Lack of endogenous modulation and reduced decay of prolonged heat pain in older adults

Abstract

This study supports the hypothesis that healthy older adults exhibit decreased endogenous pain inhibition compared to younger healthy controls. Twenty-two older adults (56-77years of age) and 27 controls aged 20-49 participated in five experimental sessions following a training session. Each experimental session consisted of five 60-s trials in which the experimental heat stimulus was presented to the thenar eminence of the left palm with or without a conditioning stimulus (cold-water immersion of the foot). The temperature for the palm (44-49 degrees C) and foot (8-16 degrees C) was customized for each subject. The intensity of experimental pain produced by the contact thermode was continuously measured during the 60-s trial with an electronic visual analogue scale. No significant associations were found between subjects rating of concentration and the overall inhibitory effect. Older subjects failed to demonstrate conditioned pain modulation (CPM) and showed facilitation in the trials using painful concurrent immersion of the foot. A novel aspect of the study was that we recorded "pain offset" (i.e., after-sensations) and found that ratings for the older sample decreased at a slower rate than observed for the group of younger adults suggesting increased central sensitization among the older sample. Decrements in CPM could contribute to the greater prevalence of pain in older age. Since a number of neurotransmitter systems are involved in pain modulation, it is possible age-related differences in CPM are due to functional changes in these systems in a number of areas within the neuroaxis.

Figure 1

Temporal profile of continuous ratings during the non-concurrent sessions with trials without the water bath (NoB, asterisk) or with a 45-second cold-water immersion of the foot, which was either presented prior to (PRIMED, open circle) or alternated with (ALT, closed circle) the focal heat stimulus. Intensity of heat pain at the palm was averaged across the five thermal trials for (a) younger and (b) older subjects.

Figure 2

Comparison of non-concurrent conditioning stimuli on focal heat pain in younger and older adults. Subjects rated heat pain without a water bath (NoB, open bar) or after immersion of their foot into a painfully cold-water bath. For the CPM sessions, subjects either repeatedly immersed their foot for five times followed by heat pain (PRIMED, gray bar) or alternated foot immersion with the heat stimulus (ALT, darkened bar). AUC (A) and peak pain ratings (B) of heat pain were recorded during a 30-second testing trial.

Figure 3

Temporal profile of continuous ratings during the concurrent sessions with trials without the water bath (NoB, asterisk) or with a concurrent 45-second water immersion of the foot at 23°C (NeutralC, open circle) or painfully cold (NoxC, closed circle) water. Intensity of heat pain at the palm was averaged across the five thermal trials for (a) younger and (b) older subjects.

Figure 4

Comparison of concurrent conditioning stimuli on focal heat pain in younger and older adults. Subjects rated heat pain without a water bath (NoB, open bar) or during immersion of their foot into 23°C (NeutralC, darkened bar) or painfully cold (NoxC, gray bar) water. AUC (A) and peak pain ratings (B) of heat pain were recorded during a 30-second testing trial.

Figure 5

For the NoB session, temporal profile of continuous ratings following cooling of the thermode to the resting baseline temperature of 33.0°C for younger (open circle) and older adults (closed circle). Ratings were obtained 10 seconds following the 30-second heat trial. Older adults reported pain longer than younger adults during this period, which might serve as a measure of “pain offset.”