HYPERALGESIA IN A HUMAN-MODEL OF ACUTE INFLAMMATORY PAIN - A METHODOLOGICAL STUDY

The aim of the study was to examine reproducibility of primary and sec ondary hyperalgesia in a psychophysical model of human inflammatory pa in. Mild bums were produced on the crura of 12 volunteers with a 50 x 25 mm thermode (47 degrees C, 7 min). Assessments of (i) cold and warm detection thresholds, (ii) mechanical and heat pain thresholds, (iii) pain to heal (43 degrees C and 45 degrees C, 5 s), (iv) secondary hyp eralgesia, and (v) skin erythema were made 1.75 and 0.5 h before, and 0, 1, 2, 4, and 6 h after a burn injury. Sensory thresholds and hypera lgesia to heat and mechanical stimuli were examined by contact thermod es and von Frey hairs, and pain intensity was rated with a visual anal og scale (0-100). To describe between-day reproducibility, the subject s were examined three times at intervals of 21 days. Within-day compar isons showed that a 20% change could be detected as significant for al l variables with fewer than 12 subjects in a crossover design (2 alpha = 5% and power = 80%). Between-day comparisons demanded up to 25 subj ects to detect changes of the same magnitude. The burns caused mild to moderate pain (VAS: mean 29, SD 14) and the subjects (all right-hande d) were more sensitive to heat pain on their left side (P < 0.03). Hyp eralgesia was induced instantaneously by the burn and outlasted the st udy period (6 h). However, no spontaneous pain was observed after the injury, and a brief period of hypoesthesia to warm and cold stimuli wa s induced by the burn. The painful measurements themselves evoked hype ralgesia to heat and mechanical stimuli on the arm, but only to mechan ical stimuli on the legs, including secondary hyperalgesia. Hyperalgre sia evoked by the measurements was significantly less intense than tha t induced by injury. Habituation to the painful stimuli was demonstrat ed by significantly higher pain thresholds and lower pain responses on the second and third day of the study. The burn model is a sensitive psychophysical model of acute inflammatory pain, when cross-over desig ns and within-day comparisons are used, and the model is suitable for double-blind, placebo-controlled studies of analgesics. In similar mod els, we recommend that analgesic and placebo are evenly divided betwee n right and left sides and study days. (C) 1998 International Associat ion for the Study of Pain. Published by Elsevier Science B.V.