Reduced pressure pain thresholds in response to exercise in chronic fatigue syndrome but not in chronic low back pain: an experimental study.

OBJECTIVE The aims of this study were to examine: (i) baseline pressure pain thresholds in patients with chronic fatigue syndrome and those with chronic low back pain compared with healthy subjects; (ii) the change in mean pain threshold in response to exercise; and (iii) associations with exercise-induced increase in nitric oxide. PARTICIPANTS Twenty-six patients with chronic fatigue syndrome suffering of chronic pain, 21 patients with chronic low back pain and 31 healthy subjects. METHODS Participants underwent a submaximal aerobic exercise protocol on a bicycle ergometer, preceded and followed by venous blood sampling (nitric oxide) and algometry (hand, arm, calf, low back). RESULTS Patients with chronic fatigue syndrome presented overall lower pain thresholds compared with healthy subjects and patients with chronic low back pain (p < 0.05). No significant differences were found between healthy subjects and patients with chronic low back pain. After submaximal aerobic exercise, mean pain thresholds decreased in patients with chronic fatigue syndrome, and increased in the others (p < 0.01). At baseline, nitric oxide levels were significantly higher in the chronic low back pain group. After controlling for body mass index, no significant differences were seen between the groups at baseline or in response to exercise. Nitric oxide was not related to pain thresholds in either group. CONCLUSION The results suggest hyperalgesia and abnormal central pain processing during submaximal aerobic exercise in chronic fatigue syndrome, but not in chronic low back pain. Nitric oxide appeared to be unrelated to pain processing.