Age-related differences in the endogenous analgesic response to repeated cold water immersion in human volunteers

Recent animal studies using stress-induced analgesia have suggested a general age-related decline in endogenous pain inhibitory systems. The aim of the current study was to examine age-related differences in the magnitude of endogenous analgesia in human volunteers, using psychophysical measures of neuroselective electrical, and thermal CO(2) laser induced pain thresholds, before, immediately after and 1 h after repeated cold water immersion of the hand. Sensory detection thresholds did not differ between age groups indicating that the functional integrity of primary afferent sensory fibres appears to be intact in older people. Consistent with many previous studies, older adults required a higher intensity of noxious stimulation in order to first report the presence of pain. The cold water immersion task was effective in eliciting a powerful analgesic response, regardless of age; pain thresholds were shown to increase by up to 100% immediately after the cold pressor test. This effect was relatively transient with thresholds returning to baseline within 1 h. The magnitude of analgesic response, however, was found to be significantly less in older people. Age differences in the efficacy of endogenous analgesic systems may be expected to reduce the ability of older adults to cope with severe persistent pain states and may help explain some of the variation in the literature on pain report.