Is temporal summation of pain and spinal nociception altered during normal aging?

Abstract

This study examines the effect of normal aging on temporal summation (TS) of pain and the nociceptive flexion reflex (RIII). Two groups of healthy volunteers, young and elderly, received transcutaneous electrical stimulation applied to the right sural nerve to assess pain and the nociceptive flexion reflex (RIII-reflex). Stimulus intensity was adjusted individually to 120% of RIII-reflex threshold, and shocks were delivered as a single stimulus or as a series of 5 stimuli to assess TS at 5 different frequencies (0.17, 0.33, 0.66, 1, and 2 Hz). This study shows that robust TS of pain and RIII-reflex is observable in individuals aged between 18 and 75 years and indicates that these effects are comparable between young and older individuals. These results contrast with some previous findings and imply that at least some pain regulatory processes, including TS, may not be affected by normal aging, although this may vary depending on the method.

Figure 1

Temporal summation protocol. In total, 3 trials of 5 electrical stimuli (black vertical lines) for each frequency (0.17, 0.33, 0.66, 1, and 2 Hz) and fifteen single stimuli (control trials) were applied in a pseudorandom order with an ITI of 15 to 30 seconds. Part of these trials (T1, T2…T9) is illustrated. The intensity of the stimulation was 120% of RIII-reflex threshold. Participants rated pain intensity after each trial (red arrow) during the ITI. ITI, intertrial interval.

Figure 2

Temporal summation of pain. The plot represents a comparison of pain ratings (y axis) of young and elderly participants relative to the single-stimulus control condition (100%) after the 5 stimulations administered at each frequency tested. Mean values and SDs of absolute pain ratings are reported in Table 3. There was a significant effect of frequency and a significant increase for each condition relative to the control trials. However, the effect was comparable between groups (interaction P = 0.79) (planned contrasts with control trials: *P < 0.05, **P < 0.01 for pooled data from both groups; see main text for detailed statistical results).

Figure 3

Temporal summation (TS) of the RIII-reflex. (A) An individual example of a raw recording of the RIII-reflex showing TS at 2 Hz. The top row indicates the timing of the electrical stimulation. The second row shows the raw electromyographic signal, where a typical RIII-reflex is observed beginning at ∼90 milliseconds after each stimulus onset. The third row represents the root mean square of the electromyographic signal used to measure RIII-reflex amplitude. (B) In this graph, RIII-reflex amplitude is plotted using the geometric mean of the 3 trials at 2 Hz in the same subject shown in (A). The black line represents the linear best fit of the curve from which the TS slope was calculated. Note that responses are reported here as mean t-scores rather than z-score for display purposes only (ie, t-score = z-score (10 + 50); this has no effect on the inference tests) (see main text for detailed statistical results).

Figure 4

Temporal summation of the RIII-reflex for the 5 frequencies, for young and elders. RIII-reflex amplitude increased with frequency. It also increased from the first to the fifth stimulus of the series, and this effect was significantly greater at higher frequencies. However, RIII-reflex amplitude was not significantly different between groups across frequencies or stimulus rank. In addition, the temporal summation effect characterized by the interaction between stimulus rank and frequency was not affected by the age group (**Ps < 0.01 for pooled data from both groups; see main text for detailed statistical results).

Figure 5

Slope of temporal summation effect. Comparison of RIII-reflex temporal summation slope for each frequency between groups. A significant effect of the frequency was observed, but this effect was not significantly different between groups. Error bars represent SEM (**P < 0.01 for pooled data from both groups; see main text for detailed statistical results).