TIL in Medical School - Doctors have myths too.

He says that myth is often taught resident-to-student, but it isn’t actually backed up by any research.

Lorber B, Swenson RM. Bacteriology of aspiration pneumonia. A prospective study of community- and hospital-acquired cases Ann Intern Med. 1974 Sep;81(3):329-31.

Brook I, Finegold SM. Bacteriology of Aspiration Pneumonia In Children, Pediatrics. 1980 Jun;65(6):1115-20.

Finegold SM. Aspiration Pneumonia Rev Infect Dis. 1991 Jul-Aug;13 Suppl 9:S737-42.

Bartlett JG. Anaerobic bacterial infections of the lung and pleural space Clin Infect Dis. 1993;16 Suppl 4:S248.

Yamashita Y et al. Anaerobic respiratory infection--evaluation of methods of obtaining specimens. Kansenshogaku Zasshi. 1994;68(5):631.

El-Solh AA et al. Microbiology of severe aspiration pneumonia in institutionalized elderly. Am J Respir Crit Care Med. 2003 Jun 15;167(12):1650-4. Epub 2003 Apr 10.

The kicker - if the doctor would stop to think about it, it should jump out as unintuitive – it would take some serious changes inside the lung to make an anaerobic infection – an infection of bacteria that thrive in areas with no oxygen. In reality it takes frequent aspirations over a long period of time to block off an area of the lungs.

The normal flora of the skin, mouth, and upper respiratory tract contains anaerobic bacteria; this alone should be sufficient to disprove overly simplistic "But there's oxygen, you can't have anaerobic bacteria there!" theories. Anaerobes are actually more common than aerobes in the mucus membranes of the mouth and URT and in fact outnumber aerobes in saliva about 10:1. According to The Anaerobic Microflora of the Human Body:

It is not surprising that anaerobes are present in large numbers in the flora of the intestinal and geintourinary tracts because oxygen concentrations are low in these regions. However, it appears unusual that they are also prominent members of the flora of the skin, mouth, nose, and throat - regions that are continuously exposed to air. The presence of anaerobes in these areas is explained first by the activity of the aerobic and facultatively anaerobic components of the flora that reside in association with the anaerobes and consume oxygen in their metabolism and second, by the colonization of anaerobes of microhabitats protected from air.

I think the moral of this story all people, be they doctors or kindergarteners, don’t usually check facts they’re taught, especially when being taught by an authoritative teacher.

Okay, here's what actually happened. In the 1970s and 1980s, a lot of people worked really hard studying the microbiology of aspiration pneumonia and all of them found lots of anaerobes. In the late 1990s, some other people, especially a guy named Paul Marik, tried the same thing using more modern techniques and found very few anaerobes. They concluded that the old studies had been wrong.

Some other people objected that anaerobes are really hard to detect and that maybe Marik and his supporters had just screwed up and not been able to find them even though they were there. This seems to be the view of Rene et al, who claimed to have repeated Marik's experiments using better technique and found lots of anaerobes just like the old theories would have predicted. It was then counterclaimed that Marik's experiment had been unusually rigorous and well-conducted, plus it was also easy to screw up the other way - that sometimes samples might have been contaminated by anaerobes in the upper respiratory tract that weren't responsible for the pneumonia at all. Everyone had a nice big fight about it which as far as I know still has not been fully resolved. UpToDate, which I tend to trust on this sort of thing, pushes the pro-anaerobe line, but emedicine, which is also pretty good, pushes the anti-anaerobe line. I do get the feeling the anti-anaerobe people now have the upper hand.

The rationalist thing to do would be to let the microbiologists fight it out among themselves and just study which antibiotics are most effective against aspiration pneumonia. The answer to that is very complicated, but the oversimplified answer as given by UpToDate is clindamycin, an antibiotic known for its efficacy against anaerobes, which seems to maybe suggest there was some kind of anaerobic component after all - but I am reading between the lines here on a subject I'm not really qualified to read-between-lines on and am probably completely wrong about this.

I agree that many doctors don't know anything about this (I hadn't heard of it until you brought it up and I checked the literature). The average doctor just checks every so often to see what antibiotic is recommended for aspiration pneumonia and then prescribes that one. As far as I know the antibiotic recommendations are still correct. This seems like a pretty efficient system, given how many things doctors have to know.

Anyway, as far as I can tell the real moral of this story is that medicine is really really hard and complicated and, like all science, often changes as technology improves and better experiments become possible. This is a less fun narrative than "Doctors are incredibly stupid and just by knowing about this one study I can totally outdo all of them" (YES, EVERYONE ON LESS WRONG, I AM TALKING ABOUT YOU) but fun narratives are wrong suspiciously often and this one is no exception.

Doctors have epistemic hygiene just as terrible as the general populace. People sometimes point out that they have incentives such as legal liability to follow AMA/FDA best practices even when there is evidence that this is wrong. That's true, but many many doctors believe in AMA/FDA best practices.

Even if you discover or derive the fact yourself you can still get it wrong. Truely knowing stuff is hard.

What does actually happen to people who get food or water in their lungs, then get very sick or die, and are diagnosed with aspiration pneumonia?

What, you didn't think doctors had medical myths too? Although Yvain seems to have voided your particular example, it should be pointed out that there are a ridiculous number of doctors in any first-world country, and based on what we know about the sanity waterline it seems absurd to assume anything other than, "Most of them are probably rather irrational."

Now, you frequently don’t have enough time to “learn it the hard way” or derive an answer yourself. If I had to read every single research publication that populated the facts in my textbooks, I might not ever graduate.

Reading publications is the easy part. To really increase your degree of belief, you would have to repeat all the crucial experiments yourself. Including the experiments validating the instruments you use.

many doctors believe that aspiration (having stuff go down into the lungs) causes anaerobic pneumonia

The NIH seems to believe that, too:

Aspiration pneumonia is inflammation of the lungs and airways to the lungs (bronchial tubes) from breathing in foreign material.