Occam's razor and Hickam's dictum are both helpful tools when engaging in diagnostic reasoning.
Lifelong Learning in Clinical Excellence | October 14, 2019 | 4 min read
By Kittane "Vishnu" Vishnupriya, MBBS, Johns Hopkins Medicine
I frequently say to medical students and myself that it is always great to have a differential diagnoses 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 one of the differentials and their clinical judgement. This helps sharpen our diagnostic acumen over time.
I recently took care of a pleasant 60-year-old woman. She had a known history of diabetes, HFpEF, obesity, and GERD, who 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 admitted to the ICU. She had a CT scan of chest with contrast initially which showed a mixed picture of infectious process versus aspiration in setting of cardiogenic pulmonary edema. She was admitted to the ICU and placed on IV diuresis and antibiotics. She had a troponin bump of 3.29, which was thought to be due to “demand.” She initially improved, but then worsened again with hypertensive emergency and worsening pulmonary edema requiring endotracheal intubation. A repeat troponin was 1.2 during the episode of decompensation. She was subsequently treated with additional diuresis and extubated after 24-hours. She was transferred to the medicine floor within 48-hours of initial presentation.
There, I began to care for her and observed that she was still short of breath, especially with exertion. After review of the blood work and her ECGs (inferior ST segment depressions), I thought that her presentation was likely a true Type 1 NSTEMI and not just “demand.” It easily explained the shortness of breath and the pulmonary edema. I requested a cardiology consultation and the cardiac team agreed and recommended a coronary CTA to further assess. The coronary CTA was requested and at our facility it is read in two parts—one part by a cardiologist who reads the coronary anatomy, and the second part by a radiologist who reads the lung windows.
I got a call later from the radiologist who said that the lung windows showed bilateral pulmonary emboli! I was stumped as I had not expected this—the patient had other “explanations” for the problem and she had just undergone a CT chest, which showed no PE and had been on DVT prophylaxis all along! PE was never on my differential. It was not the end of things. . . . I later was called by the cardiologist, who said that the patient had severe CAD in the left circumflex and posterior descending artery and the nature of the plaque/anatomy suggested ongoing ischemia! I was flummoxed. This patient had a “quadruple hit”! A pneumonia, pulmonary edema, bilateral PE, and a type I NSTEMI all at the same time!
Each one of them is sufficient to explain the original presentation and should be on the differential diagnosis of any ill patient with acute dyspnea, but all 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 some phenomenon is more likely to be accurate than more complicated explanations.” This has held well for several centuries.
A counter argument in medicine is Hickam’s dictum, named after Dr. John Hickam: “a patient can have as many diseases as they damn well please.”
We live in an era with increased focus on diagnostic errors. There is a temptation to overthink and overtest. This has to be balanced with appropriate resource utilization and “choosing wisely.” This particular patient was indeed unusual.
We are more likely to choose single disease process causing multiple issues in a patient, rather than multiple diseases all acting at the same time to explain the same problem. So, how do we minimize this potentially harmful oversight? After reflecting a lot on the matter, I think of the following as guidelines:
1.Occam’s razor is useful but not infallible. It is a tool, not a dictum.
2.In patient populations with multiple chronic illnesses, one should expect more than one disease process at the same time given the complexity of the baseline, multiple medications, interventions, and many problems becoming overt at the same time.
3.Evidence-based and established clinical pathways about investigating a problem are excellent and should always be used. However, one should be ready to recheck assumptions if the patient is not progressing along the usual pathway thinking that he/she is a “slow responder” to standard treatment. Always consider another disease that might be occurring at the same time.
4.Spending time at the bedside, rechecking the history & physical exam, the presentation, can help clear the picture and guide additional testing.
5.Get a fresh pair of eyes to look at the patient—a colleague is the best the resource.
6.Before ruling out a dangerous problem, think of Hickam’s dictum as just another tool and see if there is another process that fits the picture.
In the real world of practicing medicine today, we all probably need to come down somewhere between Occam’s razor and Hickam’s dictum. I have now modified my teaching a little bit. I now say, “always do a differential diagnosis and do hang your hat onto something, but don’t let go of the hat . . . sometimes you might end up eating it.” I have had enough indigestion over the years to prove it.