Age-related decrease in heat pain tolerance is associated with a simultaneous decline in C-fiber functionality and local cutaneous vasodilation

This study aimed to investigate the role of cutaneous vasodilation in heat pain tolerance during aging. We hypothesized that reduced vasodilation in response to heat would lead to a less efficient heat dissipation, and thus be associated with diminished heat pain tolerance. Due to their efferent role in vasomotor function, we hypothesized that C-fiber functionality would be associated to the efficacy of cutaneous vasodilation. Twenty younger and forty older subjects participated in a 15-min heat pain tolerance test, during which pain ratings were continuously measured, along with skin temperature. Participants could terminate the test at any time if the pain became unbearable. A local thermal hyperemia protocol was conducted to assess cutaneous vasodilation using laser Doppler flowmetry. Warm detection and heat pain thresholds were measured to evaluate small fiber functionality. Older subjects were divided into two groups according to their pain tolerance duration. The older MAX group (n = 22; i.e., 55 %) completed the tolerance test, while the older LOW group did not. Older MAX had preserved cutaneous vasodilation compared to young subjects, whereas older LOW had reduced heat-induced vasodilation. In addition, the skin temperature of older LOW subjects reached a higher level during the pain tolerance test, which was associated to higher pain ratings compared to the two other groups. Older LOW also had higher warm detection threshold, pointing to diminished C-fiber functionality. Reduced cutaneous vasodilation, possibly linked to impaired C-fiber functionality in aging, affects heat pain tolerance due to less efficient heat dissipation.