Suppression of first pain and slow temporal summation of second pain in relation to age.

BACKGROUND: Thermal stimuli delivered to skin of the arms or legs can produce a sensation of two distinct pains. These pains have been associated with activity in A-delta (first pain) and C-fiber (second pain) nociceptive fibers, respectively. Under appropriate conditions first pain decreases in intensity (adaptation) while second pain increases in intensity (slow temporal summation). Change in first and second pain to repeated stimulation of skin has not been assessed in relation to age.
METHODS: Ten younger (M = 25 years) and ten older (M = 65 years) subjects participated in a study of first and second pain intensity elicited by nociceptive range heat pulses (.7 second pulses; adapting temperature 39 degrees C to a stimulus temperature of 51 degrees C) delivered to skin of arms and legs. Response times to sensations of first and second pain were assessed to provide evidence that subjects were responding to first and second pain.
RESULTS: Age groups did not differ on pain intensity ratings of initial stimuli to previously unstimulated skin for either first or second pain. Older subjects failed to evidence slow temporal summation of second pain at the leg. Response times to first, but not second pain from the leg were delayed in the elderly. Response times to first and second pain at the arm did not differ with age. Auditory response times were slower in the older group.
CONCLUSIONS: Age did not influence pain intensity to unadapted and unsensitized skin. Slow temporal summation of second pain was not observed at the leg in the older group, suggesting that mechanisms subserving C-fiber mediated sensitization of second order nociceptive neurons may fail with age. Longer response times to first, but not second pain in older subjects may represent an age effect on myelinated (A-delta; first pain) and not unmyelinated (C-fiber; second pain) nociceptive afferents and may represent a type of small fiber peripheral neuropathy.