Why Brain Blood Flow Is Back in Focus After COVID
Long after the acute infection has passed, many patients report clusters of neurological and autonomic syWhy Your Brain May Not Be Getting Enough Blood: Cerebral Blood Flow and Long COVID.
You had the brain MRI. You were told it looked normal. And yet the fog, the pressure behind your eyes, the way your thinking dims the moment you stand up, none of it went away.
Normal structural imaging does not mean normal function. And for many people with Long COVID, the two have turned out to be quite different things. Research into cerebral blood flow in Long COVID is helping explain why many patients experience brain fog, dizziness and cognitive symptoms despite having a normal brain MRI.
A growing body of evidence now points toward blood flow in and out of the brain as a key piece of what is going wrong after COVID. And a newly published imaging technique that can map the brain’s entire venous drainage system in under seven minutes at extraordinary resolution may be about to change how researchers can study it.
This is not a story about a cure. It is a story about a door that has been closed for a long time, and the early signs that it is beginning to open.
Why brain fog, head pressure and dizziness are not in your head What we already know about cerebral blood flow after COVID The venous side of brain circulation and why it has been mostly ignored The new 7T MRI technique that changes what researchers can see Why symptoms worsen upright and what tilt table studies have found The glymphatic system, waste clearance and why this matters for cognition What comes next Frequently asked questions
Why brain fog, head pressure and dizziness are not in your head
The cluster of neurological symptoms that many people with Long COVID describe is strikingly consistent. Cognitive slowing that people often call brain fog. Pressure or heaviness in the head. Visual disturbances. Dizziness. A sharp worsening of all of these when upright, when exerting, or when heat is involved, followed by a return to something more bearable when lying down.
This pattern is not random and it is not imagined. It follows a physiological logic that researchers studying Long COVID, ME/CFS and dysautonomia are increasingly recognising. The brain is not consistently receiving enough blood, particularly under orthostatic stress, meaning the demands of being upright.
Critically, standard brain MRI does not detect this. Structural imaging is designed to find lesions, tumours, bleeds and tissue damage. It was never designed to measure how well blood flows through the organ under different conditions. A brain can look entirely normal on a routine scan while its perfusion is significantly impaired.
What we already know about cerebral blood flow after COVID
The evidence here is already substantial. Several independent research groups using different methods have found the same thing: reduced brain blood flow in subsets of people with Long COVID, and particularly when they are upright or under stress.
A study using arterial spin labelling, a specialised MRI technique that tracks blood flow without contrast agents, found widespread reduced perfusion across the frontal, parietal and temporal cortex in people with Long COVID and cognitive complaints, with the right hemisphere particularly affected, as reported in this study on cerebral hypoperfusion in post COVID subjects.
Tilt table studies have been even more revealing. A group at the Brigham and Women’s Faulkner Hospital in Boston evaluated 143 Long COVID patients alongside 170 ME/CFS patients using transcranial Doppler monitoring of cerebral blood flow velocity during tilt testing. Their data, published in 2026, found an almost identical autonomic pattern in both groups, demonstrating that reduced cerebral blood flow during upright posture is not merely a subjective report but a measurable, objective finding, as documented in this study on shared autonomic phenotype of Long COVID and ME/CFS.
Critically, these blood flow reductions are not always tied to the things doctors traditionally look for during a tilt test. Earlier work by van Campen, Rowe and Visser found a 26% reduction in cerebral blood flow in ME/CFS patients during tilt, compared with 7% in healthy controls, and importantly this reduction was seen even when heart rate and blood pressure appeared normal, as shown in this study on cerebral blood flow during orthostatic stress in ME/CFS. The same group confirmed these patterns held in Long COVID patients, finding their cerebral blood flow reductions were nearly identical to those seen in ME/CFS. In plain language, a normal heart rate during a tilt test does not mean the brain is receiving normal blood flow.
Transcranial Doppler studies in COVID survivors have also found evidence of chronic endothelial dysfunction in the middle cerebral artery persisting over a year after infection, as referenced in this review on cerebromicrovascular mechanisms in Long COVID. This fits with the broader picture of vascular injury after COVID, where the lining of blood vessels remains impaired long after the virus itself is gone.
The venous side of brain circulation and why it has been mostly ignored
Almost all the research described above focuses on arterial flow, meaning the blood going into the brain. The other half of the picture has received far less attention, despite being physiologically essential.
Once blood reaches brain tissue and delivers oxygen, it needs to drain out through the venous system. This is not passive. The cerebral venous system actively regulates intracranial pressure, controls how much blood is retained in the skull at any moment, and plays a critical role in clearing metabolic waste through what researchers call the glymphatic system.
The glymphatic system is the brain’s waste clearance pathway. It flushes out proteins and metabolic byproducts during sleep and during low cognitive demand states. If venous drainage is compromised, this clearance becomes impaired too. Some researchers have suggested that disrupted glymphatic function, linked to poor venous outflow, may be part of what drives persistent cognitive fog in conditions like Long COVID and ME/CFS.
Evidence for elevated intracranial pressure without visible structural cause has been reported in subsets of Long COVID patients. A review of cerebrospinal fluid findings in COVID associated neurological presentations noted that almost half of patients with persistent headache had increased opening pressure in the absence of other CSF abnormalities, with venous drainage disruption proposed as one possible mechanism, as discussed in this review on cerebrospinal fluid findings in COVID neurological presentations.
Why has the venous side been so hard to study? Because the technology to image it properly, at the resolution and speed needed to be clinically useful in sick, fatigued patients, has not existed until very recently.
The new 7T MRI technique that changes what researchers can see
In January 2026, a team at Maastricht University published a paper in Science Advances describing a new imaging protocol that maps the entire brain’s venous network at a resolution of 0.35 millimetres, the level of detail needed to see the smallest intracortical veins and how they connect to the larger drainage vessels, all in under seven minutes of scan time, as described in this whole brain mesovein imaging study.
Until now, this kind of imaging required 20 to 40 minutes in the scanner, making it impractical for patients with severe fatigue, sensory sensitivity or orthostatic intolerance. The breakthrough here is not just the resolution. It is the speed.
That speed matters enormously for Long COVID research in particular. Many patients cannot tolerate extended scan times. Some cannot lie still for long periods due to autonomic symptoms. The compression of a detailed venous imaging protocol into under seven minutes means that, for the first time, this population can realistically be studied.
The researchers at Maastricht also developed new methods for distinguishing and categorising different vessel types, separating leptomeningeal vessels, pial vessels and intracortical veins in ways that were previously not possible from a single short scan. They describe the capability as allowing reproduction of the kind of detailed vascular anatomical maps that were previously only available from post mortem tissue.
To be clear about where this sits in the research timeline: this paper describes the imaging technique itself. Its application to Long COVID patients is the next step, not a completed one. But the research community now has the tool. And the patient population to study is large, well characterised, and consistently reporting symptoms that point toward precisely the kind of venous dysfunction this technique can visualise.
Why symptoms worsen upright and what tilt table studies have found
The symptom pattern many people with Long COVID describe, better lying down, worse when upright, worse with heat or exertion, fits the physiology of impaired cerebral venous return.
When a healthy person stands up, the venous system adapts rapidly. Skeletal muscle contraction, autonomic regulation and pressure gradients work together to push blood back toward the heart and brain. In dysautonomia, this adaptation fails or is delayed. Blood pools in peripheral veins, venous return to the brain drops, and the reduction in cerebral perfusion that follows can be measured directly using transcranial Doppler during a tilt table test.
A 2025 review in the Journal of the American Heart Association synthesised the available evidence on cerebral blood flow during orthostatic stress across multiple conditions including Long COVID and ME/CFS and concluded that cerebral blood flow velocity is a meaningful biomarker that current clinical protocols are routinely failing to measure, as outlined in this review on cerebral blood flow in orthostatic intolerance. The review specifically noted that reduced cerebral blood flow can occur even when heart rate and blood pressure appear normal on a standard tilt test, which means many patients with real physiological impairment are currently receiving normal test results.
This is worth sitting with for a moment. It means that the test being used to rule out a problem in many Long COVID patients may not be measuring the thing that is actually wrong.
The glymphatic system, waste clearance and why this matters for cognition
Brain fog in Long COVID is not just a vague complaint. Researchers are increasingly linking it to specific biological processes, including disrupted waste clearance from the brain.
The glymphatic system works largely during sleep and low arousal states, using cerebrospinal fluid to flush proteins and inflammatory byproducts out of brain tissue. This clearance pathway drains into the venous system. If venous outflow is impaired or pressure gradients are disturbed, this clearance slows.
Sleep disruption is extremely common in Long COVID, and sleep is when most glymphatic clearance occurs. Poor sleep, autonomic instability and impaired venous drainage may together create a situation where the brain accumulates metabolic waste faster than it can clear it. This is consistent with the slow, cumulative quality of brain fog that many patients describe, where the cognitive picture tends to worsen through the day and across days rather than fluctuating in a simple pattern.
This hypothesis has not yet been tested directly in Long COVID cohorts. But the pieces are converging. The venous system, the glymphatic pathway, sleep disruption and autonomic dysfunction are all present in this condition. Imaging tools like the one published in Science Advances may soon make it possible to test whether they are connected in the way the biology would predict.
Why This Matters for Patients
Many people with Long COVID spend years being told that their scans are normal despite experiencing disabling brain fog, dizziness, head pressure and cognitive slowing. Research into cerebral blood flow does not prove that every neurological symptom in Long COVID has a single cause, but it does provide objective evidence that circulation to the brain can be impaired even when standard tests appear normal. For patients, this matters because it shifts the conversation away from whether symptoms are real and toward understanding the biological mechanisms that may be contributing to them.
What comes next
The combination of a large patient population with unexplained neurological symptoms and a newly available imaging tool that can study the relevant physiology in under seven minutes is significant.
Several research directions are now plausible that were not practical before. Imaging the cerebral venous system in Long COVID patients compared with healthy controls at the population scale needed to draw conclusions. Combining venous imaging with tilt testing, so that venous drainage can be assessed in supine and simulated upright positions together. Tracking changes in venous architecture over time as patients improve or relapse.
Most brain imaging in research and clinical settings is still done with the patient lying flat. Many of the symptoms being studied worsen, or only appear, upright. Bridging that gap, whether through upright MRI facilities where available or through careful protocol design, is one of the next frontiers.
None of this will produce a quick diagnostic test or a rapid treatment. Research moves slowly, and Long COVID researchers are working against a backdrop of underfunding relative to the scale of the problem. But the direction is clearer than it has been.
For patients who have been told their brain looks normal, the significance of this work is not that it offers immediate answers. It is that it takes the physiological experience of living with Long COVID seriously enough to look for the mechanisms that explain it, and provides tools to do that looking more precisely than before.
Frequently asked questions
What is cerebral blood flow and why does it matter in Long COVID?
Cerebral blood flow is how much blood the brain receives at any given moment. Research has now shown that this can be measurably reduced in Long COVID patients, particularly during upright posture, which helps explain symptoms like brain fog, dizziness and cognitive slowing.
Why does my brain MRI come back normal when I clearly have cognitive symptoms?
Standard brain MRI looks for structural damage, lesions or changes to brain tissue. It is not designed to measure blood flow or how the circulation adapts to posture and stress. Normal structure does not mean normal function.
What is the new 7T MRI technique and what does it actually show?
It is a protocol published in Science Advances in January 2026 that images the entire brain’s venous drainage system at very high resolution, 0.35 millimetres, in under seven minutes. That speed makes it accessible for fatigued patients who previously could not tolerate longer scans.
Has this new imaging been used on Long COVID patients yet?
The technique itself was published in early 2026. Its application to Long COVID research is the next logical step and not yet completed. The significance is that the tool now exists and the patient population is ready to be studied.
Why do my symptoms get worse when I stand up?
When the autonomic nervous system is not working properly, blood can pool in peripheral vessels rather than returning efficiently to the brain. Studies using transcranial Doppler have found measurable reductions in brain blood flow on standing in Long COVID and ME/CFS patients, even when heart rate and blood pressure look normal.
What is the venous side of brain circulation?
Arteries carry blood into the brain. Veins carry it back out. If venous drainage is impaired, it can raise pressure inside the skull, reduce perfusion, and disrupt the glymphatic clearance system that flushes metabolic waste from the brain. Until recently, the tools to image this system clearly did not exist at the speed needed to study sick patients.
What is the glymphatic system and how does it connect to brain fog?
The glymphatic system is the brain’s waste clearance pathway, which works largely during sleep. It relies on cerebrospinal fluid flowing through brain tissue and draining into the venous system. If venous outflow is impaired and sleep is disrupted, this clearance slows, which may contribute to the gradual build up of cognitive difficulty that many people with Long COVID experience.
Is this related to dysautonomia?
Yes. Dysautonomia affects how the autonomic nervous system regulates blood flow, including venous return to the brain. Many of the cerebral blood flow findings in Long COVID patients overlap with those seen in dysautonomia and POTS more broadly.
Could these findings lead to new treatments?
Potentially. If specific patterns of venous impairment or cerebral hypoperfusion can be identified in patient subgroups, it becomes possible to design and test treatments targeting those specific mechanisms, whether that means blood volume support, autonomic regulation or other approaches.
Are tilt table tests used to measure brain blood flow?
They can be. When a transcranial Doppler device is used during a tilt test, cerebral blood flow velocity can be measured in real time. This reveals reductions in brain perfusion during upright posture that a standard tilt test reading heart rate and blood pressure alone would completely miss.
Is cerebral hypoperfusion the same as brain fog?
Not exactly. Brain fog is the subjective experience and cerebral hypoperfusion is one of the biological mechanisms that may produce it. The link between the two is increasingly supported by research, but brain fog in Long COVID is likely multifactorial, involving immune, metabolic and vascular factors alongside impaired perfusion.
What should I do if I think this might apply to me?
If your cognitive symptoms are position dependent or worsen with exertion, it is worth raising cerebral blood flow and orthostatic intolerance specifically with a clinician. Asking for a tilt table test with transcranial Doppler monitoring, if available, rather than a standard tilt test alone, is the most direct way to get objective data on whether brain perfusion is involved in your symptom picture.
Sources and further reading
The imaging breakthrough Whole brain mesovein imaging in living humans using fast 7T MRI, Science Advances, January 2026: https://pmc.ncbi.nlm.nih.gov/articles/PMC12787541/
Cerebral blood flow after COVID Cerebral hypoperfusion in post COVID subjects, arterial spin labelling MRI: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10086005/ Orthostatic symptoms and cerebral blood flow in Long COVID and ME/CFS: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8778312/ Shared autonomic phenotype of Long COVID and ME/CFS, Brigham and Women’s Hospital: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12829881/Cerebral blood flow alterations and neuroimaging in Long COVID: https://pmc.ncbi.nlm.nih.gov/articles/PMC11572096/
The venous and orthostatic picture Cerebral blood flow in orthostatic intolerance, a 2025 review in the Journal of the American Heart Association: https://www.ahajournals.org/doi/10.1161/JAHA.124.036752 Long haul COVID patients with persistent cerebral blood flow abnormalities: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9602558/Cerebromicrovascular mechanisms in Long COVID: https://pmc.ncbi.nlm.nih.gov/articles/PMC11872997/
Venous pressure and cerebrospinal fluid Cerebrospinal fluid findings and elevated opening pressure in COVID neurological presentations: https://pmc.ncbi.nlm.nih.gov/articles/PMC9648929/
Disclaimer
This article is for educational and research discussion purposes only. It does not constitute medical advice or diagnostic guidance. Clinical decisions should be based on individual assessment and current professional standards.