This study looked at how the hearts and nervous systems of 24 ME/CFS patients with orthostatic intolerance (dizziness or fatigue when standing) behaved on their "good days" versus "bad days." Researchers found that on bad days, patients' hearts raced more when standing up, they couldn't stand as long, and their hearts pumped less blood—suggesting their nervous systems were working abnormally, sometimes overactive and sometimes underactive.
Why It Matters
Understanding how ME/CFS patients' autonomic nervous systems fluctuate between good and bad days could explain the characteristic symptom variability and inform better diagnostic criteria and treatment strategies. This research provides objective physiological evidence that orthostatic intolerance in ME/CFS involves complex, inconsistent autonomic dysfunction rather than a single fixed mechanism.
Observed Findings
Resting heart rate was significantly higher on bad days compared to good days.
21% of patients (5 of 24) could not maintain upright posture for the full 10-minute standing test on bad days, whereas all 24 maintained it on good days.
43% of patients (10 of 24) showed postural orthostatic tachycardia (≥30 bpm heart rate increase) on bad days despite not having it on good days.
21% of patients (5 of 24) showed attenuated or absent postural orthostatic tachycardia on bad days compared to good days.
Left ventricular end-diastolic diameter and stroke volume index were significantly lower on bad days compared to good days.
Inferred Conclusions
ME patients with orthostatic intolerance show paradoxical and variable sympathetic nervous system activation between good and bad days.
The sympathetic nervous system may shift from exaggerated activation to impaired activation when exposed to additional physiological stress (reduced preload/blood volume return).
Central nervous system dysfunction and fluctuating autonomic balance appear to underlie orthostatic intolerance in ME/CFS.
Remaining Questions
What triggers the shift between good and bad days, and can these triggers be identified or controlled?
What This Study Does Not Prove
This study does not establish causation—only that physiological changes correlate with symptom severity days. The small sample size (n=24) and lack of a healthy control group limit generalizability. Self-identified "good" and "bad" days are subjective; this study cannot prove the observed differences cause symptom changes rather than reflect them.
Tags
Symptom:Orthostatic IntoleranceFatigue
Biomarker:Blood Biomarker
Method Flag:Weak Case DefinitionNo ControlsSmall SampleExploratory Only