• Anneleen Malfliet, Roselien Pas, Raf Brouns, Joris De Win, Samar M Hatem, Mira Meeus, Kelly Ickmans, Robbert-Jan van Hooff, and Jo Nijs.
• Department of Rehabilitation Sciences and Physiotherapy, Faculty of Medicine and Health Sciences, Ghent University, Ghent, Belgium; Department of Physiotherapy, Physiology and Anatomy, Faculty of Physical Education & Physiotherapy, Vrije Universiteit Brussel, Brussels, Belgium; "Pain in Motion" international research group, www.paininmotion.be.
• Pain Physician. 2018 Jan 1; 21 (1): E13-E24.

BackgroundPain, fatigue, and concentration difficulties are typical features of chronic fatigue syndrome (CFS). The exact underlying mechanisms of these symptoms are still unknown, but available evidence suggests an important role for impaired pain modulation. As evidence also suggests that pain modulation is related to cardiovascular mechanisms, it seems logical to investigate whether cerebral blood flow (CBF) and heart rate variability (HRV) are altered in these patients.ObjectivesWe aimed to investigate the role of the cardiovascular system in pain modulation and symptoms of CFS; the response of CBF and HRV to physical stress and their relation to the change in temporal summation (TS) of pressure pain and self-reported symptoms was evaluated.Study DesignA controlled, randomized cross-over trial.SettingUniversity Hospital Brussels.MethodsTwenty CFS patients and 20 sedentary healthy controls were included in this study. In both of the groups, the change in TS of pressure pain, CBF (using transcranial Doppler), and HRV (using finger plethysmography) was examined during physical and emotional stress (to control for potential bias), as well as their association mutually and with self-reported symptoms of pain, fatigue, and concentrations difficulties.ResultsThere was no significant interaction or group (F-values ranging from .100 to 1.862, P-values ranging from .754 to .181) effect in CBF or HRV parameters. HRV and CBF did change during physical exercise, but the changes did not differ between patients and controls. While pain scores during TS at the trapezius site reduced in the control group after the physical exercise protocol (P = .037), they did not change in the CFS group (P = .108), suggesting impaired pain modulation. There were no significant correlations between CBF, HRV, TS, and self-reported symptoms (all P-values of correlation analyses > .01).LimitationsAlthough effect sizes were medium to large, the study sample was relatively low. Also, the mild nature of the exercise bout is discussable. Nonetheless, this mild exercise was able to provoke endogenous pain modulation in the control group, which endorsed a proper execution of the cycling exercise. Moreover, mild exercises are more applicable to daily physical activities in CFS patients than vigorous exercises.ConclusionThese results seem to refute the previously suggested alterations of CBF/HRV in CFS patients. These cardiovascular parameters appear not to explain pain before, during, and following exercise.Key WordsChronic pain, physical exercise, emotional stress, pain modulation, cardiovascular systems, temporal summation, pain pressure thresholds, transcranial Doppler, plethysmography.

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