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Polli A.1,2, Van Oosterwijck J.2,3, Ickmans K.1,2, De Wandele I.3, Paul L.4, Meeus M.2,3, Nijs J.1,2
1Vrije Universiteit Brussel, Department of Physiotherapy, Human Physiology and Anatomy, Faculty of Physical Education & Physiotherapy, Bruxelles, Belgium, 2Pain in Motion, Bruxelles, Belgium, 3Ghent University, Department of Rehabilitation Sciences and Physiotherapy, Faculty of Medicine and Health Sciences, Ghent, Belgium, 4University of Glasgow, Nursing and Health Care, School of Medicine, Glasgow, United Kingdom
Background: Contrarily to what happens in healty controls (HCs), Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) patients show a worsening of their pain following exercise. The exact mechanisms underlying this phenomenon is largely unknown. Oxidative Stress (OS) has been associated to and proposed as contributor of pain. OS enhances sooner in ME/CFS patients during incremental exercise and anti-oxidant response is delayed, when compared to HCs. OS responses are linked to Autonomic Nervous System (ANS) activity. Experimentally-induced increased levels of OS reduces the blood flow that should flows to working muscles.
Purpose: We explored whether OS is associated to pain symptoms or to pain changes following exercise, and the possible relations between OS and ANS functions in ME/CFS and HCs.
Methods: Only women, between 18 and 65 years old, were included in the study. HCs were matched to patients´ characteristics (age, sex, BMI, levels of physical activity). Subjects underwent a standardized sub-maximal exercise test with continuous cardiorespiratory monitoring. We measured Heart Rate Variability (HRV) as measures of ANS function and TBARS levels as measure of OS. Before and after the test, subjects were asked to rate their symptoms using two VAS scales for bodily pain and headache. A blood sample was also taken before and after the test, in order to measure OS and exercise-induced OS changes. We performed both between-groups and within-group comparisons for pain and OS measures. Correlation analysis were used to examine the relation between subjective pain and TBARS measures, and to to analyze associations between ANS function and OS. Parametric or non-parametric test were chosen according to the results of descriptive analysis.
Results: Eighteen patients and eighteen HCs were recruited. No statistically significant differences at baseline were found for age, BMI, anxiety and depression. Significant between-groups differences were found for pain and headache at both baseline and following exercise (all p .000). Within-group analysis showed that pain significantly improved in HCs (p=.002) following exercise; and that HRV significantly decreased following exercise in ME/CFS patients (p=.039) Correlations revealed that only in patients with ME/CFS, TBARS levels were significantly correlated with pain and headache at baseline (r= .540, p=.021; and r=.569, p=.014, respectively) and with pain after exercise (r=.524, p=.024). We found a strong inverse correlation, in HCs only, between exercise-induced changes in HRV and TBARS changes
(r= -.725, p=.001).
Conclusion(s): Pain was associated to OS level only in ME/CFS patients both before and after the sub-maximal test. Exercise reduced ANS function in patients, but this did not seem to change either pain symptoms or OS. HCs showed a significant reduction in pain symptoms and a strong association between OS and ANS activity. That is, the more the ANS increases its activity during exercise, the more OS decreases, and vice-versa.
Implications: Our data helps the understaning of complex interactions between different system thought to play a role in nociception and pain. HCs and patients carachterized by chronic widespread pain (ME/CFS) showed differences in OS stress, ANS response to exercise, and in the association between ANS activity and OS during exercise.
Funding acknowledgements: This study was funded by Ramsay Research Fund of the ME Association (UK).
Topic: Pain & pain management
Ethics approval: The study was approved by the Ethical Committee of Ghent University, Belgium.
All authors, affiliations and abstracts have been published as submitted.
