I thought this was part of MG, do you recognise it?
My other muscles get affected in this fluctuating way too. An example:
11.30 am I walk the stairs at home. Face looks quite normal, and I can even take some milk or eat some peanuts without problems.
2.00 pm I can't even sit up straight, let alone stand or walk. My eyes are both closed, and communication is almost impossible due to not being able to talk, write or even type a message. Breathing is so difficult I just lay there.
(It's not only when I've done something to tire the muscles)

The test I was talking about is indeed a spirometer. No idea what atelcatasis is, going to google this. My ladt x-ray has been a few years, guess it was 2011.
My breathing isn't stable, I mean I can't do much mostly due to these problems. Weak face, hands, legs etc I can manage because I have so many things like a wheelchair and stuff. The breathing has influence on a whole other level, if the breathing isn't right, nothing is.
Problem is, this isn't all the time.
Anyway, thanks to you guys I have an appointment soon for a bunch of tests (feb 25 or 26 top of my head)