We asked the neurologist about it, and she assured us it was just part of her ataxia, and that when she woke up her muscles were just spasming because she didn't have control of them yet. That's what we wanted to hear, since we really didn't want her to be having seizures. It seemed to make sense, too, since she could look around and wasn't holding her breath when it was happening, and someone somewhere told us people won't look at you when they're having a seizure (not so sure if that's true anymore, but that's what I heard at the time).
Anyway, now I'm not so sure she wasn't having seizures. I think we've already mentioned the two separate mornings when Becca woke up a complete noodle, just very worn out and moany. She'd mostly recover after half an hour or so, but it would mess up her development somewhat, and we were really worried that those actually were seizures, so we talked to the neurologist again, and ended up getting a 48-hour EEG scheduled.
Fast-forward to now. Becca doesn't do the shaking thing in her sleep anymore. At all. Her ataxia is just as bad as it's ever been (we were playing piano together today and I had to hold her just right or she'd shake at her trunk and lose control of her arms), so I doubt that she's just "better", but I can't get her to shake in her sleep like she used to. I've actually been trying for a few weeks to get a video of it to post, because we can't seem to get a definitive answer on what it is, but she's actually a really deep sleeper now. I just went into her room, flicked on the light (she flinched at that, but that was all), made some really loud noises, and even sat her up in bed, but she wouldn't wake up at all. I finally got her to open her eyes, but there was no startling into shaking like there used to be.
I know there's probably confounding factors. Maybe Keppra makes you sleep more soundly or something, or maybe she's just moved out of that phase. But I really can't help but suspect that those were little seizures she was having. The last time we spoke to the neurologist (Dr. Benedict) she said she wasn't convinced that the episode they recorded wasn't a seizure, and I'd probably say the same thing. Especially since she hasn't had the little drops in development that she used to have.
Anyway, the moral of the story is we don't actually know if Becca's ever had a seizure. We're pretty sure she has, but nothing's been officially observed. And we wonder if maybe those small shaking episodes were seizures as well. The good news is that Rett cases tend to have fewer seizures as they get older (although by older they tend to mean adolescence or beyond, not two and a half), and of course that whatever those shaking spells were, they're gone now -- can't say as we were the biggest fans of them, whether seizures or not.