Background & Aims
Pain tolerance is frequently assessed by thermal stimuli that induce a local vasomotor response (i.e. vasoconstriction or vasodilation). Local skin vasodilation is triggered by an axonal reflex mediated by C-fibers [1;2], which also conduct nociceptive impulses. In aging, local skin vasodilation is impaired and pain tolerance is reduced compared to young adults [3]. A previous study has shown that in elderly subjects, local skin vasodilation may contribute, along with activation of cutaneous thermoreceptors, to the pain sensation caused by the thermal stimulus [4]. This study aims at investigating whether impaired vasodilation may contribute to the decline in pain tolerance in elderly subjects.
Methods
Forty elderly (mean age: 68.7 years ; SD = 5.2 years ; 20 female) and 20 young subjects (mean age: 22.4 years ; SD = 1.9 years ; 10 female) participated in the study. First, we measured subjects’ warm and heat pain thresholds. Subsequently, participants underwent a pain resistance test, during which they placed their non dominant hand inside an airtight box with the air temperature set at 65°C (resulting in an average skin temperature of 43.5°C). They maintained this position for 15 minutes, continuously rating their pain on a visual analog scale. Participants had the option to terminate the test if the pain became too intense. We monitored the skin temperature during the tolerance test. After the skin temperature has returned to its basal level, we performed laser Doppler flowmetry on the dorsum of the hand during a local skin heating protocol, in which the skin was heated at a non-nociceptive temperature (i.e. 40°C) for 30 minutes.
Results
About half (47%) of the elderly subjects terminated the resistance test before the 15-min limit, compared with only 3% of the young subjects. Pain ratings were higher and pain onset was earlier in the elderly than in the young group. Interestingly, the skin temperature at the pain onset was 40.7°C on average in both groups, which is below the activation threshold of the nociceptors. This suggests that the vasomotor response may be involved in the generation of a painful sensation. We subdivided the elderly subjects into two groups according to their tolerance time for the following comparisons. We found that blood flow was reduced in the elderly with low pain tolerance compared with the other two groups. The warm detection threshold was increased in elderly subjects with low tolerance compared with the other two groups, whereas the pain threshold did not differ between the three groups.
Conclusions
These results suggest that the generation of painful sensations may involve not only cutaneous nociceptors but also local vasodilation. In addition, the reduction in cutaneous vasodilation observed in elderly subjects could potentially have an impact on their pain tolerance. This reduction in cutaneous vasodilation in some elderly participants could be associated with C-fibre damage, as suggested by the warm detection threshold data.
References
[1] Magerl, W., Treede, R. D. 1996. Heat-Evoked Vasodilatation in Human Hairy Skin: Axon Reflexes Due to Low-Level Activity of Nociceptive Afferents. The Journal of Physiology 497 (Pt 3): 837?48.
[2] Minson, C. T., Berry, L. T., Joyner, M. J. 2001. Nitric Oxide and Neurally Mediated Regulation of Skin Blood Flow during Local Heating. Journal of Applied Physiology 91 (4): 1619?26.
[3] Minson, C. T., Holowatz, L. A., Wong, B. J., Kenney, W. L., Wilkins, B. W. 2002. Decreased Nitric Oxide- and Axon Reflex-Mediated Cutaneous Vasodilation with Age during Local Heating. Journal of Applied Physiology 93 (5): 1644?49.
[4] Devanne, J., Dufour, A., Després, O., Pebayle, T., Lithfous, S. 2023. Interaction between local blood flow and tolerance to prolonged pain in the elderly. European Journal of Applied Physiology. doi: 10.1007/s00421-023-05294-1
[5] Brownlee, M. 2001. Biochemistry and molecular cell biology of diabetic complications. Nature 414: 813-820.