Because it's so apt.
People who have fibromyalgia and other chronic illnesses talk about "brain fog" a lot. It isn't a technical medical term. I doubt they even have a term that could cover what brain fog is exactly. But that is a very good description of what it is like. It is like a slow mist settling into the nooks and crannies of your brain and it seeps in and suddenly you can't see, but you can't recall it happening either. Sounds echo and depths cannot be measured.
My fog comes in a few different forms, when it's mild it's a matter of walking into a room and not recalling why, and then doing it three more times. It's not being able to say a name or word that was in your head seconds ago. It's loosing the threads of a conversation or repeating something you just said.
When I have a bad flare up, especially when my fatigue is the main issue, then that fog isn't just a small misty puff but a full-on cloud bank. This last week has been one of those. Most of the time I feel like at the very least I can understand and prepare a mental response even when my body is too tired to actually form the words my brain thinks. Sometimes though I can feel my brain becoming fatigued, and not from some herculean mathematical effort. I can't focus or even process, words in my head don't even make sense. I can't read or write or communicate and my body starts falling back on muscle memory, only often times it's not even current memory. I keep trying to put something down in a place it used to belong, food and dishes in the wrong places, or dropping entirely because I thought there was a counter there. Reaching out for switches that don't exist. Running into furniture because I forget where I am. Forgetting to turn the stove off or shut a door -I have left my car and garage door open all day this way. Not recalling if I took my pills or even if I refilled them. Driving past a place only to turn around and drive past again because I forget where I'm going or why I'm driving. Sometimes I will look in my review mirror and not be able to interpret what I'm seeing, or less scary but equally ridiculous, I will look into my hairbrush and not understand why I can't see my reflection. I have all the anxiety and stress that comes with that constant vague sense of missing something.
-it's my brain!
It is way more pervasive than that though. I've noticed mood and personality changes, I become snappish and emotional and also impulsive. I will make big decisions that I know I'm unprepared for or impulsive buys that I cannot afford, I will act in ways I know I wouldn't normally act. I say things to people I don't mean or will later regret or feel embarrassed about (nothing too scandalous just not me). Relationships are strained because of irrational or antisocial behavior and it's very hard to explain.
This was one of the main stresses when I was working. How do you explain to your boss that the whole last week was not really you. Things you normally understand to keep to yourself you expose to everyone, or burst into uncontrollable sobbing fits because your boss is disappointed you keep making the same stupid spelling mistake in every paragraph of copy he has to revise. Or how you managed step one and two but can't recall step three in a sequence you've done so many times before. How do you explain you forgot what he asked you to do between his office and your desk three times?
I know everyone goes through emotional ups and downs and everyone experiences lack of concentration at times, but this is something entirely different. Your brain doesn't feel like yours and it feels wrapped in cotton and placed on a faraway shelf for long periods of time. I normally consider myself a very rational person, not when this is happening. Brain fog is a very unique and devastating experience and it is not always as simple as just being spacey and forgetful.