The 2025 3rd International ME/CFS and Long COVID conference, put on by the Charité Fatigue Center at Charité —Universitätsmedizin and supported by the ME/CFS Research Foundation from May 12-13, 2025, was impressive indeed. As fascinating as many of the findings were, equally impressive was the fact that most of them came not from the US or the UK but from Europe, which, not too long ago, was not exactly leading the pack in ME/CFS research. The rise of Europe as a significant center for ME/CFS and long-COVID research is encouraging, as is the involvement of many younger researchers.

Understanding I: Cardiovascular dysregulation and mitochondrial pathology

I always thought the muscles were going to be a big part of these diseases…but that’s getting ahead of things. First, a top-down look from David Systrom.

David Systrom – “Circulatory Dysfunction in ME/CFS”

David Systrom’s presentation was an excellent choice to start off this section because it provided a more systemic overview of what’s happening in these diseases. We’ll see that later presentations may be able to explain, on a more microscopic level, why Systrom has found what he has.

Systrom’s invasive exercise test results have redefined what we know about these diseases. Because the invasive exercise test examines what happens to the blood as it makes it way from the heart to the muscles and then back to the heart, it provides an unparalleled way to understand the energetic system in ME/CFS and long COVID, in broad strokes.

Systrom came to ME/CFS independently of the field when he began examining people with unexplained exercise intolerance. He first showed up on the ME/CFS scene with a poster at the 2014 IACFS/ME International Conference. His first paper on the subject, “Unexplained exertional dyspnea caused by low ventricular filling pressures: results from clinical invasive cardiopulmonary exercise testing“, contained data from over 600 people with exercise intolerance and did not mention ME/CFS (apparently because he did not have diagnostic data on all of them).

Systrom’s macro model of energy production.

The next data set in 2021 was even larger (@ 1,500 people) and did focus on ME/CFS. He identified several subsets: in one subset, impaired blood flows caused by preload failure were the major contributor to their depressed energy output during exercise.

Preload Failure – THE Ubiquitous Finding in ME/CFS and Long COVID

Systrom noted that the one ubiquitous finding in ME/CFS and long COVID is “preload failure”. Preload failure refers to reduced “filling pressure” when patients are upright, and particularly during maximum exercise. Preload failure occurs when the veins that return the used-up blood to the heart fail to constrict properly and drive normal amounts of blood to the heart.

The preload failure sets up reduced stroke volume, which refers to reduced amounts of oxygenated blood flowing out of the right side of the heart into the arteries.

Preload Failure + Impaired Oxygen Uptake

The other major subset has preload failure plus an inability to take up oxygen in the muscles. This group is not pumping enough blood out, and the oxygen in the blood they are pumping out is not being taken up – a double whammy – as Systrom put it. (An upcoming paper from Systrom will show that the same thing is happening in long COVID.)

Systrom’s more extensive study of small fiber neuropathy (SFN) bumped the incidence to about 60%. SFN could be impairing blood flows to the tissues.

The big question, of course, is what is happening here? Possibly, enter small fiber neuropathy (SFN) which refers to damage to the small unmyelinated nerve fibers that transmit sensory and autonomic nervous system signals. Systrom has a new, and yes, large (n=407; Systrom seems to specialize in large studies) SFN study coming out.

There’s no shortage of SFN studies, particularly in fibromyalgia, but this isn’t your normal SFN study. Systrom went deeper to access the sweat glands as well, and when he did, so he found the highest prevalence yet of SFN – a whopping 50% increase from 40 to 60% prevalence- in these diseases.

Systrom noted that he initially thought that a peripheral “left-to-right shunt” was to blame. A left-to-right shunt occurs when oxygenated blood from the left side of the heart flows back into the right side of the heart, thus bypassing the lungs. That would explain how the high oxygen levels he’s found are leaving the heart and then entering the heart.

Some left-to-right shunting does appear to be occurring, but a host of other explanations have shown up over time, including a small fiber neuropathy that shunts blood away from the muscles, capillary problems – now a major possibility (see below), and mitochondrial issues that prevent them from taking up oxygen.

 Anouk Slaghekke’s “Skeletal Muscle in ME/CFS and long COVID” Poster and Presentation

“I’ve never seen collagen deposition like this before.” Slaghekke

I was on the edge of my seat with Anouk Slaghekke’s “Skeletal Muscle in ME/CFS and long COVID” presentation.

Slaghekke’s poster, “Microvascular Dysfunction and Basal Membrane Thickening in Skeletal Muscle in ME/CFS and Post-COVID: from Pathology to Diagnosis“, may have been the hit of the conference and was one of three to win an award at the conference.

Wust said Slaghekke had called him over, saying, “I’ve never seen collagen deposition like this before”. Wust said he was “super-excited” by her findings because they showed the most clear-cut difference seen yet in his lab between healthy controls and the ME/CFS patients.

Slaghekke pointed out that the amount of oxygen and nutrients reaching the muscles is not just a matter of blood flow, but that something called diffusion is equally important. Once the blood gets to the muscles, oxygen and nutrients need to be able to diffuse across the capillaries into the tissue itself.

Using an electron microscope, Slaghekke assessed muscle biopsies for capillary levels, collagen IV content, and structure. First she found “increased collagen IV deposition in the capillary basement membranes”.

Four types of collagen exist. Collagen IV provides the main structural support for the “basement membranes” – connective tissues that underlie the blood vessels, muscle fibers, and epithelial (skin) cells. Too much collagen deposition in the capillaries would make them rigid and impair their ability to provide blood, oxygen, and nutrients to the muscles (and remove waste products from them).

Note the clear capillaries in the healthy controls vs the collagen-loaded and cloudy capillaries in the ME/CFS and long-COVID patients. Then check out how well differentiated the ME/CFS/long-COVID patients were from the healthy controls. Finally, see the narrowing of the lumen – the passage through which blood flows – in the ME/CFS/long-COVID patients. (The basement membrane is in blue).

Check out the massive basement membrane (BM) found in an ME/CFS/long-COVID patient.

Digging deeper, Slaghekke found, using electron microscopy, at times massive basement membrane thickening, vacuoles, and endothelial cells that were so thickened that no lumen (no interior) was present through which the blood could move (!).

All that spelled problems diffusing the oxygen and nutrients out of the capillaries into the muscles and removing waste products from the muscles.

While Slaghekke’s finding was in the muscle capillaries, one wonders if increased collagen deposition is occurring in other capillaries or other tissues. Increased collagen deposition in the blood vessels could be impairing blood flows in the brain, disrupting the blood-brain barrier, and allowing waste products such as amyloid proteins from being cleared. Increased collagen deposition has been found in liver, kidney, and lung disease, scleroderma, vascular diseases, and aging.

A 2023 German study found a reduction in the number of capillaries feeding the muscles, increased expression of extracellular matrix (connective tissue) genes, thickened cell basement membranes, as well as in what the authors described as a “remarkable” number of inflammatory macrophages (CD169+) and complement proteins. The authors proposed that extracellular matrix problems were also likely to be found in the muscle fibers.

These findings align well with Wust and Slaghekke’s observations of reduced capillary density, damaged capillaries, and evidence of immune infiltration, which may contribute to the damage.

Amyloids (strangely shaped, difficult-to-break-down proteins) provided an interesting side note as they could reflect and inability to remove unwanted materials. Wust noted they were investigating an amyloid product they’d found in the muscles, and amyloid proteins have shown up in ME/CFS studies before.

Wust noted they are looking for collaborators to validate their findings.

Jürgen Steinmacker – Changes in the Mitochondria in the Muscle

Prof. Dr. Jürgen Steinacker (University of Ulm) has been engaged in the EPILOC study that’s been charting the long-term effects of long COVID. EPILOC, which has mostly focused on symptom persistence thus far, has changed gears a bit, and recently delved into a most interesting part of ME/CFS and long-COVID pathophysiology: the mitochondria.

Four (or five, depending on how you are counting) complexes exist in the electron transport chain. Studies have suggested that different complexes are impacted in these diseases.

Steinacher published “Functional and Morphological Differences of Muscle Mitochondria in Chronic Fatigue Syndrome and Post-COVID Syndrome” in 2024. The study found decreased activity of the first complex in the electron transport chain in the mitochondria in both long-COVID and ME/CFS patients, as well as morphological changes in cristae (folds). Steinacher noted that the coronavirus downregulates mitochondrial genes, and, in fact, many viruses, including cytomegalovirus, hepatitis C, and respiratory syncytial virus (RSV) have been found to do that.

Decreased activity of the first complex results in reduced ATP production (energy production) as well as increased oxidative stress. Decreased complex I activity in aging – which may be happening prematurely in these diseases – is associated with ataxia (clumsy movements) and microglial activation.

(Findings concerning the complexes in the electron transport chain have been variable, however. Missailidis found reductions in Complex V activity and increased Complex I activity.)

The lack of a correlation between mitochondrial function and aerobic capacity in the long-COVID group suggested, as did Rob Wust’s recent paper, that mitochondrial problems, while present, may not be the be-all and end-all of the energy production problems in these diseases.

Steinacher, though, proposed that stress reduced levels of heat shock protein 70 (Hsp70) could “immediately” reduce mitochondrial energy production in these diseases. Heat shock proteins protect the cells from oxidative stress, and indeed, in 2009, Jammes, in a small study (n=18), found that ME/CFS patients with low levels of Hsp70 evidenced the highest amount of oxidative stress following exercise.

Karl Johan Tronstad – Changed Patterns of Blood Metabolites

Lost in all the excitement over Rituximab was all the other work that Fluge, Mella, and Tronstadt have been doing elsewhere with ME/CFS. They were early investigators exploring the metabolic basis of the energy production problems in ME/CFS, assessed endothelial functioning, and were one of the first, if not the first, to find that something in the ME/CFS serum caused increased oxygen consumption and triggered lactate production in muscle cells.

Back in 2021, Dr. Tronstad and his Norwegian colleagues published a large study, “A map of metabolic phenotypes in patients with myalgic encephalomyelitis/chronic fatigue syndrome“, suggesting that “elevated energy strain” caused by exertion-triggered reductions in tissue oxygen levels (tissue hypoxia) played a core role in ME/CFS. That same year, Tronstad, Fluge, and Mella introduced a working hypothesis in “Pathomechanisms and possible interventions in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS)”

Their pathomechanism paper proposed that three main factors were present:

1. An infection triggered an immune response, and featuring the B-cells, was present
2. The B-cells produced autoantibodies that damaged the blood vessels and the autonomic nervous system.
3. Attempts to compensate for the damage resulted in an increased sympathetic tone (increased fight-or-flight response) and metabolic adaptations that attempted to restore energy production.

Now, four years and a lot of ME/CFS and long-COVID research later, he (and Fluge and Mella) were back with an updated version of their understanding of what’s happening in these diseases.

Tronstad’s Pathomechanism model for ME/CFS.

Tronstad quickly skipped over the blood vessel findings – four years later, it seems fairly clear that the circulation is impaired, not just in the muscles but also in the brain.

At the metabolic level, Tronstad focused on the increased lactate levels observed at a very low workload (2019 study) and the utilization of unconventional energy sources (e.g., amino acids). As they proposed four years ago, hypoxia (low oxygen levels) causes a shift from aerobic to anaerobic energy production.

Energy-sensing pathways, such as mTOR, AMPK, and SIRTs, then instruct the cells to reduce oxygen consumption. Glycolysis (anaerobic energy production) ramps up. Instead of pyruvate going to acetyl-CoA (and aerobic energy production), it produces lactate, and ultimately, exercise intolerance. Glucose is conserved, leaving amino and fatty acids to produce energy. In some cases, dyslipidemia and insulin resistance pop up

Variations of this model explain why some patients respond to efforts to improve oxygen levels (such as hyperbaric oxygen treatment, drugs, supplements, or physical exercise) while others don’t.

While they believe that tissue hypoxia is found in everyone with these diseases, three “metabotypes” are found:

1. Metabotype 1 – has increased NEFAs and ketone bodies; low triglycerides. This metabotype occurs when the body shifts from using carbohydrates to using fats as its primary energy source. It’s associated with starvation or fasting, prolonged exercise, impaired carbohydrate metabolism, or mitochondrial dysfunction. (On a personal level, this metabotype interested me because of the very low body weight I experienced during my ME/CFS onset, which resulted in gynecomastia (thickening of the breast tissue). While the metabolic changes in this metabotype do not directly produce gynecomastia, they can suppress male hormone production and result in gynecomastia.)
2. Metabotype 2 – high triglycerides, low NEFAs – similar to dyslipidemia; in this metabolite, impaired utilization of fatty acids by the mitochondria diverts lipids toward storage. Insulin resistance is present, particularly at the tissue level. Fat deposition in the liver, muscles and other tissues drives inflammation and cellular stress. “Pseudostarvation” occurs at the tissue level. This group is the worst off.
3. Metabotype 3 – the smallest subset (@ 15%), metabotype 3 overlaps with 1 & 2; the patients are less affected overall.

In their latest work, soon to be published, a proteomic analysis of ME/CFS patients’ serum uncovered major changes in the immune, secretome, and metabolic proteins. They believe the secretome proteins could be a key finding as they can affect the entire body. All in all, the findings point to immune dysregulation that affects the blood vessels and the metabolism.

Christian Puta – Mechanisms of Post-Exertional Malaise

“I’ve never seen anything like this before.” Wilhelm Bloch

Now we turn to Dr. Christian Puta at the University of Jena for his model of how post-exertional malaise (PEM) is produced.

Earlier this year, Puta and 23 other researchers from Universities across Germany published “Towards an understanding of physical activity-induced post-exertional malaise: Insights into microvascular alterations and immunometabolic interactions in post-COVID condition and myalgic encephalomyelitis/chronic fatigue syndrome“.

Christian Puta started his presentation off with a short stand-to-sit exercise.

Explaining that he will soon get to the exercise test, Christian Puta started his presentation off by leading the audience in 10 “sit to stand” (arms crossed on chest) squats. Afterwards, like Tronstadt, he warned there is “no universally valid explanatory mechanism” but that a number of different factors work together in various ways to produce ME/CFS in different people. As with Systrom, Slaghekke, Wust, Tronstad, Fluge and Mella, he proposed that problems with oxygen – in this case oxygen extraction – plays a central role in these diseases.

Puta started off by suggesting that immune activation, including deformed red blood cells, were driving microvascular (capillary) problems which prevented blood/oxygen from getting to the mitochondria in the proper amounts.

Factors produced during exercise (lactic acid, reactive oxygen species (free radicals), prostaglandins, Na+ K+ ATPase dysregulation, b2 AdR autoantibodies) might be systemically dysregulating the immune system.

He turned to David Systrom’s work showing that peak venous succinate and peak venous lactate were correlated with peak energy production (VO2 max) at about 4 “METS” of activity. (Four METS corresponds to the sit-to-stand exercise.) Because succinate is released into the bloodstream during periods of high metabolic stress and reflects impaired mitochondrial function, the study suggested that ME/CFS patients’ mitochondria would have buckled doing the sit-to-stand exercise.

Deformed Red Blood Cells

He then turned to call a colleague, Wilhelm Bloch, who told him, referring to the deformed red blood cells he was finding in COVID-19, “I’ve never seen anything like this before”.

In 2021, Bloch and colleagues published an incredibly complex paper, “The oxygen dissociation curve of blood in COVID-19“, which, if I have it right, demonstrated an increased affinity of oxygen in hemoglobin in COVID-19. This suggests that oxygen is binding very tightly to hemoglobin (probably caused by increased levels of methemoglobin) – making it more difficult for oxygen to get off the red blood cells and diffuse into the tissues.

If that wasn’t bad enough, Bloch’s 2022 paper suggested that increased oxidative stress and stiff, less deformable red blood cells were also impairing oxygen diffusion into the tissues in people with mild long COVID. (Bloch’s finding was recently validated, particularly in females, in an Italian study which concluded:

“Persistent alterations in RBC functionality may underlie the microvascular dysfunction and symptoms of post-acute COVID-19 syndrome (PACS), including fatigue and cognitive impairment.”

Laughing, Puta thanked a patient who informed him that Leslie Simpson, a New Zealand researcher, found problems with red blood cell deformability in ME/CFS over thirty years ago. Actually, the first mention of red blood cell deformability problems in ME/CFS appears to have occurred in a Letter to the Editor of Lancet, “Abnormal Red-Blood Cell Morphology in Myalgic Encephalomyelitis” in 1987 (!).

Interest in the subject died with Lloyd’s failure, in an achingly small study (n=22), to validate Simpson’s findings. Simpson, however, persisted and after assessing red blood cell morphology in over 2,000 patients, wrote in a 1997 paper: The results reported here “would support a proposal that ME is a hemorrheological disorder in which symptoms are manifestations of the consequences of impaired capillary blood flow.” Twenty-two years later, a 2019 Stanford/San Jose University study found “stiffened” red blood cell membranes in ME/CFS.

Conclusion

What an interesting session this was! It indicated that European researchers are pushing the ME/CFS field when it comes to the muscles, mitochondria, blood flows, diffusion issues, and red blood cells.

While the blood flow findings, from the heart to the capillaries, appear to be mounting, the mitochondrial problems at the heart of these diseases seem to recede somewhat from center stage. Both Wust and Steinmacher found mitochondrial abnormalities but concluded that more was going on.

Note how many blood flow (read oxygen/nutrient flow) problems showed up (reduced preload/stroke volume, reduced number of capillaries, collagen deposition in the capillaries, deformed red blood cells, tightly held oxygen molecules on hemoglobin).

Next came numerous possible reasons that could explain this, including thickened cell basement membranes in the capillaries and muscle fibers, endothelial cells with narrowed or even missing passages for blood flow, and reduced capillary density. All of these suggest that barriers to oxygen and nutrient perfusion are impairing energy production in these diseases.

As the body tries to make energy without using oxygen, Wust’s finding of increased glycolytic fibers at the muscle level, Tronstad’s glycolytic model, and Puta’s glycolytic findings make sense.

When you add in deformed and stiffened red blood cells, as well as red blood cells that cling very tightly to their oxygen molecules, it appears that multiple barriers to oxygen and nutrient diffusion may exist in numerous tissues in these diseases. Multiple barriers would make perfect sense in diseases that can produce such debility.

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