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Keywords:

  • intradermal capsaicin;
  • neuropathic pain;
  • pain model;
  • sciatica

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• The intradermal capsaicin model of neuropathic pain has been previously developed and used in healthy volunteers.

• Few previous studies have used this model in pain patients.

WHAT THIS STUDY ADDS

• We have compared the response to intradermal capsaicin in the painful and nonpainful legs of patients with unilateral sciatica and compared these with healthy, pain-free control subjects. Pain and hyperalgesia responses were enhanced in both legs of patients with unilateral sciatica compared with healthy controls. The time course of hyperalgesia was different in sciatica patients, demonstrating a slower and larger evolution.

• Qualitative and quantitative differences between pain patients and healthy controls suggest that the use of a neuropathic pain model such as intradermal capsaicin to screen for novel antineuropathic agents might be superior in patients with pre-existing neuropathic pain syndromes.

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 µg and 10 µg capsaicin, into each calf. For each dose, spontaneous pain (10 cm VAS), area of flare (cm2) 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 µg. 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 µg. 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 µg 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 µg.

CONCLUSIONS

The responses to intradermal capsaicin are quantitatively and qualitatively different in unilateral sciatica patients compared with pain-free controls.