What do we actually know about orthostatic intolerance?

Your pattern could still relate to OI if you're doing a lot of orthostatically challenging things during the day. I'm curious, do you have prominent OI? As in, do you get dizzy spells/times when you nearly faint or actually faint? No need to answer if you don't want to.

There are certainly people with ME/CFS who can do orthostatic feats I couldn't dream of eg sitting upright in a wheelchair for hours and hours...and talking for significant portions of that time. They'll feel crap afterwards but I'd have fainted within the first half hour.

 

A few years back I tried to distinguish between my POTS and my OI, but I just go with the flow these days.

I was tested for POTS day and night - about once an hour or so in the day at least - for four days. Abnormal HR and BP within 10 minutes of standing plus tachycardia. But I don't know if I really feel that. Can't say.

OI I can. Standing bothers me. I have never fainted, but it gets harder to stand the longer I'm up. I prefer to lean on stuff or use a support. The longer I'm upright - including sitting in a chair, the worse I slide. This is both cognitively and physically/strength-wise. The cognitive decline is disconcerting. I struggle with segues when talking or writing, I mix up prepositions, my inferences are dangerously off at times, etc - and I used to write for a living. These daily burgeoning deficits I ascribe to OI - rightly or wrongly.

As for dizziness...No. But bad balance problems. I tilt more as the day ages. It's like gravity pulls me more to the left, like I am starting to fall (a gentle pull of a falling sensation), but I usually don't fall. The NIH said I have profound bilateral vestibular shit, and I'm sure these contributes to gait issues etc, but I still embrace OI as a primary culprit.

 

"I have the same issue. Standing, sitting and slow walking are a problem. Fast walking is fine, which points to cerebral perfusion"

Ditto

Postural swaying with low gait speed and standing still. I feel much better when I start moving quickly.

 

That all sounds horrible to deal with. I think the confusion is that you and I are using the term OI differently. I use OI as an umbrella/parent term, with POTS, postural hypotension, neurally-mediated hypotension etc as types of OI.

My difficulty is different from yours on testing - takes longer to make me hypotensive (at least when tested in the afternoon), but evenutally my BP drops spectacularly and reproduces the symptoms I experience during near-faints at home exactly. The people who have tested me are only concerned with whether I'm actually at risk of passing out and falling and hitting my head. But my concern is why do we feel so rotten before that, ie for me, pretty much all the time when upright? Why do I feel hideous for the rest of the day if I "push through" on orthostatically challenging activities? Why does @NelliePledge feel awful when upright even though she's hypertensive (notwithstanding that someone with high blood pressure could still have episodes of rapid drops in blood pressure)?

I think the questions @Jonathan Edwards asks are pertinent, such as whether that the difficulties people with ME/CFS have being upright are really about blood pressure plus or minus heart rate or something else entirely. (Hope I haven't misrepresented your questions, JE!) We're getting signals to lie down, perhaps in the same way that we seem to get signals to stop squeezing the dynamometer in the NIH study. Why? Or more interestingly, how?