CHARACTERIZATION OF CAPSAICIN-INDUCED MECHANICAL HYPERALGESIA AS A MARKER FOR ALTERED NOCICEPTIVE PROCESSING IN PATIENTS WITH RHEUMATOID-ARTHRITIS

Rheumatoid arthritis (RA) is characterised by pain and tenderness not only over inflamed or damaged joints, but also over apparently normal tissues. Experimental models suggest that these features result from c hanges of sensitivity within both peripheral and central neurones, but direct evidence from human disease is lacking. At present, most clini cal studies have evaluated overall pain experience rather than activit y within components of the nociceptive pathway. Therefore, the aim of this study was to assess the use of a capsaicin-based technique to qua ntify changes of neuronal sensitivity in patients with RA. First 20 mu l of capsaicin in solution (0.03 mg/ml) was applied topically for 30 min to apparently normal skin on the forearm of control subjects and p atients with RA. The subsequent development of mechanical hyperalgesia to pinprick stimuli was then measured at various time points using a 74.4-mN von Frey hair. The relationship between the area of hyperalges ia and a number of clinical measures was determined. Capsaicin-induced mechanical hyperalgesia was found to decline with age in normal subje cts (r = 0.47, P < 0.01). The development of hyperalgesia had a simila r time course in normal subjects and patients with RA. The maximum are a of hyperalgesia, however, was substantially larger in 35 RA patients ; 254.3 +/- 20.7 cm(2), compared with 35 normal controls; 109 +/- 7.5 cm(2) (P < 0.001). An association was apparent between hyperalgesic ar ea and a composite score of joint tenderness (r = 0.47, P < 0.01), but not with overall pain score ora systemic marker of inflammation. Thes e results provide evidence for enhanced sensitisation of a population of sensory fibres in RA. Peripheral sensory activity over the forearms of rheumatoid patients has previously been shown to be normal and the results suggest the presence of enhanced central mechanisms in this d isorder. The correlation between capsaicin-induced hyperalgesia and jo int tenderness in the RA patients implies that joint symptoms arise pa rtially as a result of central, and not exclusively peripheral, factor s. The study supports the use of capsaicin-based techniques to explore nociceptive mechanisms in clinical disorders characterised by chronic pain. (C) 1997 International Association for the Study of Pain.