The Hickam's Dictum

"Patients can have as many diseases as they damn well please"
(John Hickam, MD, Duke University, circa 1950)

The Zebra Aphorism has lead the medical community to adopt the Hickam's Dictum, the thinking that it is statistically more likely that a patient has several common diseases rather than a single rare disease. Furthermore, this thinking emphasizes that even when statistically unlikely, patients can prove to have multiple diseases. 

Why are so many rare diseases not well-known by the average medical professional?

  1. For some, it was forgotten a long time ago. Others skipped the chapters teaching the less common conditions in medical school and got enough of the other questions right to pass their exams.
  2. The Zebra Aphorism gets overused.
  3. No definitive pharmacological or surgical treatment exists. Many health care professionals resort to the concept of "Why look for it if I can't do anything about it?"
  4. There is a lack of general awareness and understanding for most of these conditions

Each rare disease community is represented by a color. For example, the EDS community is represented by the Turquoise Zebra because the awareness color for EDS is turquoise. The rainbow zebra represents all identified rare conditions.

The Zebra Aphorism

"When you hear hoofbeats, think horses, not zebras."
(Theodore E. Woodward, MD, University of Maryland,1950)

Earlier Versions of the Zebra Aphorism include:                          
"When you hear hoofbeats behind you, don't expect to see a zebra."                  
"Don't look for zebras on Greene Street."                    

Overuse of the Zebra Aphorism has led to unwarranted skepticism regarding acceptance of a legitimate rare diagnosis. The Zebra has become a symbol for rare conditions, leading several rare disease communities to adopt the Zebra as their official mascots. The Zebra Aphorism is meant for medical novices, but has become adopted by fully trained health care professionals.

Copyright 2010-2023. EDSers United. All Rights Reserved.

Bottom Line: Diagnosticians Must

Recognize Diseases/Disorders When Present!

"In making the diagnosis of the cause of illness in an individual case, calculations of probability have no meaning. The pertinent question is whether the disease is present or not. Whether it is rare or common does not change the odds in a single patient. If the diagnosis can be made on the basis of specific criteria, then these criteria are either fulfilled or not fulfilled."

(A. McGhee Harvey, James Bordley II, Jeremiah Barondness)

While the rainbow zebra represents all 7000 rare conditions that have currently been identified. There are several rare diseases that have yet to be identified. Those who are undiagnosed or have conditions that have not yet been named are represented by the black and white zebra. 

EDSers United



Rare Disease Awareness 

Because the zebra represents ALL rare conditions, it is important to understand the variety of ways rare disease groups differentiate themselves from each other, while embracing their zebra pride.