Measurement of flare responses in patients with pain

Pain is a sensory and emotional experience and, as such, is always subjective. However, for many patients pain results from stimulation of nociceptors in somatic structures. Polymodal nociceptors also mediate flare responses to noxious stimuli. In this study skin flare responses overlying the painful region in patients with pain from degenerative spine disease (16), cancer (12) and osteomyelitis (1) were examined, and compared with control areas in the same patients. Flares were induced with capsaicin (0.2 g/L in 70% ethanol) and the area was measured by planimeter after 30 minutes of exposure to the chemical. Patients with chronic pain had reduced flare responses over the pain site compared with control areas (p less than 0.1; Wilcoxon two-sample test). There was no difference between patients with acute on chronic pain and with chronic pain alone. Acute pain alone produced an increased flare response. The results suggest increased nociceptor activity in acute pain and reduced activity in chronic pain. This may reflect sensitisation and desensitisation of the nociceptor, as has been demonstrated in experimental studies in animals.