When you hear hoofbeats behind you, don't expect to see a zebra.

-- Theodore E. Woodward, MD

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).

At this point, the hardcopy version of Zebra Cards shifts focus to the question: What can Zebra Cards teach us, and why is that material important? The answer has five parts, but rather than include the entire text here, it has been placed in another file. Click a link to see the discussion.

1. Zebra Cards serves as a motivating force.
2. Zebra Cards teaches medical facts.
3. Zebra Cards will sharpen clinical skills.
4. Zebra Cards conveys the boundless subtlety of the human organism.
5. Zebra Cards helps train the mind to remember disjoint facts.

Zebra Cards should be enjoyable to physicians and students at all levels of clinical experience. They were compiled from an urban North American internist's point of view. Pediatric conditions are omitted unless the internist has occasion to encounter them. Although neurology's heritage lies in internal medicine, it has developed into a field so arcane and highly refined that it seems as if everything in that domain is a zebra (15). I have, therefore, deliberately restricted the number of neurologic cards.

Readers with a philosophical or historical interest in zebras may wish to read the extended discussion in the printed version of Zebra Cards. An excerpt of the philosophical discussion is available on-line.

Happy hunting.

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