A∂- and not C-Fibers Mediate Thermal Hyperalgesia to Short Laser Stimuli After Burn Injury in Man

It remains unclear which nerve fibers are responsible for mediating hyperalgesia following skin injury. Here, we examined the role of A∂- and C-fibers in inflammatory hyperalgesia following a first-degree burn injury. A CO2-laser delivered ultrafast short constant-temperature heat pulses to the upper part of the lower leg, to stimulate selectively the relatively fast conducting thinly myelinated A∂ and the slowly conducting unmyelinated C-fibers. Participants were asked to respond as fast as possible whenever they detected a thermal stimulus. Thresholds and reaction times to selective Aδ- and C-fiber activations were measured in the conditioned and the surrounding intact skin, at pre-injury, and 1- and 24-hours post-injury. First-degree burn injury caused a significant decrease in Aδ-fiber detection thresholds and a significant increase in the proportion of Aδ-fiber mediated responses in the inflamed area 24 hours, but not 1 hour, following burn injury. No changes in heat perception were observed in the intact skin surrounding the injury. No group differences in C-fiber mediated sensations were observed. Our findings indicate that quickly-adapting Aδ-fibers but not quickly-adapting C-fibers are sensitized when activated by short and ultra-fast heat stimuli following skin burn injury. Our results further show that this change occurs between 1- and 24-hours post-injury and that it does not extend to the skin surrounding the injury.