Hunting Zebras

In medical school, we learned about rare medical conditions, but we were often chastised, “when you hear hoofbeats, think horses, not zebras”. This means that patients presenting with symptoms of a common condition will usually end up being diagnosed with that common condition rather than a rare one. Therefore, it makes the most sense to focus your diagnostic testing on the most common conditions, rather than wasting medical resources searching for rare conditions. Although this is sensible advice, if you don’t look for rare conditions, you will never find them. In residency, I had a young adult patient with a pheochromocytoma, which is an extremely rare, benign, but hormonally active, adrenal tumor which was causing episodic severe hypertension and tachycardia with chest pain. A pheochromocytoma occurs in 1 out of 50,000 patients. Although I may never encounter another case of pheochromocytoma, I occasionally test for it in my Direct Primary Care (DPC) practice since I can do so for less than $50.

The prevalence of Hypermobile Ehlers-Danlos Syndrome was previously thought to be 1 in 5,000 patients, but now it’s estimated at 1 in 500 patients. It may actually be even more common, since the prevalence in my practice is 1 in 50. Most of the EDS patients I’ve diagnosed have Hypermobile Ehlers-Danlos Syndrome (Type 3), but one of them had Classic EDS (Type 2), which is thought to occur in less than one in a million patients. I don’t consider myself an EDS expert; I simply have the time to perform a thorough history and physical on my patients. In the insurance-based system, I barely had time to perform a cursory exam for billing purposes. Never once, did I perform a thorough head-to-toe musculoskeletal exam and calculate a Beighton score, so it’s no surprise that my first Ehlers-Danlos diagnosis occurred shortly after opening my DPC practice.

Hereditary Hemochromatosis occurs in 1 out of 500 patients, but I have recently diagnosed two patients in a panel half that size in my DPC practice. The cost of testing is just $15, but I would not have been able to justify iron testing for an insured patient without anemia while working in the system. When I was working in the insurance-based system with a panel of over 3500 patients, I didn’t diagnose a single case of Ehlers-Danlos Syndrome or Hereditary Hemochromatosis, which means that, statistically, I missed at least 7 cases of each condition, probably more given that prevalence rates are based only on the diagnosed cases and don’t count all of those patients who are still awaiting diagnosis. Although it’s certainly possible that more medically complex patients seek out DPC, it’s also possible that zebras aren’t as rare as we thought. 

(Editor’s Note: Does anyone else have a zebra story they can share in the comments?)