This is going to be controversial.
I'm in ID, which means I went through IM first. During residency it was never a question when an elderly person came in with altered mental status, a positive UA, and a urine culture with something. We never looked up the data. We never cited a paper. It was just one of these core tenets of medicine. AMS was as good as dysuria as a symptom of UTI. It was obvious. Or so I thought.
Let me give context first: there are a couple of flavors of ID doc. Some love HIV, some are big into fungi, etc. I'm into sepsis. It's often claimed by ICU folks -which is totally appropriate- but I believe it should really be owned by ID. That's just me.
Learning about sepsis means learning about the immune system. Indeed, the encephalopathy that happens in sepsis is mediated by a number of things, sometimes with that favorite neurology catch-all term "toxic metabolic encephalopathy." An old mind is more susceptible than a young one. In the absence of major hemodynamic upset (e.g. shock), the encephalopathy of sepsis is mediated by cytokines. It's been well studied in animal models, this thing called "sickness behavior" that causes them to hide away from others and act all sorts of unusual. Older brains, damaged brains, heavily medicated brains…maybe they don't respond to the cytokines in the same way. And why should they? Primitive humans didn't routinely live to ripe old age. As far a Mother Nature knew, your "sickness behavior" was crafted for a sick 20-something, not an 80-year old on several meds from the Beer's list.
All that is to say is this: if your demented old lady doesn't have any other systemic manifestations of an infection -or even an inflammatory response whatsoever- then i don't believe the bacteruria is at fault because I don't think her cytokines are even bumping to the level that her brain realizes that is going on. In my mind, that's like saying that Grandpa Joe cut his finger and now there's some pus there, and suddenly he's getting naked and screaming at the neighbors.
Let me go a step further. Demented old people are capable of feeling pain. You can assess them with GCS all day. There is no reason they can't feel cystitis or urethritis unless you've somehow blocked their ability to feel anything down there. So if they don't have painful urination- if the inflammation from bacteria doesn't even register on the most local of levels- how on Earth is it going to cause a delirium?
tl;dr When encephalopathy co-occurs with infection, it is a consequence of a susceptible brain exposed to a systemic inflammatory response. If the inflammatory response is local, there's not going to be an encephalopathy. Probably one of the 10 other interventions you simultaneously did on admission is what fixed them.
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