When you hear hoofbeats behind you, don't expect to see a zebra.
Despite its absence from medicine's formal lexicon, the term zebra is used in the medical community both commonly and with near-universal comprehension (2). On the rare occasions when it does appear in print, understanding of its meaning is usually assumed or is given by reprinting an analogue of the canonical quotation above (3). Thus, lacking a formal definition for the term, I will try to provide one here.
Zebra is frequently equated with a rare disease or condition. This conceptualization is too restricted, however. Consider the following events, reported by a physician who took his daughter to the 1978 Worcester, Massachusetts science fair and carnival (4):
We heard a sudden sound of hoofbeats, followed by screams from the crowd.
Expecting horses, I was surprised to see that two small zebras,
pulling a cart, had gotten away from their handlers and were running amok.
The italics are added to emphasize surprise as the essence of the experience.
"Surprise" is defined as: 1. an unexpected encounter, or 2. a feeling of astonishment
or amazement (5). This leads directly to our definition for zebra: A medical
condition whose appearance at a particular time and place, in a particular person,
is both unexpected and astonishing. The "condition" may be an item from the history,
a physical finding, a laboratory result, or a diagnosis. Casual reflection reveals
that rare diseases do, in general, satisfy this definition (6); slightly more thought
shows that commoner afflictions may also qualify, if they present in an unusual
fashion (for example, card HE-004).