Are Pain Reports as well as Autonomic and Motor Reflex Responses Changed in Parallel while Ageing?

Most studies of experimentally induced pain revealed an increase of pain threshold and a decrease of tolerance threshold in the elderly. Physiological indicators have not yet been studied and compared with the verbal reports very often. Therefore, we intended to find out whether the assumed changes in the pain report are similar to the nociceptive evoked autonomic and reflex responses. Forty healthy young persons (age: 24.2±3.2 years) were compared to forty healthy elderly persons (age: 70.7±5.0 years). At first, the nociceptive flexion reflex (NFR) threshold was assessed by applying electrical suralis stimulation and employing a staircase method for threshold determination. Thereafter, ten consecutive supra-threshold reflex recordings (5 mA above reflex threshold) were averaged and the reflex parameters were calculated (latency, amplitude and reflex area). At the same time evoked heart rate responses (area of the heart rate increase in 3 time intervals, 0–3s, 3–6s and 6–9s) and the sympathetic skin response (SSR, latency and amplitude) were assessed. Following each stimulation, the pain report was measured with a 6 step categorical scale (no pain, slight pain, moderate pain, intense pain, very intense pain and unbearable pain). The verbal pain report at different stimulation levels and the nociceptive flexion reflex threshold were found not to differ significantly between age groups. Significant decreases of the reflex area (p<0.05), the SSR amplitude (p<0.001) and the heart rate area within each time interval (p<0.05) were found in the group of the elderly compared to the group of the young (Student's t-test). The group differences of SSR latency, NFR latency and amplitude were not significant. The present study revealed a decrease in supra-threshold nociceptive evoked autonomic and motor reflex responses in the elderly, while the pain report and the nociceptive flexion reflex threshold remained unchanged. Accordingly, an age-related dissociation of the different reaction systems can be seen, suggesting that the underlying ageing mechanisms in these systems differ. One can conclude that the elderly feel the same „pain“, but are limited in their defensive reactions.