Spatial mapping of the zone of secondary hyperalgesia reveals a gradual decline of pain with distance but sharp borders

The purpose of this study was to examine how pain to punctate mechanical st imuli varies with position within the zone of secondary hyperalgesia. Secon dary hyperalgesia was produced by an intradermal injection of capsaicin (50 mu g) into the volar forearm of human volunteers (n = 9). Before and at 20 , 60 and 100 min after the capsaicin injection, a computer-controlled elect romechanical stimulator was used to deliver controlled-force stimuli to the skin via a 12-mm wide, 100-mu m thick blade probe. Three forces (16, 32 an d 64 g; 1 s) were each applied in a random order to 10 sites spaced in l-cm increments along a line starting 1 cm from the injection site and ending n ear the wrist. At 40 and 80 min after capsaicin injection the 'zone of hype ralgesia' was determined with use of a hand-held 20-g von Frey probe. Where as, before capsaicin, the blade probe produced little or no pain, after cap saicin the 32-g and 64-g stimuli evoked pain consistently within but not ou tside the border of secondary hyperalgesia determined with the von Frey pro be. Within the zone of hyperalgesia the average pain ratings to the 64-g st imulus decreased exponentially with distance from the injection site. Surpr isingly, the space constant for this exponential decay was large (about 18 cm), and thus the decrease in pain ratings from the center to the edge of t he secondary zone was small (37%). However, pain ratings dropped precipitou sly just outside the zone of secondary hyperalgesia. This finding unlikely reflects a ceiling effect because pain ratings within the zone of secondary hyperalgesia increased linearly with force. The relatively uniform pain ra tings to the blade stimuli within the zone of secondary hyperalgesia and th e sharp border that delimits the zone of hyperalgesia indicate that this se nsory disturbance approaches being an 'all-or-nothing' phenomenon. Thus, a two-state model for central plasticity is needed to explain secondary hyper algesia. (C) 2000 International Association for the Study of Pain. Publishe d by Elsevier Science B.V. All rights reserved.