Occum’s razor and Hickam’s dictum: Strange new diseases or something much more interesting?

I have an unusual fascination with the concept of medicine as both an art and a science. Superficially, the two disciplines have little to do with each other – Where medicine is sterile, factual, statistical, and specific, art is paradoxically relaxed, fluid, and imaginative. The role for creativity in the mind of a doctor is limited. We are thinkers and memorizers of minutia. Artists are thoughtful and philosophical. The proverbial war of the left versus right-brained prevails.

The ancient Greek physician Hippocrates is credited with being the first to truly separate medical science from religion and philosophy, based on his belief that disease was not a punishment inflicted by the gods but rather the product of environmental factors, diet, and living habits. For this, he is touted as being the “father of medicine.”

Fast-forward a thousand years, and we meet a lesser known, but equally influential ancient physician, named Claudius Galenus. Galenus studied Hippocrates’s understanding of pathology, but was also heavily influenced by the writings of the great Greek and Roman thinkers like Plato and Aristotle. He recognized medicine as an interdisciplinary field that was best practiced by utilizing theory, observation, and experimentation in conjunction.

One of Galenus’ most famous writings is the treaty entitled “That the best physician is also a philosopher”, which called for a melding of rationalist and empiric medicine sects. Galenus believed the best doctors were the best because they really thought about what they did.

My obsession with medical philosophy introduced me to the interesting and opposing theories called Occam’s razor and Hickam’s dictum. No, these strange-sounding terms do not refer to unusual diseases, but rather to two important, yet contradictory fundamental philosophical ideals of medicine, with specifics regarding the approach to the diagnosis of disease.

Occam’s razor refers to the idea of “diagnostic parsimony”, which means when diagnosing a given injury, ailment, illness, or disease, a doctor should strive to look for the fewest possible causes that will account for all the symptoms.

Hickam’s dictum counterbalances Occam’s razor by stating more often, it’s statistically more likely that a patient has several common diseases, rather than having a single, rare disease that explains a myriad of symptoms.

In veterinary school, we learn when that we hear hoofbeats, we should look for horses, not zebras. The adage refers to more than just the sounds made by the limbs of our patients. Essentially, we’re trained to think more like Occam, and base a diagnosis on the idea that “common things happen commonly”.

An unvaccinated puppy with vomiting, bloody diarrhea, and a fever likely has parvovirus. An older cat with vomiting, weight loss, a rough hair coat, and a lump on its throat likely has hyperthyroidism. In other words, keep it simple, explain things in the most uncomplicated way, and you will have your answer.

There are times, as a veterinary oncologist, I find Occam’s razor to be a bit cumbersome. Don’t get me wrong – I’m a huge fan of giving my patients as few problems as possible and I’m happy to leave Hickam’s mentality to my internist friends who love to come up with dozens of differential diagnoses for each and every one of a patient’s clinical signs. But I can’t ignore the innumerable instances where I’ve seen Occam’s line of thinking not only to be wrong, but also downright dangerous for my patients.

Perhaps the worst “Occam offense” occurs when a veterinarian concludes a pet’s most likely diagnosis is cancer based solely on demographic characteristics such as their age or breed, in lieu of performing diagnostics designed to verify their suspicion.

I understand owners are reluctant to spend money on expensive tests when the outlook for their beloved companion may be poor, and vets are often expected to offer their professional opinion as to a diagnosis prior to having the evidence they need to be sure. This doesn’t excuse a doctor from their responsibility for knowing and stating their limitations and their obligation to divulge an inability to predict an outcome off of limited information.

Occam’s razor also fails owners when we “blame” cancer, or anti-cancer treatments, for a pet’s adverse signs, behavior changes, or lab work alterations that actually result from a completely separate illness or condition. Cancer most commonly strikes older to geriatric pets, a population of animals predisposed to a wide variety of chronic, progressive diseases.

Sometimes a cat with lymphoma isn’t eating well because they have worsening kidney disease unrelated to their cancer. Sometimes dogs have diarrhea because they’re fed a remarkable amount of table scraps and it has nothing to do with their chemotherapy.

I don’t think it’s an end all be all West Side Story type of analogy where a veterinarian must choose whether they’re an Occam or a Hickam. That’s an oversimplification of the point I’m trying to make.

The more important issue is how we need to open up some of the tunnel vision that blinds us to the bigger picture of what’s really going on with our patients. We’re obligated to treat the whole animal, and their entire family, with equal parts respect and knowledge. Occum’s razor isn’t an excuse for laziness or ineptitude.

If we sway too far towards diagnostic parsimony, we could easily miss important signs of other diseases or conditions that are unrelated to our primary concern. Likewise, it’s possible to knit pick each and every detail of a pet’s case to the point where we are blinded by the possibilities of their pathology.

Finally, pet owners should keep an open mind when it comes to their veterinarian’s recommendations for further testing when a definitive diagnosis has not yet been achieved. Opinions and experience account for a great deal of the art of medicine, but there’s also tremendous amount of data garnered from the plain old science part of the discipline as well.

Maybe the hoofbeats are zebras more often than we think.

Maybe I sound a bit too philosophical for a doctor.

Maybe they said the same thing about Galenus…







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