This post is about our article on Hickam's dictum, just published online (open access!) today.
I don't know if I read the 2004 NEJM CPC that mentioned Hickam's Dictum (HD - "A patient can have as many diseases as he damn well pleases") and popularized it, but knowing me, I probably did. My interest in HD piqued over the past 5-10 years because it has been increasingly invoked in complex cases, and whenever this happened, I always thought it was more likely that a unifying diagnosis (according with Ockham's razor) was present, and that the case had not yet been completely solved. So, we set out to investigate HD formally using three lines of evidence that you can read about in the article. We learned much more than we were able to report in the article because of word limitations, so I will report other interesting findings and insights gained along the way here (if you want to skip over the summary info to the "other interesting findings", scroll to the bold "other insights" subheading).
First, a summary of our results. As should be obvious, we confirmed that patients get multiple diagnoses, but case reports alleging to instantiate HD did not document random diagnoses occurring in a patient - there was a pattern to their occurrence. The vast majority of the time, there was a primary diagnosis which explained the patient's chief complaint and acute presentation, as well as one or more of the following:
- An incidentaloma (about 30% of cases)
- A pre-existing, already known condition (about 25% of cases)
- A component of a unifying diagnosis (about 40% of cases)
- A symptomatic, coincident, independent disease, unrelated to the primary diagnosis, necessary to fully explain the acute presentation (about 4% of cases)
As we explain in the discussion, finding an incidentaloma during investigation of the chief complaint represents a spurious coincidence. Finding a new disease superimposed upon a chronic one and being surprised suggests that clinicians anchored to the chronic condition and forgot that new diseases can be superimposed on it; e.g., they failed to recognize that having recurrent CHF does not preclude development of CAP this admission. When authors report a primary disease and its complications, epiphenomena, or underlying cause, they appear to have failed to realize that those are all components of a unifying diagnosis! In the three (3!) cases we categorized as #4, we actually were being generous and honestly these probably represent one of the other categories but we didn't have sufficient information to confidently confirm that.
