diseases on anyone, but we do enjoy looking for them. And you should be glad. If doctors don’t take pleasure in the search for uncommon diagnoses, they miss things. Without that intellectual curiosity, they stop paying attention to the important details of your story—the details that can provide clues that a headache is a symptom of brain cancer, or that your child’s abdominal pain could be due to diabetes.
From the minute we knock on the door, our minds are going in two directions: #1: What’s the most common explanation for these symptoms?, and #2: What other things could be causing these symptoms that I can’t afford to miss? We’re thinking of things like cancer, meningitis, or autism—things you don’t want your child to have (and we don’t either), but things that we will miss if we’re not watching.
With experience, doctors develop a sense of familiarity with the stories patients tell. More often than not, we could recite your story before we walk in the room. A 4-year-old with a few days of nasal congestion followed by a fever makes us think “ear infection.” A teenager with declining academic performance and social isolation screams “depression.” And severe abdominal pain with fever and vomiting sounds a lot like “appendicitis.” But every once in a while, the story doesn’t fit. There’s a new twist that we haven’t heard before, or that reminds us of a totally different story.
Some doctors lose this vigilance after years in practice, and some never develop it at all. One of my most memorable hospital admissions was a 7-year-old boy with asthma that I admitted for an asthma exacerbation. His mom had picked him up from school because he was having trouble breathing. She took him to his pediatrician, who gave him a breathing treatment that helped. He went home and got worse again, and his mother took him to an emergency room. They gave him a couple more treatments, which seemed to help, but he quickly worsened again, so they decided to admit him.
When he finally made it to the hospital, I sat down and talked to him (he had come by ambulance, and his mother wasn’t there yet). He told me that his symptoms had started at school, after a class party that involved eating different types of bread from around the world. He had thrown up twice, and his skin was itchy. He felt a little better after his mother gave him Benadryl. I asked him what he thought caused his symptoms, and he told me “I think it was the bread with the nuts.”
He had been suffering not from an asthma exacerbation, but from a life-threatening allergic reaction. I gave him epinephrine, steroids, and more Benadryl and watched him overnight. He got better and went home the next morning.
Anaphylaxis isn’t all that uncommon, but it’s also not the first thing you think when a kid with asthma has trouble breathing. Among other even less common zebras, I’ve seen an abdominal tumor disregarded for a month as constipation, autism that was blown off as ADHD and speech delay, abdominal pain that was a result of heart failure, and child abuse masquerading as any number of symptoms. What did these cases have in common? There was something about the story that just didn’t fit. They had stripes. And when a horse has stripes, we should think “zebra.”
So if your pediatrician is a zebra hunter, be grateful. You don’t want to know what he’s thinking. Indulge him with answers to seemingly unrelated questions, and try not to let the questions worry you. The most common diagnosis is the most likely diagnosis. But if your child has a zebra, he’ll find it.