I haven't read all the other replies yet but wanted to answer this while I was thinking about it.
I have experimented with standing still in spite of these warnings and the urge to sit down becomes almost unbearable. Still, it has never led to me blacking out, as you might expect if it were truly low blood pressure. The heart rate does goes up, but perhaps only because the body is signalling an emergency situation
That sounds very familiar. I remember when I first got sick and the doctors were asking me about why I needed to sit down. And I tried to explain to them that feeling of "I HAVE TO SIT DOWN NOW."
This feeling is different from weakness or fatigue but I didn't have the right words for it. After I had the tilt table test I realized that A LOT of my symptoms - nausea, dizziness, lightheadedness - were pre-syncope (pre-fainting) symptoms. And several other symptoms - muscle twitches (fasciculations), muscle pain, feeling short of breath, increased heart rate (but not enough for POTS diagnosis when I first got ill) - were reproduced during the tilt table test. So I think they must also be related in some way (maybe indirectly?).
These symptoms came one by one the longer I had to stand on the tilt table test. But until I did this test I had no idea that any of these symptoms could be related to dropping blood pressure.
@Forbin, have you ever had a tilt table test? I ask because I have never fainted in day to day activities but I passed out during both my tilt table tests. On the first test I was fidgeting, moving my feet around without realizing it, which I guess was keeping my blood pressure from dropping. The person running the test had to ask me to stop moving. Within a few seconds I passed out.
Apparently there are many ME/CFS patients who do have this blood pressure drop but never faint in normal life. From the
Johns Hopkins patient handout on Orthostatic Intolerance:
While fainting has been considered a classic symptom of NMH, we have found that many people who develop NMH during tilt table testing do not faint in day-to-day life.
Back to the original question by
@Peter Trewhitt
Does any of the published literature on orthostatic intolerance or POTs look at it as a continuum rather than an all or nothing phenomenon? Is there such a thing as mild orthostatic intolerance?
I'm pretty sure that most doctors see POTS, and other types of Orthostatic Intolerance, as having a range from mild to severe. I posted a link to a short POTS overview video by Dysautonomia International (see
https://www.s4me.info/index.php?threads/what-is-pots-video-by-dysautonomia-international.1070/ ) and it specifically mentions a range of severity (at about 2:10 in the video).
Hope this is helpful.
EDIT: I should mention why I've had two tilt table tests.
I had my first tilt table test in 1995 when trying to get a diagnosis and treatment (I first got sick in 1990 at age 29).
The second test was done for the long term disability insurance in 2003. Same results, just a slightly different length of time (20 minutes on the 1995 test, about 30 minutes on the 2003 test) before a huge plummet in my BP and I passed out.