Experimental tissue acidosis leads to increased pain in complex regional pain syndrome (CRPS)

The aim of this study was to investigate the role of local acidosis in the generation of pain in complex regional pain syndrome (CRPS). We investigate d ten patients with CRPS of the upper extremity with a mean duration of the disease of 43 weeks (range 4-280 weeks) and ten control subjects for sensi tivity to infusion of fluids with low pH (pH 6.1). Another group of five CR PS patients and three healthy controls was investigated using the same prot ocol but neutral infusion fluid (pH 7.4). A motorized syringe pump was inst alled for a constant infusion of synthetic interstitial fluid (SIF, either acidified (pH 6.1) or neutral) into the skin at the back of the hands and, thereafter, into the interosseus I muscle on both sides. A flow rate of 30 ml/h was chosen for intradermal and 7.5 ml/h for intramuscular infusion ove r a period of 10 min. The magnitude of pain was rated on an electronic visu al analogue scale. Patients were requested to give their ratings every 10 s during the whole stimulation period. The ratings were normalized as fracti ons of individual grand mean values. We found significantly increased pain perception during infusion of acidified SIF on the affected side in CRPS pa tients. Low pH fluid into the skin was significantly more painful between 4 and 6 min (ipsi 1.27 normalized rating (NR) (0.19-1.94), contra 0.31 NR (0 .03-0.51), P < 0.02) and between 8 and 10 min (ipsi 1.38 NR (0.19-1.94), co ntra 0.08 NR (0-0.27), P < 0.03) on the affected side, while analysis over the whole stimulation period just failed to reach statistical significance (ipsi 281 area under the curve (AUC) (187-834), contra 87 AUC (28-293), P = 0.059). Low pH infusion into the muscle was significantly more painful on the affected side during the whole infusion time (ipsi 861 AUC (308-1377), contra 190 AUC (96-528), P < 0.01). The quality of the deep pain during inf usion into the muscle was described by the patients as very similar to the CRPS-related pain. In controls we found no side differences of pain intensi ty during low pH stimulation. Neutral SIF evoked no pain at all, neither in CRPS patients (ipsi O AUG, contra 0 AUG) nor in healthy controls. Our resu lts suggest that hyperalgesia to protons is present in patients with CRPS. Further, we could demonstrate that pain is not only restricted to the skin but is also generated in deep somatic tissue of the affected limb. (C) 2000 International Association for the Study of Pain. Published by Elsevier Sci ence B.V. All rights reserved.