Rare diseases

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

Over the last two weeks in Dr. HPK’s clinic, I have seen many rare diseases, ranging from dermatomyositis, tendinous xanthomas, to bacterial endocarditis. This is my first time seeing many of these diseases and I am in awe of Dr. HPK’s ability to so confidently make the diagnosis. I have found myself wondering: how much time should be devoted to teaching rare diseases in the medical curriculum? How does one gain the skills necessary to make the diagnosis of a rare disease? And how many cases of a rare disease must we see to confidently diagnose? 

As I did some reading in the area, I was surprised to see that novices are much more likely to make the diagnosis of a rare disease. They are also much more likely to be wrong. This is based on fundamental principles of cognitive biases known as the “availability heuristic,” that the most easily remembered diseases are also the most probable.  This, combined with the fact that rare diseases tend to be the most easily recalled by novices (why else would they be called fascinomas?) suggests that novices are predisposed to making the diagnosis of a rare disease. Having seen a case of dermatomyositis in the clinic, I hope that I will not attribute every case of proximal muscle weakness to this diagnosis. 

Rare diseases should really only be taught fleetingly in medical schools. Medical schools should allot an amount of time to teaching a specific disease that is directly proportional to the prevalence of that disease.

  

Experts can accurately diagnose rare diseases because they are better able to weigh probabilities and are much more likely to have seen all of the uncommon presentations of common diseases. They use pattern recognition much more than the novice clinician, and are therefore usually able to accurately assign a diagnosis. In this way, experience is the best teacher of rare disease. Dr. HPK explained to us that you only need to see one single case of a disease to diagnose it correctly. I think what he meant was that you only need to see a single case to think of the disease – diagnosing is much more difficult.

-Carolyn