Occam’s razor and Hickam’s dictum 

By Kittane Vishnupriya, MBBS, Johns Hopkins Medicine 

I frequently tell medical students—and remind myself—that it’s always valuable to develop a differential diagnosis for a patient’s presenting problem. I also used to say that doctors should try to “hang their hat” on a particular diagnosis based on arguments for and against each of the differentials and their clinical judgment.  

A patient encounter 

A few years ago, I cared for a pleasant 60-year-old woman with a known history of diabetes, HFpEF, obesity, and GERD. She presented with acute shortness of breath, cough, and subjective fevers. She was dyspneic enough to require BiPAP for respiratory support in the emergency department and was admitted to the ICU. 

An initial contrast-enhanced CT scan of the chest showed a mixed picture of infection versus aspiration in the setting of cardiogenic pulmonary edema. She was treated with IV diuresis and antibiotics. Her troponin rose to 3.29, which was attributed to “demand.” She initially improved but then deteriorated, developing a hypertensive emergency and worsening pulmonary edema that required endotracheal intubation. A repeat troponin was 1.2 during this episode of decompensation. She was treated with additional diuresis, extubated after 24 hours, and transferred to the medicine floor within 48 hours of her initial presentation. 

On the medicine floor, I took over her care and noted that she was still short of breath, especially with exertion. After reviewing the blood work and her ECGs (inferior ST-segment depressions), I believed her presentation was likely a true type 1 NSTEMI and not just “demand.” This could readily explain her shortness of breath and pulmonary edema. I requested a cardiology consultation; the cardiac team agreed and recommended a coronary CTA for further assessment. At our facility, coronary CTA is read in two parts: one by a cardiologist who interprets the coronary anatomy and another by a radiologist who interprets the lung windows. 

Later, I received a call from the radiologist, who reported that the lung windows showed bilateral pulmonary emboli. I was stumped—I had not expected this. The patient had other explanations for her presentation, had just undergone a CT of the chest that showed no PE, and had been on DVT prophylaxis all along. PE was never on my differential. It wasn’t the end of it. I was later called by the cardiologist, who said the patient had severe CAD in the left circumflex and posterior descending artery, and the plaque morphology/anatomy suggested ongoing ischemia. I was flummoxed. This patient had a “quadruple hit”: pneumonia, pulmonary edema, bilateral PE, and a type 1 NSTEMI—all at the same time. 

Occam’s razor vs. Hickam’s dictum 

Each one of these conditions is sufficient to explain the original presentation and should be on the differential diagnosis for any ill patient with acute dyspnea, but all of them at the same time flies in the face of the often-quoted Occam’s razor. One version of it states: “The simplest explanation for a phenomenon is more likely to be accurate than more complicated explanations.” This principle has held well for centuries. 

A counterargument in medicine is Hickam’s dictum, named after Dr. John Hickam: “A patient can have as many diseases as they damn well please.” 

Balancing thoroughness and resource stewardship 

We live in an era with an increased focus on diagnostic errors. There’s a temptation to overthink and overtest, which must be balanced with appropriate resource utilization. This particular patient was indeed unusual. 

Avoiding diagnostic oversight: practical guidelines 

1. Occam’s razor is useful but not infallible. It’s a tool, not a hard and fast rule. 

2. In patient populations with multiple chronic illnesses, be prepared for more than one disease process to occur simultaneously, given baseline complexity, multiple medications and interventions, and the potential for several problems to become overt at once. 

3. Evidence-based clinical pathways for investigating a problem are excellent and should be used. However, be ready to reevaluate assumptions if the patient isn’t progressing along the usual pathway—avoid attributing this to being a “slow responder” to standard treatment. Always consider another disease that might be occurring concurrently. 

4. Spend time at the bedside. Recheck the history and physical exam; revisiting the presentation can clarify the picture and guide additional testing. 

5. Get a fresh pair of eyes on the patient—a colleague is the best resource. 

6. Before ruling out a dangerous problem, remember Hickam’s dictum as another tool, and ask whether another process also fits the picture. 

Adjusting my approach 

In the real world of practicing medicine today, we probably need to land somewhere between Occam’s razor and Hickam’s dictum. I’ve modified my teaching a bit. I now say, “Always develop a differential diagnosis and hang your hat on something, but don’t let go of the hat . . . sometimes you might end up eating it.” I’ve had enough indigestion over the years to prove it. 

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This piece expresses the views solely of the author. It does not necessarily represent the views of any organization, including Johns Hopkins Medicine.