Neurogenic flare responses in chronic rheumatic pain syndromes.

Chronic rheumatic pain syndromes such as the fibrositis syndrome, 'whiplash' syndrome, low back pain syndrome and regional pain syndrome are common clinical disorders of unknown cause. The presence of tender points in predictable anatomical locations is essential to their diagnosis. Exaggerated dermatographia or flare response to mechanical stimulation is also a commonly observed physical finding. Dermatographia is thought to be a local axon reflex mediated phenomenon, and, as such, is a component of the neurogenic inflammatory response. Because neurogenic inflammation may be mediated by polymodal nociceptors we examined the flare response to topical capsaicin, a chemical method of stimulating local axon reflexes, in 12 patients with chronic rheumatic pain syndromes and in 10 controls. There was a significant correlation (rs = 0.61; p less than 0.01) between the area of flare induced by mechanical stimulation and the area of flare induced by chemical stimulation for all subjects. Patients with chronic rheumatic pain syndromes had a lower threshold for capsaicin-induced flare responses compared with controls. They also had larger flares at capsaicin concentrations of 0.02 and 0.033 mg/mL (p less than 0.05) applied as 20 microL aliquots over 30 minutes. It is concluded that neurogenic flare responses are increased in patients with chronic rheumatic pain syndromes.