"Cerebral blood flow is reduced in ME/CFS during head-up tilt testing even in the absence of hypotension or tachycardia... van Campen et al, 2020

Moreover, the clinical importance of this observation is reflected in the strong association identified between the number of orthostatic symptoms reported during head-up tilt test and the degree of cerebral blood flow reduction.
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Although they were able to find a relationship between orthostatic intolerance symptoms and a reduction in cerebral blood flow, there was still a lot of variation:
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we would caution that clinical symptoms alone may be insufficient to diagnose orthostatic intolerance, as some individuals who did not endorse orthostatic symptoms before the study nonetheless had substantial reductions in cerebral blood flow. This is evident from Figure 3 in which patients, denying orthostatic intolerance symptoms in daily life, still had a mean cerebral blood flow reduction of 12% (reduction range between 1 and 46%).
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The exact cause of cerebral blood flow reduction remains to be determined, but may involve reductions in cardiac output and the presence of hypocapnia. Similarly, the pathophysiology of orthostatic intolerance symptoms (possibly related to increased catecholamines, metabolic changes or inflammatory changes) needs to be addressed in future studies.
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No talk of deconditioning/similar in the paper that I recall
 
This is just spelling out the corollary of the findings:
Fourth, using the lower limit of normal in cerebral blood flow reduction, 90% ME/CFS (95% CI 87-92%) patients showed a more than 13% reduction. The findings confirm and extend previous reports of a high prevalence of orthostatic intolerance in the ME/CFS population. Our results show that 98% of delayed orthostatic hypotension patients and 100% of POTS patients had a more than 13% cerebral blood flow reduction, inferring that in those with documented delayed orthostatic hypotension and POTS, clinically important reductions in cerebral blood flow could be assumed to be present. On the other hand, the largest group of ME/CFS patients were those with a normal heart rate/blood pressure response. Of this group, 82% had an abnormal cerebral blood flow decrease as defined by the lower limits of normal. These patients would have been misclassified as having normal hemodynamics using just heart rate and blood pressure changes during head-up tilt test. Our data suggest that for a more comprehensive categorization of the circulatory dysfunction in ME/CFS patients cerebral blood flow measurements should be included.
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Limitations
We acknowledge that referral bias by the general practitioner may have played a role, selectively referring patients with orthostatic symptoms. Conversely, our study did not enroll those who were bedbound, and we elected not to expose those with more severe functional impairments to tilt testing.
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Figure 3: 95% confidence intervals of the %decrease in cerebral blood flow of healthy controls and different ME/CFS patient groups at end tilt. The %decrease of all ME/CFS patient groups and subgroups was significantly larger than that of healthy controls. Abbreviations: Atyp: atypical; CBF: cerebral blood flow; CI: confidence interval; Daily life OI: clinician estimate of orthostatic intolerance symptoms; DisDur: disease duration; dOH: delayed orthostatic hypotension; HC: healthy controls; NormHR/BP: patients with a normal heart rate and blood pressure response; Pat: patients; PetCO2: end-tidal CO2 pressure; POTS: postural orthostatic tachycardia syndrome; yrs: years. **: P<.0005; *: P<.01 between groups.
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cerebral blood flow comparisons.png
 
The study mentions this elephant in the room (hypocapnia when upright but not when supine) but does not discuss it or its implications as far as I can see.

Among ME/CFS participants, patients with daily life orthostatic intolerance symptoms, and those with an end-tilt PetCO2 ≤ 30 mmHg showed a greater % cerebral blood flow decrease.
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Ryan31337 said:
There's prior research suggesting it can't be blamed solely on hypocapnia. CBF disruption occurs first, the hypocapnia follows and reinforces it. You also don't get the same symptoms from voluntary hyperventilation.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4016155/
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I'm aware, I wasn't suggesting hyperventilation. Still, it's a significant finding which should be investigated IMO. Why is it happening only upright?
 
Commonly held view is that orthostasis results in central hypovolemia, particularly if there's neuropathic factors contributing to that. Knock-on effects lead to respiratory activation.

Various other reasons for hypocapnia are possible, hyperventilation features in mito disease and other myopathies for example. But I guess the focus is on mechanical changes due to it being reliably triggered on standing.
 
I think this study is really important. I often wondered why researchers weren’t looking into this. I don’t know of any other conditions (except perhaps PoTS alone - and even then, not to the extent of ME and PoTS together), where patients have to continuously be lying flat and feel absolutely dreadful when upright even just a few seconds or a minute upright for those with severe ME. It stops you from doing anything at all. I’ve never heard of it even for example in the most severest lupus or severest autoimmune conditions on the forums I went on a few years ago. Someone I knew who was very very sick, terminally ill, only had to lie flat in the last few weeks of her life.

But why could this be happening to us for ME? @Ryan31337 when you say being upright causes central hypovolemia, does that mean the brain is not getting enough of a volume of blood? And why could this happen - what do you mean by mechanical changes?

and I also wonder - are there not medicines to help bring more blood back to the brain?
 
Ryan31337 said:
You also don't get the same symptoms from voluntary hyperventilation
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Sid said:
I'm aware, I wasn't suggesting hyperventilation.
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This may or may not be relevant, but my low CO2 was attributed to hyperventilation by at least two prominent ME/CFS researchers. Both offered me the services of a breathing coach. I declined.
 
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