Millions of individuals diagnosed with Long COVID, Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS), Postural Orthostatic Tachycardia Syndrome (POTS), and related post-viral conditions experience severe fatigue, orthostatic intolerance, and cognitive dysfunction. Despite growing recognition across public health organizations, clinical trials and standard care pathways frequently face challenges by evaluating patients primarily through broad symptom clusters. This approach can bundle distinct physiological processes together, complicating research efforts and therapeutic evaluation.
In theoretical and clinical research, physicians and researchers are exploring whether a mechanism-anchored framework can help organize and evaluate these complex post-viral presentations. In conceptual work published in Frontiers in Medicine, Dr. Robert Groysman describes post-viral conditions as multisystem network disorders that may involve overlapping dysfunctions, including hypotheses surrounding fragile mitophagy, autonomic dysregulation, and microvascular changes. In this Q&A, Dr. Groysman discusses the distinction between symptom-level classification and biological stratification, the preliminary nature of current cellular models, and how defining disease endotypes could inform future clinical trial design.
Q: In your conceptual paper on network disorders, you discuss the limitations of symptom-based phenotyping in post-viral conditions. Why is there a push toward identifying measurable biological mechanisms?
Robert Groysman: Symptom-based phenotyping remains essential for clinical documentation and understanding patient experience, but it does not necessarily explain underlying pathophysiology. Fatigue, cognitive complaints, orthostatic tachycardia, dizziness, post-exertional symptoms, and gastrointestinal issues represent clinical endpoints. The same clinical presentation can theoretically stem from multiple distinct biological drivers.
This presents a challenge in Long COVID and ME/CFS research when heterogeneous patient cohorts are grouped solely by shared clinical features. Two individuals presenting with debilitating fatigue may harbor different dominant physiological abnormalities: one might show marked autonomic and hemodynamic impairment, another may exhibit metabolic or mitochondrial recovery deficits, while another might display immune or mast-cell-associated activation. Grouping them solely under a broad symptom category risks obscuring these potential distinctions.
The field is actively investigating measurable biological parameters: autonomic function testing, endothelial and microcirculatory markers, immune profiling, metabolic indicators, mitochondrial assays, and gut barrier integrity measures. It is important to emphasize that many of these potential biomarkers remain experimental or investigational, and routine clinical assays cannot yet fully capture every proposed mechanism. However, biological stratification represents an important direction for academic research.
Clinically, mechanism-based evaluation aims to help researchers understand which physiological domains are disrupted. In clinical trials, defining biological endotypes helps reduce cohort heterogeneity, lowering the risk that a mechanistically targeted compound fails simply because it was tested in an unstratified population.
Q: You describe Long COVID within a multisystem network model. What hypotheses exist regarding how dysautonomia, endothelial dysfunction, and mast cell activation might interact?
Robert Groysman: Post-viral syndromes frequently cross traditional organ-system boundaries. Patients may report concurrent autonomic, vascular, gastrointestinal, and metabolic symptoms, which presents diagnostic and coordination hurdles in standard specialty-divided medical systems.
Current literature investigates several interacting pathways:
- Autonomic Dysregulation: Alterations in autonomic tone can disrupt hemodynamics, cerebral perfusion, and gastrointestinal motility.
- Vascular and Endothelial Factors: Emerging studies evaluate whether persistent endothelial activation or microcirculatory impairments influence tissue oxygenation and nutrient delivery.
- Mast Cell and Immune Pathways: Mast-cell-derived mediators are being investigated for their potential contributions to vascular permeability, vasodilation, and systemic inflammatory signaling.
- Metabolic and Gut Barrier Integrity: Cellular metabolic stress and shifts in the gut microbiome may feed into ongoing systemic inflammation.
Rather than viewing these as isolated clinical findings, network models hypothesize that they operate in bidirectional feedback loops. For example, autonomic stress can impact peripheral blood flow, while vascular or inflammatory signaling may further disrupt autonomic regulation. These models remain working hypotheses, and ongoing research is required to map precisely how these systems intersect across patient subgroups.
Q: Your paper introduces “fragile mitophagy” as a conceptual recovery-failure endotype. What is the biological hypothesis behind this, and how does it relate to post-exertional malaise (PEM)?
Robert Groysman: Fragile mitophagy is a theoretical framework proposed to describe a potential vulnerability in cellular quality control following metabolic stress.
In healthy cellular physiology, physical exertion or physiological demand temporarily stresses a portion of the mitochondrial network. Under homeostatic conditions, damaged mitochondria are targeted and degraded through autophagic and lysosomal pathways—a process termed mitophagy—followed by mitochondrial biogenesis.
The fragile mitophagy hypothesis proposes that in a specific subset of patients with ME/CFS or Long COVID, this clearance and recovery pathway functions adequately under low resting states but becomes overwhelmed when metabolic demand surges. If the lysosomal clearance capacity is exceeded, structurally compromised mitochondria and associated oxidative products could temporarily accumulate, potentially contributing to secondary inflammatory signaling and prolonged recovery times.
This hypothesis offers a conceptual biological model for the delayed nature of post-exertional malaise, where functional decline often manifests 12 to 48 hours after activity rather than exclusively during exertion. It remains an exploratory concept that requires formal validation through specialized cellular assays and translational studies.
Q: Many patients are historically told their exhaustion reflects simple deconditioning. How does a cellular recovery-failure model differ from that view?
Robert Groysman: Deconditioning typically presents with proportional exertional fatigue that resolves with standard rest and responds predictably to graded physical activity. In contrast, post-exertional malaise (PEM)—the cardinal feature recognized in consensus criteria for ME/CFS and observed in many Long COVID cohorts—is characterized by a delayed, multisystem symptom exacerbation following minimal exertion.
A recovery-failure hypothesis suggests that PEM is not merely rapid fuel (ATP) exhaustion during exercise, but rather an impairment in the cellular restorative process that occurs afterward. If mitochondrial clearance and biogenesis pathways are functionally fragile, metabolic demand could trigger cellular stress that the recovery apparatus clears slowly.
While this model helps conceptualize why symptoms worsen significantly hours or days after an event, it is currently a theoretical mechanism. Researchers are working to identify concrete, reproducible laboratory markers that can distinguish distinct cellular recovery deficits from deconditioning in clinical settings.
Q: In patients with suspected POTS or dysautonomia who have normal routine blood panels, how does mechanism-guided evaluation approach the workup?
Robert Groysman: Routine diagnostic tests—such as complete blood counts, basic metabolic panels, and standard thyroid screenings—are designed to detect overt organ failure, acute infection, or primary endocrine disease. They are generally static resting tests and do not assess dynamic physiological adaptation.
Postural Orthostatic Tachycardia Syndrome is defined hemodynamically by an excessive heart rate increase upon standing in the absence of orthostatic hypotension. However, that hemodynamic response can have varied contributing factors across individuals. Published literature identifies multiple potential phenotypes, including:
- Neuropathic mechanisms (such as peripheral autonomic or small-fiber denervation),
- Hyperadrenergic states (associated with elevated standing norepinephrine levels),
- Hypovolemia or altered renin-angiotensin-aldosterone signaling,
- Co-occurring conditions involving mast cell activation or joint hypermobility.
Mechanism-guided assessment focuses on dynamic physiological testing—such as formal tilt-table testing, autonomic reflex screens, blood volume assessments, and hemodynamic monitoring—rather than relying solely on resting labs. Clarifying the primary contributing physiology can assist clinicians in tailoring standard management protocols to the patient’s individual presentation.
Q: What are the implications of mechanistic stratification for future clinical trials in post-viral illness?
Robert Groysman: The primary challenge in evaluating treatments for post-viral syndromes is biological heterogeneity. If a therapeutic candidate targets a specific pathway—such as mast-cell stabilization, microvascular perfusion, or mitochondrial cofactors—testing it across an unselected, broad cohort can dilute observable efficacy. If only 20% of participants harbor the targeted biological abnormality, a clinically meaningful response in that sub-cohort may be washed out in the aggregate statistical analysis.
Incorporating biomarker-driven and physiological stratification into trial design allows researchers to enrich cohorts with individuals whose biological profiles match the drug’s mechanism of action. This approach also aids in interpreting trials that yield neutral results, clarifying whether an intervention was ineffective overall or simply tested in a mechanistic endotype to which it was poorly matched.
Current Directions in Post-Viral Research
Understanding Long COVID, ME/CFS, and related dysautonomias requires moving beyond general diagnostic categories toward mapping the biological diversity within these populations. Hypotheses linking mitochondrial quality control, microvascular integrity, autonomic regulation, and neuroinflammation provide a framework for future investigation, but large-scale replication and biomarker validation remain necessary.
As research advances, defining validated biological endotypes will be essential for replacing empiric trial-and-error strategies with targeted, evidence-based therapies.
Selected Scientific References and Reading
- National Academies of Sciences, Engineering, and Medicine (NASEM). (2015). Beyond Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Redefining an Illness. Washington, DC: The National Academies Press. DOI: 10.17226/19012
- Davis, H. E., McCorkell, L., Vogel, J. M., & Topol, E. J. (2023). Long COVID: major findings, mechanisms and recommendations. Nature Reviews Microbiology, 21(3), 133–146. DOI: 10.1038/s41579-022-00846-2
- Sheldon, R. S., et al. (2015). 2015 Heart Rhythm Society Expert Consensus Statement on the Diagnosis and Treatment of Postural Tachycardia Syndrome, Inappropriate Sinus Tachycardia, and Vasovagal Syncope. Heart Rhythm, 12(6), e41–e63. DOI: 10.1016/j.hrthm.2015.03.029
- Appelman, B., et al. (2024). Muscle abnormalities worsen after post-exertional malaise in long COVID. Nature Communications, 15(1), 17. DOI: 10.1038/s41467-023-44432-3
- Choutka, J., et al. (2022). Unexplained post-acute infection syndromes. Nature Medicine, 28(5), 911–923. DOI: 10.1038/s41591-022-01810-6
Medical Disclaimer:
This article is intended solely for educational and informational purposes and does not constitute medical advice, diagnosis, or treatment recommendations. The biological mechanisms discussed—including models of fragile mitophagy and cellular recovery failure—represent preliminary hypotheses undergoing active scientific investigation. Individuals experiencing post-viral symptoms, orthostatic intolerance, or unexplained fatigue should consult a qualified healthcare professional for formal clinical evaluation.