• Astrid J Terkelsen, Janne Gierthmühlen, Nanna B Finnerup, Anders P Højlund, and Troels S Jensen.
• From the Danish Pain Research Center and Department of Neurology, Aarhus University Hospital, Aarhus, Denmark (A.J.T., T.S.J.); Department of Neurology, Division of Neurological Pain Research and Therapy, University Hospital of Schleswig-Holstein, Kiel, Germany (J.G.); Danish Pain Research Center, Aarhus University Hospital, Aarhus, Denmark (N.B.F.); and Department of Orthopedic Surgery, Odense University Hospital, Odense, Denmark (A.P.H.).
• Anesthesiology. 2014 May 1;120(5):1225-36.

BackgroundComplex regional pain syndrome is multifactorial. Exaggerated inflammatory responses to limb injury may be involved. The authors hypothesized that capsaicin-induced pain and neurogenic inflammation (skin perfusion and flare area) are increased in patients with complex regional pain syndrome compared with that in controls.MethodsTwenty patients with unilateral upper-limb complex regional pain syndrome and 20 age-, sex-, and body mass index-matched controls participated. Topical capsaicin 5% was applied to the back of both hands for 30 min, and pain intensity was assessed on a visual analogue scale. A laser Doppler perfusion imager scanner estimated capsaicin-induced skin perfusion and flare area. Autonomic and small-fiber function was assessed by sensory testing, quantitative sudomotor axon reflex test, and vasoconstrictor responses.ResultsThe authors found bilateral hypersensitivity to capsaicin (P ≤ 0.02), skin fold (P = 0.001), joint pressure (P < 0.0001), cold (P ≤ 0.01), and heat pain (P ≤ 0.04) in patients compared with that in controls and thermal and mechanical hyperalgesia in the complex regional pain syndrome-affected hand compared with that in the unaffected hand (P ≤ 0.001). The patients had normal capsaicin-induced flare areas, thermal detection thresholds, quantitative sudomotor axon reflex test, and vasoconstrictor responses.ConclusionsThe main finding is bilaterally increased capsaicin-induced pain in patients compared with controls. The flare response to capsaicin was normal, suggesting that the increased pain response was not due to increased neurogenic inflammation. The bilateral hypersensitivity to painful chemical, thermal, and mechanical stimuli not confined to the innervation area of a peripheral nerve or root cannot be explained by a regional change and may partly be due to central sensitization.

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