I had no issues at all with NMH (one type of OI) before I got sick in 1990 (post-viral onset).
My initial symptoms were definitely related to NMH - dizziness, nausea, suddenly needing to sit down, and feeling short of breath, especially when climbing a flight stairs. This was a huge contrast to my previous ability to do 45 minutes on the stairmaster at level 8 or 9, hike on steep trails, and so on. I also had heat intolerance.
I don't know if I developed POTS on top of CFS. Or is POTS is part of CFS. Or if it was always POTS and not CFS. Any input is appreciated.
I'm not sure whether anyone knows *for sure* but autonomic symptoms are listed in at least two different ME/CFS criteria, the Canadian Consensus document and the International ME document (sorry, I don't remember their full official names, nor do I have a link right now). But I think the current belief is that orthostatic intolerance is a common in ME/CFS patients, or at least in a sub-group of patients.