Category: Pathophysiology

Total blood volume and red blood cell volume are not associated with orthostatic intolerance in adults with myalgic encephalomyelitis/chronic fatigue syndrome

Abstract:

Orthostatic intolerance is common in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and may worsen daily functioning. Although hypovolemia has been proposed as a contributor, its relationship to objective orthostatic abnormalities remains uncertain. We examined whether measured blood-volume abnormalities were associated with standardized lean-test outcomes.

Adults meeting the 1994 ME/CFS criteria and reporting substantial post-exertional malaise underwent a 10-min lean test with capnography, followed 8 days later by total blood volume (TBV) measurement using the Daxor BVA-100. Supine hypocapnia was defined as baseline end-tidal CO2 (eTCO2) <34 mmHg; postural orthostatic syndrome of hypocapnia (POSH) as normal supine eTCO2 with any leaning value <34 mmHg; and postural orthostatic tachycardia syndrome (POTS) as a heart-rate increase ≥30 beats/min or absolute heart rate ≥120 beats/min.

Among 49 participants, TBV was hypovolemic in 35%, normovolemic in 43%, and hypervolemic in 22%; red blood cell volume was deficient in 49%. Overall, 55.1% had at least one lean-test abnormality, most commonly POSH (40.8%). Abnormalities did not differ by TBV or red blood cell volume category. Static blood-volume categories were not associated with lean-test-defined orthostatic abnormalities in ME/CFS, suggesting contributions from autonomic, vascular, respiratory, or cerebrovascular mechanisms.

Source: Yamazaki T, Mancini DM, Blate M, Quan P, Norweg A, Cook DB, Natelson BH. Total blood volume and red blood cell volume are not associated with orthostatic intolerance in adults with myalgic encephalomyelitis/chronic fatigue syndrome. Physiol Rep. 2026 Sep;14(17):e71087. doi: 10.14814/phy2.71087. PMID: 42717554. https://physoc.onlinelibrary.wiley.com/doi/10.14814/phy2.71087 (Full text)

Skeletal muscle properties in long COVID and ME/CFS differ from those induced by bed rest

Abstract:

Patients with long COVID and myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) suffer from post-exertional malaise. The accompanying physical inactivity may contribute to a lower aerobic capacity and may explain skeletal muscle adaptations in these patients. Here, we compare whole-body exercise responses and skeletal muscle adaptations after strict 60-day bed rest in healthy people with those in long COVID and ME/CFS patients, and healthy age- and sex-matched controls.

Bed rest alters respiratory and cardiovascular responses to maximal exercise, which are dissimilar in patients. Bed rest causes muscle atrophy without altering fiber type. Both patient groups have more glycolytic fibers, and ME/CFS patients display type I-specific atrophy. Only after bed rest is oxidative phosphorylation capacity associated with maximal oxygen uptake.

As skeletal muscle characteristics differ between patients and healthy individuals after bed rest, physical inactivity cannot solely explain the lower exercise capacity and skeletal muscle adaptations in long COVID and ME/CFS patients.

Source: Charlton BT, Slaghekke A, Appelman B, Eggelbusch M, Huijts JY, Noort W, Hendrickse PW, Bloemers FW, Posthuma JJ, van Amstel P, Goulding RP, Degens H, Jaspers RT, van Vugt M, Wüst RCI. Skeletal muscle properties in long COVID and ME/CFS differ from those induced by bed rest. Nat Commun. 2026 Jul 28;17(1):9125. doi: 10.1038/s41467-026-75725-y. PMID: 42649155. https://www.nature.com/articles/s41467-026-75725-y (Full text)

A cardiometabolic perspective on post-exertional malaise in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and Long COVID

Abstract:

Post-exertional malaise (PEM), the defining feature of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), is increasingly recognized in individuals with Long COVID. Although traditionally viewed as symptom exacerbation and/or emergence of new symptoms following exertion, PEM may reflect disrupted coordination across interconnected physiological systems operating over distinct post-exertional timescales. Such disruption may result in altered post-exertional physiological trajectories that contribute to multisystem manifestations and potential cardiometabolic consequences.

This perspective outlines a conceptual approach for investigating PEM through longitudinal assessment of post-exertional physiological trajectories while considering disease severity and underlying cardiometabolic health, including obesity and type 2 diabetes. Integrating these dimensions may help contextualize post-exertional responses, inform future longitudinal cardiometabolic profiling, facilitate patient stratification, and provide a framework for future mechanistic and interventional studies in ME/CFS and Long COVID.

Source: Westermeier F, Pretorius E, Untersmayr E, Bertinat R, Sepúlveda N, Fisman E. A cardiometabolic perspective on post-exertional malaise in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and Long COVID. Cardiovasc Diabetol. 2026 Aug 20;25(1):250. doi: 10.1186/s12933-026-03316-8. PMID: 42625219. https://link.springer.com/article/10.1186/s12933-026-03316-8 (Full text)

Experimental hypoxia to probe neuro-metabolic and vascular dysregulation in ME/CFS: a multimodal proof-of-concept MRI study

Abstract:

Background: Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) is a poorly understood, debilitating multisystem condition. Converging evidence implicates impaired cellular bioenergetics, neuroinflammation and defective neurovascular coupling that may manifest as “virtual hypoxia” only under physiological stress.

Methods: We performed a single-session multimodal 3T MRI study combining brain volumetry, arterial spin labelling (ASL) and multivoxel proton magnetic resonance spectroscopy under normoxia and two controlled hypoxic challenges (oxygen saturation 87 ± 3%) in 26 ME/CFS patients and 27 age- and sex-matched healthy controls.

Results: After intracranial-volume normalization, patients showed a reduced brainstem volume (1.46 ± 0.14 vs. 1.55 ± 0.18 % of eTIV; p = 0.013, FDR-p = 0.039), whereas deep grey matter and whole-brain parenchymal fraction did not differ between groups. Whole-brain cerebral blood flow (CBF) rose under hypoxia in both groups (controls +4.8 ± 13.0%, patients +3.7 ± 11.7%), with greater initial inter-individual variability in patients (patient-to-control variance ratio up to 6.94; FDR-p = 0.001). Thalamic lactate-to-creatine (Lac/tCr) ratios increased with hypoxia in controls (FDR-p = 0.028) but were already elevated at normoxia in patients (0.171 vs. 0.135; FDR-p = 0.021) and did not rise further (FDR-p = 0.38). In exploratory analyses, patients showed exaggerated inverse coupling between thalamic total N-acetylaspartate (tNAA/tCr) and white-matter CBF.

Conclusions: These findings provide in vivo evidence of impaired neuro-metabolic and vascular adaptive capacity in ME/CFS, supporting the virtual hypoxia hypothesis and highlighting candidate imaging markers for stratification that warrant validation.

Source:  Viola Bader, Katharina Estermann, Eva Niess, Tobias Zrzavy, Florian Fischmeister, Teresa Haider, Birgit Ludwig, Frederik Barkof, Henk JMM Mutsaerts, Gregor Kasprian, Fabian Niess, Wolfgang Bogner, Kathrin Kollndorfer, Lukas Haider. Experimental hypoxia to probe neuro-metabolic and vascular dysregulation in ME/CFS: a multimodal proof-of-concept MRI study. medRxiv 2026.08.10.26359935; doi: https://doi.org/10.64898/2026.08.10.26359935 https://www.medrxiv.org/content/10.64898/2026.08.10.26359935v1.full-text (Full text)

Cerebrospinal fluid opening pressure in relation to symptomatology and craniocervical anatomy in patients with myalgic encephalomyelitis/chronic fatigue syndrome

Abstract:

Background: Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a debilitating disease that affects millions worldwide. Its cause remains unknown, although many physiological mechanisms are impaired. Structural abnormalities in the craniocervical region seem to be more prevalent in patients with ME/CFS, with a possible impact on intracranial pressure (ICP) and lumbar puncture (LP) measurements. This study aimed to quantify the occurrence of elevated cerebrospinal fluid (CSF) opening pressure and a possible association with clinical symptoms and radiological variables in craniocervical segments in a cohort of patients with ME/CFS.

Methods: This study included 34 patients previously diagnosed with ME/CFS as part of a larger case-control study, for whom LP was performed only in the case group. Key measurements included age, sex, opening pressure from LP, symptomatic relief post-LP, and radiological variables with potential influence on CSF dynamics. Individuals were stratified into two groups according to opening pressure (OP) measurements using 20 cmH2O as the cutoff value, with subsequent statistical comparisons.

Results: Overall, the majority of participants reported moderate disease severity (65%) and concomitant self-reported low quality of life. The median CSF opening pressure was 18 (IQR 8) cmH2O, ranging from 11 to 29.5. Opening pressure stratification showed that 21 of the 34 cases (62%) had an OP measurement of <20 cmH2O (low-OP group) and 13 cases (38%) had an OP measurement of ≥20 cmH2O (high-OP group). The two OP groups did not differ significantly in baseline characteristics, including disease severity and quality of life. However, for certain radiological measurements, the high-OP group presented with measures reflecting a wider spinal canal, including a significantly larger foramen magnum diameter (33.9 mm), which was higher than the measures in the low-OP group (31.7 mm). Estimates of symptom relief 1-4 days post-LP were significantly higher among patients in the high-OP group (80%) than among those in the low-OP group (19%).

Conclusion: This study suggests that a proportion of patients with ME/CFS have elevated CSF opening pressures and may experience symptom relief following CSF subtraction. Our data indicate that some anatomical structures in the craniocervical area may be related to CSF pressure, although interpretation is challenging and merits further investigation.

Source: Jolley C, Bragée B, Soinne L, Huhmar H, Billing H, Bertilson B, Sjogren P. Cerebrospinal fluid opening pressure in relation to symptomatology and craniocervical anatomy in patients with myalgic encephalomyelitis/chronic fatigue syndrome. Front Med (Lausanne). 2026 Aug 4;13:1869714. doi: 10.3389/fmed.2026.1869714. PMID: 42614190; PMCID: PMC13481257. https://pmc.ncbi.nlm.nih.gov/articles/PMC13481257/ (Full text)

Orthostatic intolerance with small heart and/or disequilibrium in patients with myalgic encephalomyelitis/chronic fatigue syndrome: clinical update and paradigm shift

Abstract:

Orthostatic intolerance (OI) is characterized by the inability to maintain an upright posture without experiencing severe signs and symptoms, including hypotension, palpitations, light-headedness, pallor, fatigue, weakness, dizziness, impaired concentration, tremulousness, and nausea. The majority of patients with myalgic encephalomyelitis (ME) or chronic fatigue syndrome (CFS) exhibit OI, which is the primary factor restricting daily functional capacity.

During an upright posture, approximately 800 mL of blood is translocated from the intrathoracic venous compartment to the veins of the buttocks, pelvis, and legs. Under normal conditions, compensatory cardiovascular responses to this orthostatic stress include activation of the muscle pump through calf muscle contraction and a neurogenically mediated increase in heart rate and in systemic vascular resistance. The majority of the symptoms of OI are believed to be cardiovascular and related to reduced cerebral blood flow and excessive activation of the sympathetic nervous system.

However, compensatory sympathetic activation is essential for maintaining orthostasis. Many patients have a small left ventricle and associated low cardiac output. In addition, both the renin-aldosterone and antidiuretic hormone systems that regulate circulatory blood volume were downregulated.

Postural stability is necessary for maintaining the static balance critical for performing many daily activities. Recently studies have reported that postural instability or disequilibrium, potentially associated with central vestibular dysfunction, should be considered to be involved in the pathogenesis of OI among patients with ME/CFS. Disequilibrium should be recognized as an important cause of OI in those patients.

Source: Miwa K. Orthostatic intolerance with small heart and/or disequilibrium in patients with myalgic encephalomyelitis/chronic fatigue syndrome: clinical update and paradigm shift. Front Med (Lausanne). 2026 Jul 29;13:1744154. doi: 10.3389/fmed.2026.1744154. PMID: 42591853; PMCID: PMC13463185.  https://pmc.ncbi.nlm.nih.gov/articles/PMC13463185/ (Full text)

Central noradrenergic deficiency in post-infectious chronic fatigue: Neurobehavioral correlates

Abstract:

Fatigue, brain fog and post-exertional malaise are major features of post-infectious myalgic encephalomyelitis/chronic fatigue syndrome and post-acute sequelae of severe acute respiratory syndrome coronavirus 2. To date, no specific neurotransmitter abnormalities have been found in either condition. We examined the possibility of central catecholaminergic involvement and clinical correlates in post-infectious myalgic encephalomyelitis/chronic fatigue syndrome and post-acute sequelae of severe acute respiratory syndrome coronavirus 2 groups compared to healthy volunteers and, as positive controls, Parkinson’s disease patients.

In an observational, cross-sectional cohort study conducted at the National Institutes of Health Clinical Center we measured CSF levels of catecholamines and metabolites in the four groups and assessed correlations with neurobehavioral measures. CSF indices of the central Norepinephrine Pathway (norepinephrine + 3,4-dihydroxyphenylglycol + 3-methoxy-4-hydroxyphenylglycol) and Dopamine Pathway (dopamine + 3,4-dihydroxyphenylacetic acid + homovanillic acid) were measured and related to patient-recorded outcomes and physiological assessments.

Mean values for Norepinephrine Pathway activity (in pmol/mL) were lower in the post-infectious myalgic encephalomyelitis/chronic fatigue syndrome, post-acute sequelae of severe acute respiratory syndrome coronavirus 2, and Parkinson’s disease groups compared to the healthy volunteer cohort (post-infectious myalgic encephalomyelitis/chronic fatigue syndrome (-15.9, 95% confidence interval [-26.1, -5.7], P = 0.0006); post-acute sequelae of severe acute respiratory syndrome coronavirus 2 (-9.62, [-17.9, -1.4], P = 0.015); Parkinson’s disease (-19.4, [-27.5, -11.3], P < 0.0001)).

Post-acute sequelae of severe acute respiratory syndrome coronavirus 2 participants with post-exertional malaise had evidence of central noradrenergic deficiency compared to healthy volunteers (-18.3 [-31.3, -5.3], P = 0.0018). The post-infectious myalgic encephalomyelitis/chronic fatigue syndrome and post-acute sequelae of severe acute respiratory syndrome coronavirus 2 groups did not differ from the healthy group in values for the Dopamine Pathway index. Across all participants, Norepinephrine Pathway activity correlated positively with handgrip duration and general health and negatively with fatigue.

We conclude that post-infectious myalgic encephalomyelitis/chronic fatigue syndrome and post-acute sequelae of severe acute respiratory syndrome coronavirus 2 feature a specific central neurotransmitter pattern involving noradrenergic but not dopaminergic deficiency. The noradrenergic abnormality is associated with major symptoms such as post-exertional malaise.

Source: Aregawi L, Walitt B, Sullivan P, Norato G, Benjamin RN, Goldstein DS. Central noradrenergic deficiency in post-infectious chronic fatigue: neurobehavioral correlates. Brain Commun. 2026 May 13;8(3):fcag173. doi: 10.1093/braincomms/fcag173. PMID: 42205163; PMCID: PMC13202209. https://pmc.ncbi.nlm.nih.gov/articles/PMC13202209/ (Full text)

Perioperative outcomes in patients with myalgic encephalomyelitis/chronic fatigue syndrome undergoing general anesthesia: a retrospective matched-pair study

Abstract:

Background: Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a chronic multisystem disease characterized by profound fatigue, post-exertional malaise, cognitive impairment, and autonomic dysfunction. Despite features with potential relevance for anesthesia and perioperative care, empirical data on perioperative outcomes in patients with ME/CFS remains limited. We therefore performed a retrospective matched-pair analysis to generate clinical data on perioperative responses and identify areas for future research.

Methods: We conducted a retrospective matched-pair analysis at a single tertiary center. All patients with ME/CFS undergoing general anesthesia from 2015 to 2026 were identified using ICD-10-GM codes with additional manual verification and matched 1:1 to controls for comparison. Patients with confounding diagnoses or American Society of Anesthesiologists physical status above III were excluded. The analysis focused on intraoperative hemodynamic parameters, including baseline, post-induction, median, and lowest recorded systolic blood pressure and heart rate, as well as early postoperative outcomes in the post-anesthesia care unit (PACU), including maximum pain scores and requirement for rescue analgesia.

Results: Out of 189 individuals identified through ICD-10 codes, 15 matched pairs were included after application of exclusion criteria. Patients with ME/CFS exhibited lower minimum intraoperative systolic blood pressure (90.0 [82.5-95.0] vs. 100.0 [90.0-110.0] mmHg, p = 0.044) and lower minimum heart rate (50.0 [40.0-57.5] vs. 60.0 [50.0-65.0] bpm, p = 0.012). Vasopressor use and fluid administration did not differ, and no episodes of severe hypotension or perioperative adverse events were observed. Postoperative pain was higher in ME/CFS, with higher maximum pain scores (NRS 5.0 [4.0-6.0] vs. 1.0 [0.0-4.0], p = 0.008) and more frequent opioid rescue analgesia (80% vs. 33%, p = 0.039). Postoperative nausea or vomiting, oxygen supplementation, and PACU length of stay were similar between groups.

Conclusions: In this small exploratory cohort, general anesthesia was not associated with clinically relevant hemodynamic instability in patients with ME/CFS. Postoperative pain scores and opioid rescue requirements were higher in the ME/CFS group. Post-exertional malaise, a key disease feature with potentially delayed onset and significant impact, was not captured and remains an important target for future research. These findings should be considered hypothesis-generating and support prospective studies evaluating perioperative management and patient-relevant outcomes in ME/CFS.

Source: Steinkirchner FM, Kaufmann CK, Kraus RF, Käss M, Schieffer E, Graf BM, Lassen C, Kimmerling V, Dejaco A. Perioperative outcomes in patients with myalgic encephalomyelitis/chronic fatigue syndrome undergoing general anesthesia: a retrospective matched-pair study. BMC Anesthesiol. 2026 Jul 16;26(1):426. doi: 10.1186/s12871-026-04102-5. PMID: 42464211. https://link.springer.com/article/10.1186/s12871-026-04102-5 (Full text available as PDF file)

Metabolomic Classification of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome via Explainable Ensemble Learning and Pareto-Guided Feature Selection

Abstract:

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a debilitating multisystem illness characterised by post-exertional malaise, non-restorative sleep, and cognitive impairment, yet no objective diagnostic biomarkers have been established. Untargeted plasma metabolomics provides a broad view of the biochemical disturbances underlying ME/CFS; however, the high dimensionality of omics datasets and the limited interpretability of conventional classifiers nevertheless hinder translation into clinical practice. This study evaluates three ensemble classifiers-Explainable Boosting Machine (EBM), XGBoost, and LightGBM-for binary ME/CFS classification using plasma metabolomic and lipidomic profiles from 197 participants (106 ME/CFS; 91 healthy controls; 888 features).

Feature dimensionality was reduced using a Pareto-Guided Recursive Neural Network (PRNN) pipeline. Model performance was assessed via 50-repeat stratified hold-out validation. EBM achieved the highest accuracy (0.909; 95% CI: 0.868-0.949) and area under the receiver operating characteristic curve (AUC: 0.940; 95% CI: 0.909-0.983), with XGBoost and LightGBM performing comparably. Interpretability analyses revealed that pairwise metabolite interaction terms-particularly proline & indole-3-lactate, tyrosine & N-acetylornithine, and maleic acid & arachidic acid-contributed the greatest discriminative signal.

An ablation analysis comparing the full interaction-augmented EBM (AUC = 0.940) with a main-effects-only EBM (AUC = 0.882) confirmed that pairwise metabolite co-variation contributes additional discriminative value beyond individual metabolite levels, implicating amino acid catabolism, tryptophan-kynurenine pathway dysregulation, mitochondrial energy impairment, and lipid remodelling as central pathophysiological features. Global and instance-level explanations jointly demonstrated population-level metabolic signatures alongside individual heterogeneity, highlighting the added clinical value of explainable artificial intelligence (XAI) in metabolomics.

These findings support EBM-based metabolomic profiling as an internally validated approach for ME/CFS classification, subject to external validation, calibration assessment, and prospective testing.

Source: Yagin FH, Korkmaz Y, Colak C, Alzakari SA, Alkhalifa AK, Al-Hashem F, Aghaei M. Metabolomic Classification of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome via Explainable Ensemble Learning and Pareto-Guided Feature Selection. Int J Mol Sci. 2026 Jun 30;27(13):5920. doi: 10.3390/ijms27135920. PMID: 42450188. https://www.mdpi.com/1422-0067/27/13/5920 (Full text)

Disrupted glymphatic function and its relationship with sleep and cognitive impairment in ME/CFS assessed via DTI-ALPS

Abstract:

The glymphatic system is a recently discovered brain waste clearance system that is mostly active during sleep and disengaged during wakefulness. Impaired glymphatic function leads to the deposition of metabolic waste products in the brain potentially causing inflammation leading to various symptoms in ME/CFS. While the glymphatic function has been assessed in other neurodegenerative diseases using ‘diffusion tensor imaging along the perivascular space’ (DTI-ALPS), it has not been studied in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS).

This preliminary study investigates glymphatic function in 58 participants (ME/CFS = 31 and healthy controls = 27) using the DTI-ALPS index derived from DTI data acquired with 3 T MRI. The bilateral hemispheric DTI-ALPS index was estimated to assess glymphatic function, and an asymmetry index was calculated to determine interhemispheric asymmetry in glymphatic function.

We found that the global DTI-ALPS index was significantly lower in ME/CFS patients compared to healthy controls (ME/CFS: 1.44 ± 0.086; healthy controls: 1.51 ± 0.11, p = 0.014), indicating reduced glymphatic function in ME/CFS. Examining the hemispheres separately, showed the right hemisphere DTI-ALPS index was lower in ME/CFS than healthy controls (ME/CFS = 1.41 ± 0.097; healthy controls = 1.49 ± 0.12; p = 0.009) but not different on the left. Additionally, we did not find any significant difference in asymmetry index between ME/CFS and healthy controls. We observed an association between the global DTI-ALPS index and severity of ‘sleep disturbance’ (p = 0.013, r = -0.47) and “impaired concentration” (p = 0.026, r = -0.43).

This study demonstrated impaired glymphatic function in ME/CFS which may lead to symptoms such as cognitive dysfunction and sleep disturbance experienced by ME/CFS.

Source: Thapaliya K, Marshall-Gradisnik S, Inderyas M, Barnden L. Disrupted glymphatic function and its relationship with sleep and cognitive impairment in ME/CFS assessed via DTI-ALPS. Front Neurosci. 2026 Jun 19;20:1875420. doi: 10.3389/fnins.2026.1875420. PMID: 42403482; PMCID: PMC13329448. https://pmc.ncbi.nlm.nih.gov/articles/PMC13329448/ (Full text)