The human capsaicin model of allodynia and hyperalgesia: sources of variability and methods for reduction.

Intradermal and topical application of capsaicin have been used to study mechanisms of mechanical allodynia (MA) and pinprick hyperalgesia (PPH) and the efficacy of drugs in relieving these symptoms. However, it is associated with significant inter- and intra-subject variability. In order to improve the model's sensitivity, we examined several potential sources of variability of capsaicin-evoked MA and PPH in healthy volunteers, including skin temperature fluctuations, method (intradermal vs. topical) and site (volar forearm vs. foot dorsum) of administration. In study I, 12 subjects received, in a 6-session, randomized, crossover trial, 1) 250 micrograms of intradermal (ID) CAP to the volar forearm with skin temperature fixed at 36 degrees C (36 ID). 2) 250 micrograms ID CAP with varying skin temperature (VT ID), or 3) 250 microliters of l% CAP patch placed on the skin at 36 degrees C. The resulting MA and PPH areas observed with each method were measured. In study II, a 4-session, randomized crossover trial, 12 subjects were given 100 micrograms ID CAP in the volar forearm or foot dorsum and subsequent areas of MA and PPH recorded. In study I, 5/12 subjects had small MA areas (< or = 5 cm2) and one subject had small PPH areas in at least 4/6 sessions. The most consistent intra-subject responses were seen with the 36 ID method. Correlation coefficients for the two sessions using the same method of administration were: MA; 36 ID r = 0.83, VT ID = 0.19. Topical r = 0.81; PPH: 36 ID r = 0.93; VT ID r = 0.38, Topical r = 0.78. In study II, 4/12 subjects had little MA for both forearm and foot though all subjects developed PPH. However, greater intra-subject consistency (MA: foot: r = 0.84; arm: r = 0.49; PPH: r = 0.87; r = 0.39) and significantly larger areas of MA (15.8 +/- 4.2 vs 9.1 +/- 2.5, p < 0.05) were seen with the foot. (PPH: foot: 28.9 +/- 6.7; arm: 21.6 +/- 4.2, NS). Large variability exists among subjects receiving CAP, with some developing minimal MA. However, these subjects may be screened out prior to entry, increasing the sensitivity of the model, which may be further improved by clamping the skin temperature.