HYPERALGESIA AND TEMPORAL SUMMATION OF PAIN AFTER HEAT INJURY IN MAN

Temporal summation of pain occurs when repeated stimuli become increas ingly painful in spite of unchanged stimulus intensity. Summation can be quantified as the difference in pain between the first and the last stimulus in a train of stimuli. The aim of the study was to compare t emporal summation of pain in normal skin with summation of pain in ski n with primary and secondary hyperalgesia evoked by a heat injury. A h eat injury was produced on the crus of 12 volunteers with a 50 x 25 mm thermode (47 degrees C, 7 min). Measurements were made before, and 0, 1, 2, and 4 h after the heat injury, in three areas: primary and seco ndary mechanical hyperalgesia induced by the heat injury, and in a mir ror image of the injury on the opposite leg. Temporal summation of pai n was induced by repeated electrical stimuli (five stimuli at 2 Hz) an d assessed by visual analog scale (VAS). Primary hyperalgesia was eval uated by von Prey hairs and electrical stimuli, and the areas of secon dary hyperalgesia with a rigid von Prey hair (314 mN). Significant pri mary (P < 0.000001) and secondary (P < 0.00006) mechanical hyperalgesi a were evoked by the heat injury. The pain threshold to single electri cal stimuli was reduced within the injury (P < 0.03), but not outside. The pain responses to single and repeated electrical stimuli were not significantly altered by the injury. Temporal summation of pain occur red in 418 stimulus trains out of 576 (73%), but no significant change s in summation developed in skin with primary or secondary mechanical hyperalgesia compared with normal skin (baseline measurements). Tempor al summation at high stimulus intensities was more pronounced than at lower intensities (P < 0.0002). We found no correlation between either temporal summation and area of secondary hyperalgesia, or temporal su mmation and pain intensity during the induction of heat injury. We con clude that the development of primary and secondary mechanical hyperal gesia after heat injury in man was not associated with changes in temp oral summation of painful electrical stimuli. (C) 1998 International A ssociation for the Study of Pain. Published by Elsevier Science B.V.