The effect of age on A delta- and C-fibre thermal pain perception.

It has been suggested that ageing may have a differential effect on C fibre-mediated protopathic/tonic pain versus epicritic/phasic pain perception mediated by A delta fibres. The present study attempted to independently assess age-related changes in the function of A delta- and C-nociceptive fibres by examining CO2 laser-induced thermal pain thresholds before, during and after a compression block of the superficial radial nerve in 15 young and 15 healthy elderly adult subjects. Nerve block efficacy was monitored via measures of cold, warm and mechanical threshold, and simple reaction time. During nerve compression block, reaction time and mechanical threshold increased, cold sensation became impaired while warm sensation remained unaffected throughout the test in both groups. With respect to pain sensitivity, young adults exhibited significant increases in thermal pain threshold during A-fibre block while pain threshold remained relatively stable across the 3 test periods in the elderly group. It would appear that elderly adults rely predominantly on C-fibre input when reporting pain whereas younger adults utilise additional input from A delta fibres. Subsequent analysis revealed that during pre- and post-block periods, older adults exhibited a significant elevation in thermal pain threshold; however, when A delta-fibre function was impaired and only C-fibre information was available, both groups responded similarly. These findings support the notion of a differential age-related change in A-fibre-mediated epicritic pain perception versus C-fibre-mediated protopathic pain.