I've had that exactly. In fact, I've described it exactly the same way. I ended up that way from trying to push myself to keep doing things for too long.
I was really, really severe for a while, and it got to the point that just the simplest activities were too much and were constantly draining my resources. For me, I needed to cut back even further (which barely even seemed possible at the time), and focus all of my energy on just scooping food into my mouth a couple times a day, etc. Watching TV, brief conversations, lights, ambient sound, reaching out to pat my cat, etc. were all draining out my resources. Pretty much all I was actually equipped to do was lie silently. I know this may sound obvious, but I would STRONGLY suggest that you wife try very hard not to do anything that gets her near that point where her limbs won't work anymore (even if they're trivial things). Over time, she may find that she's able to do progressively more without triggering the problem. But once things get that bad, it seems easy to do damage that takes a long time to recover from if we try to keep going.
For what it's worth, I am MUCH better now than I was then. ...But that whole first year of being bed ridden was pretty awful. And I'm still bed ridden now, just much more capable when I'm here than I used to be (so long as I'm careful). I've also gotten a lot better at recognizing my symptoms as effects of overexertion rather than something constant that I just need to learn to live with. ...Which means I can often prevent them, or at least head things off before it gets really bad.
I'm so sorry for you both that you're going through this. I wish you all the best.