Intradermal capsaicin as a neuropathic pain model in patients with unilateral sciatica

AIM This study compared the responses between patients with unilateral sciatica and pain-free volunteers following administration of intradermal capsaicin. METHODS Fourteen patients with unilateral sciatica and 12 pain-free volunteers received one injection per hour over 4 h of 1 mu g and 10 mu g capsaicin, into each calf. For each dose, spontaneous pain (10 cm VAS), area of flare (cm(2)) and the sum of allodynia and hyperalgesia radii across eight axes (cm) were recorded pre-injection and at 5, 15, 30, 45 and 60 min post injection. METHODS Fourteen patients with unilateral sciatica and 12 pain- free volunteers received one injection per hour over 4 h of 1 mu g and 10 mu g capsaicin, into each calf. For each dose, spontaneous pain (10 cm VAS), area of flare (cm(2)) and the sum of allodynia and hyperalgesia radii across eight axes (cm) were recorded pre-injection and at 5, 15, 30, 45 and 60 min post injection. RESULTS Sciatica subjects experienced higher spontaneous pain and hyperalgesia responses in both legs compared with pain-free volunteers. The largest mean difference in spontaneous pain was 2.8 cm (95% CI 1.6, 3.9) at 5 min in the unaffected leg following 10 mg. The largest mean difference in hyperalgesia was 19.7 cm (95% CI 12.4, 27.0) at 60 min in the unaffected leg following 10 mg. Allodynia was greater in patients than in controls with the largest mean difference of 2.9 cm (95% CI 1, 4.8) at 5 min following 10 mg in the affected leg. Allodynia was also higher in the affected leg compared with the unaffected leg in sciatica patients with the highest mean difference of 3.0 cm (95% CI 1.2, 4.7) at 5 min following 10 mg. CONCLUSIONS The responses to intradermal capsaicin are quantitatively and qualitatively different in unilateral sciatica patients compared with pain-free controls.