I think we would need to know more to respond well. Othostatic intolerance is mostly related to your ability to stand and remain standing. Either your blood pressure drops and you go into a faint or pre-faint state, or your heart rate zooms up and stays there (POTS).Is this related to when you can not hold your head up straight? I remember during the beginning of the CFS ME it was very difficult to keep my head up. Like when I'm driving my head is laying next to the window to prop it up. Is this a sign that i'm in PEM?
I'd relate my inability to hold my head up at times more to just the ME and ME weakness and not to my severe orthostatic intollerance. OI more so causes things like the need to lay down.
Your post is confusing as its headed "orthostatic intollerance" but I just realized you are though asking about PEM. These are two different things
PEM can make one become weak and make it harder to hold up head.