Post Hoc, Ergo Propter Hoc

We understand very little about the human body. Sometimes I think that the more we learn, the more we learn how much we don’t know. Occasionally, our patients show us. Take Jim*, for example. He came to me complaining of wheezing and shortness of breath. He had been newly diagnosed and treated for asthma over the previous months, requiring multiple Emergency Department visits, oxygen and even antibiotics, all with little relief. He certainly wheezed on my exam, and had elevated eosinophils, the blood cells that are involved in defending us from parasites in the third world and cause allergic inflammation in the first. He worked in Africa and traveled frequently, so both would have to be considered.

I wasn’t sure his wheezing was caused by asthma. I systematically evaluated the possibilities. I sent blood and stool tests to look for allergies, parasites, and autoimmune disease. I did a CT scan of the chest to look for unusual inflammatory lung problems that can present this way.  I even did a bronchoscopy with biopsies of the lung to look for infections, parasites and inflammation.

“Maybe it is asthma after all,” I said to him at the last visit. The allergist saw him and agreed with me: all the tests had ruled out virtually everything else. We stepped up treatment for his asthma.

A few weeks later, at our follow-up appointment, his breathing tests were clearly normal.  I thought the treatment was working.

“Actually, I’m not on any medications,” he said. He had stopped taking his medications, he told me, because he didn’t think they were making much of a difference. Instead, he went to a local homeopath who had asked him if he ate peanuts. He did, three cans a week. He stopped eating peanuts and the problems went away.

The homeopath thought the diagnosis was “aspergillosis.” I didn’t know why Jim had responded to a nut-free diet (his peanut allergy testing had been negative), but my exhaustive work up made at least one thing clear:  this wasn’t aspergillosis.  Aspergillus is a common fungus in the soil and environment that, in some people, can result in an allergic/inflammatory reaction, or, in others, can result in invasive infection. Jim didn’t have either and I had the blood tests, CT scan, and cultures to prove it.

Both Jim and his wife were embarrassed to tell me the details of how the homeopath had come to his diagnosis. But I was even more embarrassed. This incorrect “diagnosis” had done more to help this man in two weeks than dozens of doctors and treatments had done for him over the course of the previous year. And while I resented the fact that the homeopath tried to cloak his treatment in a diagnosis borrowed from Western medicine, lending a false “scientific” credence to his claims, there was something to be learned from the experience. As Richard Dawkins put it: “There is no alternative medicine. There is only medicine that works and medicine that doesn’t work.”

I had thought that the lesson of this story was that interventions can sometimes be beneficial, even if we don’t understand why.  But it turned out that this was a story about how easily we may be misled to believe a treatment works.  Unfortunately, Jim’s symptoms returned a few weeks later despite a nut free diet.  It again seemed that the most likely explanation for his symptoms was asthma with variable allergen exposures, not peanuts.  He started the medicines again.

In medicine, as in life, we are susceptible to the post hoc, ergo propter hoc fallacy:  the logical fallacy that if something immediately preceded an outcome, then it must have caused that outcome.  The truth is there is a lot of variability in our bodies, in what we are exposed to, and in our diseases.  Our health is affected by a whole host of factors – what our genes are, how they are modified while we are in our mother’s uterus and our environment, how they interact with our environment.  It is easy to assume that a patient got better because the treatment worked.  We think, for example, that the infection resolved because of the antibiotics, but it may have been a virus that the body cleared on its own.  Diseases wax and wane, and this vicissitude makes it imperative that our medical interventions be scientifically tested in rigorous clinical trials before we submit to the compelling story about the supplement our neighbor took that seemed to cure all that ailed him.

* The patient name has been changed to protect privacy.

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  1. […] In the end, even with all our advanced medical training, we decided to do what our doctor had recommended. She hadn’t sent us there for procedures. She had referred us for the blood tests, and that is what we did. Fortunately our twins were born at term without incident, which does not necessarily mean we made the right decision. […]



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