Sympathetic overactivity in subjects complaining of unexplained fatigue.
Pagani, M, Lucini, D, Mela, G S et al. · Clinical science (London, England : 1979) · 1994 · DOI
Quick Summary
This study examined how the nervous system controls heart rate in people with unexplained fatigue compared to healthy people. Using heart rate recordings, researchers found that fatigue patients showed an imbalance in their autonomic nervous system—specifically, the part that triggers the "fight or flight" response was overactive while the calming part was underactive, even at rest.
Why It Matters
This study provides objective, non-invasive physiological evidence that ME/CFS involves quantifiable autonomic nervous system dysfunction rather than just subjective symptoms. These findings support the biological basis of unexplained chronic fatigue and suggest that autonomic testing could help identify and understand the condition, potentially improving diagnosis and treatment strategies.
Observed Findings
At rest, fatigue patients showed significantly elevated low-frequency (sympathetic) spectral power compared to controls (73±11 vs 51±10 normalized units, p<0.05).
Patients demonstrated reduced heart rate variability responsiveness during mental arithmetic stress compared to healthy controls.
Sympathetic predominance in patients occurred alongside vagal withdrawal, indicating a bidirectional autonomic imbalance.
No significant differences were found in systolic blood pressure variability between groups.
The autonomic imbalance appeared to be selective to sinoatrial node control rather than systemic.
Inferred Conclusions
Unexplained fatigue is associated with sympathetic overactivity and vagal withdrawal at baseline, suggesting a functional autonomic nervous system disorder.
Non-invasive HRV analysis may serve as an objective physiological marker to distinguish fatigue patients from healthy controls.
The reduced stress responsiveness in patients suggests impaired autonomic flexibility, which could contribute to disease mechanisms and clinical outcomes in chronic fatigue syndrome.
Remaining Questions
Does autonomic dysregulation precede fatigue onset, persist throughout disease course, or emerge as a consequence of prolonged symptoms?
What This Study Does Not Prove
This study demonstrates association between fatigue and autonomic imbalance but does not establish causation—it is unclear whether sympathetic overactivity causes fatigue or results from it. As a cross-sectional study, it cannot determine whether this autonomic pattern is present before fatigue develops or how it changes over time. The findings are also specific to heart rate control and do not necessarily explain all mechanisms underlying ME/CFS symptoms.
Tags
Symptom:Fatigue
Biomarker:Blood Biomarker
Method Flag:Weak Case DefinitionSmall SampleExploratory Only
About the PEM badge: “PEM required” means post-exertional malaise was an explicit required diagnostic criterion for participant inclusion in this study — not that PEM was studied, observed, or discussed. Studies using criteria that do not require PEM (e.g. Fukuda, Oxford) are tagged “PEM not required”. How the atlas works →
What is the relationship between the degree of sympathetic overactivity and symptom severity, functional impairment, or prognosis?
Can autonomic biomarkers predict treatment response or disease progression in ME/CFS populations?
How do these autonomic abnormalities relate to other proposed ME/CFS mechanisms such as post-exertional malaise, immune dysfunction, or mitochondrial dysfunction?