Intramuscular and intradermal injection of capsaicin: a comparison of local and referred pain

The present study compared capsaicin-induced muscle and skin pain in humans . Twelve healthy subjects received, in a randomised, balanced order, 3 intr amuscular (i.m.) injections into the brachioradial muscle: capsaicin:100 mu g/l mi, capsaicin 100 mu g/20 mu l or 1 ml solvent (Tween 80), and one int radermal injection (i.d.): capsaicin 100 mu g/20 yl. Local and referred pai n intensities and areas were assessed from 0 to 60 min after injection. Int radermal capsaicin produced more intense local pain than i.m. capsaicin in the first min (skin: 68 +/- 6, muscle: 51 +/- 6 mm VAS x min, P < 0.05). In contrast, the local pain offset was later (muscle: 38 +/- 5, skin: 23 +/- 5 min, P < 0.05) and referred pain was more frequent (muscle: 9/12, skin: 1 /12 subjects, P < 0.01) following i.m. capsaicin compared with i.d. capsaic in. Capsaicin(1 mi) produced significantly more pain than 20 mu l i.m. (pai n in the first min: 1 mi. 71 +/- 6, 20 mu l: 51 +/- 6 VAS x min, P < 0.05, offset: 1 mi: 50 +/- 4, 20 mu l: 38 +/- 5 min, P < 0.05). The different loc al and referred pain following identical noxious stimulation of muscle and skin indicates that the neurophysiological mechanisms underlying skin and m uscle pain differs; The model with identical noxious stimulation of muscle and skin may be suitable for the study of differences in deep and superfici al pain as seen in the clinic. (C) 2000 International Association for the S tudy of Pain. Published by Elsevier Science B.V.