Evidence for different mechanisms of primary and secondary hyperalgesia following heat injury to the glabrous skin.

Characteristics of primary (within the area of injury) and secondary (outside the area of injury) hyperalgesia were determined after a heat injury applied to the glabrous skin of the hand in 8 human volunteers. The heat injury consisted of two burns (53 degrees C, 30 s) applied over an area 7.5 mm in diameter separated (centre to centre) by a 2 cm interval. Following the injury, a zone of hyperalgesia to mechanical stimuli measuring 20.1 +/- 3.6 cm2 (mean +/- SEM) developed in an area surrounding and including the burns. Within this zone, the pain threshold for mechanical stimuli decreased significantly by a similar amount for all areas tested (12.0 +/- 1.1 bars to 5.2 +/- 0.5 bars). Hyperalgesia to heat occurred only within the area of the burns. The heat pain threshold decreased and total ratings of heat pain increased significantly. In contrast, there was decreased pain to heat stimuli between the two burn sites, and no change in painfulness of the heat stimuli at other areas within the zone of hyperalgesia to mechanical stimuli. Particularly notable was the coexistence of hypalgesia to heat stimuli and hyperalgesia to mechanical stimuli in the uninjured region between the two burn sites. These results indicate that the characteristics of primary and secondary hyperalgesia differ and also suggest that the mechanism for hyperalgesia to mechanical and thermal stimuli differ.