Commentary
Nutshell: The study found that 89% of ME/CFS patients had symptoms of orthostatic intolerance in everyday life, such as feeling very tired or lightheaded on standing,. Almost half of these had classical signs of orthostatic intolerance, unusually low blood pressure on standing or increased heart rate. However, just over half did not. Using an "improved", but not yet independently validated approach, the authors found is that blood flow to the brain was reduced in almost all patients with OI, regardless of heart rate/blood pressure changes.
Some of these points have already been made on this thread, particularly by
@Dolphin.
Why they did the study
It's worth understanding why the authors have done this study, using a new technology, but there's quite a lot of information here and I put it in a quote box.
Orthostatic intolerance, which is common in ME/CFS, is defined as "a clinical condition in which symptoms worsen upon a seeming and maintaining upright posture" and improve on lying back down. The standard way of investigating orthostatic intolerance is to look for abnormalities in blood pressure or heart rate either on standing or using a tilt table to achieve a near upright position.
However, not everyone with orthostatic intolerance (OI) symptoms have these abnormalities in heart rate or blood pressure. And OI symptoms are generally assumed to be related to reduced flow of blood to the brain (cerebral blood flow, CBF). Traditionally, CBF has been measured using the Doppler effect — think the changing pitch of sirens as an ambulance moves away from you — to measure the speed of blood in the brain. (Called transcranial cerebral blood flow velocity measurements). The weakness of this approach is that to measure flow, just as in a river, you need to know not just the speed but also the cross-section (of blood vessels). There is some evidence that blood vessels expand or contract when people stand up. If blood vessel cross-section/diameter does change, measuring speed alone would give an unreliable measure of blood flow.
The authors used a new technique that measures cross-section of the carotid and vertebrate arteries that feed blood to the brain as well as blood speed, to get what should be a more reliable estimate of blood flow.
What the study found
The authors analysed data on 429 patients who underwent the test out of the total of 675 patients diagnosed with ME/CFS. (The reason for excluding other patients, including because they underwent a different kind of test for good reasons, all seemed fair enough to me.) And that makes this one of the very biggest ME/CFS patient cohorts ever to feature in a research study
86% of ME/CFS patients had daily life orthostatic intolerance symptoms as assessed by questionnaire. That's a pretty high figure. The authors concede that the figure might be too high because of referral bias, where physicians are more likely to refer patients with OI to the clinic used by the authors (presumably they are specialists in OI). On the other hand, the authors also point out that they didn't include bedbound patients, who are more likely to have OI, and excluded severe patients from the test because they thought it would be too stressful for them. That could lead to an underestimate. Potentially it’s swings and roundabouts. At any rate, 86% is probably the most robust estimate we have of OI in ME/CFS patients.
Of the patients with OI symptoms, almost half (48%) had either an abnormally high heart rate on standing (postural orthostatic tachycardia syndrome, POTS) or blood pressure that dropped unusually far on becoming upright (delayed orthostatic hypotension). The other half (52%) had normal blood pressure and heart rate response to standing (where blood pressure might drop a little and heart rate goes up a bit, as it does in healthy controls).
For the 52% of patients who had normal heart rate and blood pressure responses but also had OI symptoms, 82% of these showed an abnormally large reduction in CBF (a 24% drop compared with a 7% drop for healthy controls).
So it appears that for the 82%, their OI symptoms might well be due to reduced blood flow to the brain, even though their heart rate blood pressure responses to standing are normal. Which confirms the authors' basic hypothesis.
Further support for the role of CBF in OI symptoms was the moderate correlation between the number of OI symptoms recorded during the tilt test and the degree of CBF. The authors actually described this as a strong correlation because of the low P-value, but the correlation of R = 0.4 can only really be described as moderate (see diagram below).
What does it all mean?
Although at one point the authors got carried away and said their data shows CBF causes OI symptoms, they don't include that claim in the conclusion or the abstract. They have shown an association between CBF and OI symptoms, which is very interesting all the same.
As the chart below shows, the correlation between symptom count and CBF isn't that amazing. Incorporating symptom severity as well may have given a stronger correlation. Or perhaps the true correlation really isn't that strong.
Also, 10% of patients who didn't have orthostatic intolerance symptoms had either POTS or delayed orthostatic hypotension. And some with OI symptoms show normal CBF response on standing.
In short, the authors' comments in the introduction  that “part of OI symptomology is related to" cerebral blood flow is probably a fair summary.
It is worth noting that the authors have developed a new technique here which, as they point out, needs replicating/validating by others.
Also, some traditional Doppler blood velocity measurements do support reduced blood flow in other types of patients with orthostatic intolerance. In other words, this effect is not unique to ME/CFS patients. Perhaps a more interesting question is, "why do so many ME/CFS patients have OI symptoms"?
I think Lucinda Bateman has commented that OI symptoms are highly treatable, which should be good, but I don't think this has been proven in clinical trials and, anecdotally, it seems to be a mixed picture for patients who have tried relevant medications.
