When standing is the problem.

That upright-flow drop above is POTS.¹ The brain compensates by breathing harder, which blows off CO2 and tightens brain vessels, deepening the shortfall, a self-reinforcing loop that, with the adrenaline-and-racing-heart response, is a leading explanation for the brain fog and panic on standing.² The flow drop is well measured, but whether it directly causes the symptoms is debated, which is why treatment works upstream, restoring upright blood volume with salt, fluids and compression.¹⁵ Why some people pool more is partly structural: laxer vessel walls, as in hypermobility, distend further under the same standing pressure, one reason connective-tissue laxity travels so closely with POTS.¹⁶

The largest study, in nearly 113,000 people, found a loss on the order of six IQ-equivalent points in those with unresolved symptoms.⁵ ASL-MRI and PET independently show drops in blood flow and metabolism in orbitofrontal, limbic and brainstem regions, and the leading explanation for brain fog is dynamic neurovascular failure: small-vessel and endothelial dysfunction with a leaky blood-brain barrier, often worse when flow falls on standing. Around 92 percent of patients show reduced upright cerebral blood flow even when heart rate and blood pressure look normal, a pattern shared with ME/CFS.¹⁷¹⁸ The microclot theory is plausible but contested, and no independent cohort has confirmed it.¹⁹

Stand someone with ME/CFS upright and their brain blood flow drops about 26 percent, versus 7 percent in healthy people, measured directly in the neck arteries. That large drop shows up even in patients whose heart rate and blood pressure look normal, the ones a standard POTS workup misses, and the flow does not fully recover on lying down, worsening the sicker the patient is.²⁰ Long COVID produces the same orthostatic signature, a reason the two illnesses are seen as overlapping, though the samples are small.²¹ Much of this work comes from one research group using its own method, and whether the low flow is a root cause or a downstream consequence remains unproven.²²

Loose joints often come with a loose circulation. In hypermobile Ehlers-Danlos syndrome and the wider hypermobility spectrum, the same lax connective tissue that lets joints overextend is thought to let leg and abdominal veins stretch and pool more blood when you stand, which is one proposed reason this group leans so heavily toward POTS and orthostatic intolerance.¹⁶ The association is real and runs both ways: across hypermobility clinics orthostatic intolerance sits around 74 to 78 percent versus 10 to 34 percent in controls, roughly half of POTS patients fall somewhere on the hypermobility spectrum, and in the largest hEDS cohort 79 percent showed reduced brain blood flow on standing.²³²⁴²⁵ Measured directly, ME/CFS patients who are hypermobile drop their cerebral blood flow about 32 percent on tilt versus 23 percent without, even when heart rate and blood pressure look normal.²⁶ The catch is the mechanism. Hypermobile EDS has no identified gene and no demonstrated vessel-wall defect, the extra pooling is mostly inferred rather than measured in hypermobility itself, and the autonomic testing points as much to a small-fibre, sympathetic neuropathy as to slack veins.²⁵ Most of these numbers also come from specialist clinics; at the community level hypermobile children show no clear excess of POTS, so referral bias inflates the clinic picture.²⁷ This group shares the upright-flow signature of long COVID and ME/CFS, though one head-to-head study reads them as overlapping but distinct.²⁸ The same orthostatic toolkit applies: salt, fluids, compression and recumbent-first reconditioning. One firm caution. This is the common, distensible-vessel hypermobile type, diagnosed in about 1 in 500 people and skewing female.²⁹ Do not confuse it with vascular EDS, a separate, rare disease caused by COL3A1 mutations that ruptures arteries and organs and needs specialist surveillance.³⁰