The measurement of capsaicin-sensitive sensory nerve fiber function in elderly patients with pain.

The present study was undertaken to investigate the possible involvement of peripheral nociceptors in 3 diagnostic categories of elderly patients with chronic pain. The 3 diagnostic groups consisted of: chronic lower lumbar pain of degenerative musculoskeletal etiology (LLP), post-herpetic neuralgia (PHN) and pain of non-organic origin in a group with a predominantly psychiatric disorder (PPD). The last group had pain confined to the trunk. All patients documented subjective pain using a visual analogue scale and an 8-item pain descriptor scale. Topical application of capsaicin was used to induce an axon reflex flare in 25 patients with pain and 29 control subjects. Three forms of measurement were used: flare size measured by tracing and by laser Doppler flowmetry, latency to onset of increased skin blood flow and blood flux. The groups with LLP and PHN had reduced flares at the affected site by all 3 measures, whereas no decrease was observed in the PPD group. No significant difference was found between groups when tested on a control site remote from the affected area using the 3 forms of measurement. We suggest that an alteration in the function of capsaicin-sensitive sensory fibers could be related to the subjective report of subacute and chronic pain in diagnostic groups such as LLP and PHN where there is a known organic cause.