The present study compared
capsaicin-induced muscle and skin
pain in humans. Twelve healthy subjects received, in a randomised, balanced order, 3 intramuscular (i.m.)
injections into the brachioradial muscle:
capsaicin 100 microg/1 ml,
capsaicin 100 microg/20 microl or 1 ml
solvent (
Tween 80), and one
intradermal injection (i.d.):
capsaicin 100 microg/20 microl. Local and
referred pain intensities and areas were assessed from 0 to 60 min after injection. Intradermal
capsaicin produced more intense local
pain than i.m.
capsaicin in the first min (skin: 68+/-6, muscle: 51+/-6 mm VASxmin, 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.
capsaicin.
Capsaicin (1 ml) produced significantly more
pain than 20 microl i.m. (
pain in the first min: 1 ml: 71+/-6, 20 microl: 51+/-6 VASxmin, P<0.05, offset: 1 ml: 50+/-4, 20 microl: 38+/-5 min, P<0.05). The different local and
referred pain following identical noxious stimulation of muscle and skin indicates that the neurophysiological mechanisms underlying skin and
muscle pain differs. The model with identical noxious stimulation of muscle and skin may be suitable for the study of differences in deep and superficial
pain as seen in the clinic.