Zebras and the Availability Heuristic…..

During a recent shift, I spent the better part of two hours with a patient’s family discussing the unexpected discovery of a large lesion in their child’s brain. The implications of the finding were grave, and the family was devastated. This was the first time I have made such a diagnosis, and by far the hardest news I’ve had to break in my seven years as a pediatrician. It is an experience that I will never forget, and one I’ve yet to fully recover from at this point.

The diagnosis was, as previously stated, fairly unexpected. There were red flags that appropriately led to the ordering of neuroimaging however. This wasn’t based on a hunch or fear of a lawsuit, as happens far too often in the practice of modern medicine. There was enough concern to go on the encephalic expedition, but the findings were still a shock. My emotional response to the situation was intense and, as often happens, it got me thinking.

In medical school, there is an old adage often passed down by more seasoned physicians that serves to reign in an over imaginative differential diagnosis. When one hears the sound of approaching hoofbeats, inexperienced students are frequently cautioned, one should expect to see horses rather than zebras. Horses, which can be and often are very serious, are common. Horses make the most sense in the setting of hoofbeats. Of course, what is considered a horse may change depending on your location. Measles is a horse in some areas of the world, but it is most certainly a zebra in the United States…..for now. That may change in the future if the anti-vaccine crowd gets their way. But the cranium of the poor child in my above anecdote contained a zebra, a roughly 7cm by 6cm zebra nestled in the left hemisphere of her brain.

When a medical practitioners emerges from such an encounter, they stand at a point of divergence. There are two potential paths that can be taken, and the one which is most hardwired in the human brain is, as is frequently the case, the wrong choice. It would be foolish of me to deny the plain and simple fact that in the practice of medicine our prior experiences inform our future choices. For medical students and residents, there is certainly ample opportunity to learn from mistakes made when caring for ill patients. This is a process that should never end, although the opportunities are hopefully fewer with increasing knowledge and experience.

One potential path, the right one in fact, is one of appropriate assimilation of an emotionally significant and uncommon occurrence into one’s practice. It involves the use of the occurrence as a means of teaching others, of emphasizing historical red flags and abnormal physical exam findings that should lead to reasonable evidence-based testing, and of improving communication with patients and families. And it certainly allows for the experience to serve as an impetus for introspection and growth as a physician and a person. The alternate route, unfortunately, makes more sense. And it seems intuitive considering the horrendous impact on the patient and her family. The ultimate destination of this path is the placement of more importance on personal experience than on objective evidence, and it is a dangerous direction to take.

Prior experience impacts future decisions, of this there is no doubt. But this can be taken to an extreme in the practice of medicine in part because of something described by psychologists as the availability heuristic. What is a heuristic? The term may be unfamiliar, but you have experienced the phenomenon on a daily basis. It is, in fact, an unavoidable and often quite beneficial result of the evolution of the human brain. A heuristic is a rule-of-thumb, a seemingly common sense mental technique, based on experience, that allows for more efficient understanding of our environment. But heuristics tend to sacrifice accuracy for the ability to arrive at a solution more quickly. 

The availability heuristic is a specific example that is frequently employed by physicians. It warps our perception of the likelihood of a diagnosis because we tend to place more emphasis on that which can easily be brought to mind. Emotionally charged experiences are typically the ones we recall most readily. For the remainder of my career, whenever I care for a child with a presentation remotely similar to the one mentioned at the beginning of this post, I will see her face. I will see the face of her parents. I will think of the empty feeling I felt in the pit of my stomach when I heard that the radiologist wanted to speak to me. And I will recall the images on the computer screen which revealed the fist sized tumor in this precious child’s cerebrum. My human nature, developed over hundreds of thousands of years of evolution, will demand action. That action will come in the form of more testing and more imaging. Or will it? Armed with an understanding of how good thinking can go bad, and how the human brain is hardwired to make rash judgements, will I ignore that voice in the back of my head calling for unnecessary testing and imaging, with all of the potential risks that they carry with them? I think so, but I am only human after all.

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2 comments so far

  1. Pat on

    Sad story and good post, doc. And it addresses a huge dilemma for doctors, I’m sure. With the availability of so many tests and so many possible diagnoses, it must be hard to stick to the horses and devastating to miss a zebra. And with our litigious society, which came first, the lawsuit or the blanket tests? That is easy to answer. The emotional response which you are experiencing is sure to influence a juror when a doctor is accused of NOT practicing good care by not ordering enough tests. People want simple answers, and remain unconvinced that doctors can’t know everything.

  2. theredstickskeptic on

    Overall the advances in diagnostic testing/imaging are a boon to mankind, but they carry a price both emotional and financial. My personal opinion is that fear of a lawsuit, is actually a minor reason for the overuse of these modalities. I think that the more powerful motivations are the many cognitive biases, such as hyperactive availability heuristics and many many others, that are part of our genetic makeup. That and intellectual laziness.


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