Polite Dissent

Anybody who has spent any time hanging around doctors (or reading a lot of medical blogs) will have run across the term zebra. In medical parlance, zebra is slang for a rare and unusual condition. It can have a positive connotation (usually when referring to yourself, as in “I found this incredible zebra when I was treating this patient’s cough.”) or a negative one (which is generally reserved for other people, as in ” She wastes too much time chasing zebras.”)

The term comes from one of the “laws” medical students learn in their first year of two of medical school. Unlike Sutton’s Law¹ and Occam’s Razor², this law doesn’t even have a fancy name. It’s usually referred to as the horse law, or the zebra law or the hoof beatrule.

“If you hear hoof beats, you should look for horses, not zebras.”

This rule reminds clinicians to look for the common causes of symptoms first, not the unusual ones. Sure, a nine-year old could have a thyroid condition that’s causing his sore throat, but it’s phenomenally unlikely. The odds are that he has an infectious sore throat — probably viral, maybe bacterial. A doctor should make sure all common causes are ruled out before spending time chasing rare ones.

So now the next time you hear about a medical zebra, you’ll know what it means.

UPDATE 16 Nov 2004 9:15 PM
I was watching the new medical show “House” on Fox tonight and one of the doctors (Omar Epps, I think) mentioned zebras. You heard it here first!

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¹Sutton’s Law is based on a story about Willy Sutton, a famed bank robber from early in the twentieth century. When asked why he robbed banks, he allegedly replied, “because that’s where the money is.” Therefore when performing an exam or running tests, go where the money is. If it’s a sore throat then look in the throat and check a throat culture, don’t waste time fretting over neurological reflexes or getting a urinalysis.

²Occam’s Razor (at least the medical interpretation of Occam) tells us that if a patient presents with multiple symptoms, the most likely diagnosis is the one which explains them all (or at least most of them).