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@ChrisArmstrong, I think you might find that many of your trial participants do have orthostatic intolerance (including POTS). My son and I do (my daughter is borderline). And it seems on PR that possibly even the majority do.
Chris O'Callaghan in Melbourne believes hypermobility is linked to/the cause of orthostatic intolerance - due to the stretchiness of blood vessels not providing enough resistance to ensure blood gets to the brain when a person moves from supine to standing.
It could fit with your hypothesis. I've always had a bit of a tendency to dizziness upon standing and occasionally fainting. But the full-on orthostatic intolerance didn't develop until the ME kicked in.
But Gijs is right - there does seem to be a bit of a story around autoantibodies and orthostatic intolerance.
BTW - the frequency of hypermobility in ME/CFS patients isn't pinned down all that well I think. Some people very knowledgeable about ME actually dispute that it is more common in People With ME. Do you have a percentage for how many of your trial participants are hypermobile vs the general population?
Checking how many have orthostatic intolerance wouldn't take long and could be very interesting. Chris O'Callaghan might be an useful person to chat to given you are in Melbourne too I think. (Not that I agree with all that he says - he thought ME is really just orthostatic intolerance and the answer is GET when I last saw him. But he may be open to new ideas.)