Well, when it's...not a stroke, Eisenstein.
One of the interesting things about the human brain and body is that, when you've had something go very wrong indeed in your head, that wrong thing can be reactivated by things going wrong in your body. It can even be made worse.
Let's take a typical case: your average Little Old Lady In No Acute Distress (LOLNAD) who had a stroke, say, ten years ago, that left her with a little residual left-sided weakness. Other than having to use a cane to keep from tipping over unexpectedly, she's dandy. She lives in one of those poncy-schwantzy apartments with a day nurse who comes in to help give her meds and an aide that stays overnight.
One morning, the aide notices that in addition to being more weak on the left side, Granny LOL is confused and can't talk very well. She's having difficulty following commands and seems really sleepy and irritable. The aide, being a bright person, calls 911.
The ambulance arrives and whisks Granny LOL to the nearest emergency department, where she has a thorough stroke workup: chemistries, urinalysis, a CT scan of her head to rule out a bleed, and a general exam. Everything's negative....except the urine. It's hot. She's got a UTI.
So she's given a dose of antibiotics and sent over to me at Sunnydale, where she gets an MRI to look for signs of new occlusive stroke. She also gets all the other tests and chicken-waving that are standard for our patients. At the end of the day, we discover...
...No stroke. Thank God. Just a UTI that's been allowed to grow (because most people can brew one without symptoms, and older folks are hit hard by them).
Why does this happen? Honestly, I don't know the technical explanation. I do know that in probably half the female patients I get who are over the age of 70, and about a third of the male patients that age, there's a urinary tract infection that's present. It's recognized that even a supposedly "asymptomatic" UTI can cause the sudden onset of confusion or a worsening of dementia. That same infection can cause what we call a reactivation of old stroke symptoms as well--sometimes to the point that the person's worse off than when they had the original stroke.
Moral of the story: If you have an elderly patient (or, for that matter, any patient) who suddenly manifests a worsening level of consciousness, do a UA while you're doing all the other stuff to rule out nastier things. It's a fair chance your person will have a positive dipstick, and antibiotics will vastly improve things.