• Mira Meeus, Kelly Ickmans, Filip Struyf, Linda Hermans, Kevin Van Noesel, Jorinde Oderkerk, Luc S Declerck, Greta Moorkens, Guy Hans, Sofie Grosemans, and Jo Nijs.
• Department of Human Physiology and Rehabilitation Sciences, Vrije Universiteit Brussels, Belgium. Mira.Meeus@artesis.be
• Pain Physician. 2013 Mar 1;16(2):E61-70.

BackgroundAlthough enhanced temporal summation (TS) and conditioned pain modulation (CPM), as characteristic for central sensitization, has been proved to be impaired in different chronic pain populations, the exact nature is still unknown.ObjectivesWe examined differences in TS and CPM in 2 chronic pain populations, patients with both chronic fatigue syndrome (CFS) and comorbid fibromyalgia (FM) and patients with rheumatoid arthritis (RA), and in sedentary, healthy controls, and evaluated whether activation of serotonergic descending pathways by acetaminophen improves central pain processing.Study DesignDouble-blind randomized controlled trial with cross-over design.MethodsFifty-three women (19 CFS/FM patients, 16 RA patients, and 18 healthy women) were randomly allocated to the experimental group (1 g acetaminophen) or the placebo group (1 g dextrose). Participants underwent an assessment of endogenous pain inhibition, consisting of an evaluation of temporal summation with and without conditioned pain modulation (CPM). Seven days later groups were crossed-over. Patients and assessors were blinded for the allocation.ResultsAfter intake of acetaminophen, pain thresholds increased slightly in CFS/FM patients, and decreased in the RA and the control group. Temporal summation was reduced in the 3 groups and CPM at the shoulder was better overall, however only statistically significant for the RA group. Healthy controls showed improved CPM for both finger and shoulder after acetaminophen, although not significant.LimitationsThe influence of acetaminophen on pain processing is inconsistent, especially in the patient groups examined.ConclusionThis is the first study comparing the influence of acetaminophen on central pain processing in healthy controls and patients with CFS/FM and RA. It seems that CFS/FM patients present more central pain processing abnormalities than RA patients, and that acetaminophen may have a limited positive effect on central pain inhibition, but other contributors have to be identified and evaluated.

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