Effects of topical perineural capsaicin in a reversible model of equine foot lameness.

Capsaicin is a local substance P depleter with dramatic analgesic effects. We tested the hypothesis that the topical application of capsaicin ointment over the palmar digital nerves would attenuate the clinical effects of a reversible model of equine foot lameness. Seven healthy adult horses shod unilaterally with adjustable heart bar shoes were studied in a crossover design for 2 weeks. Grade 5.0/5.0 lameness (nonweight bearing) was induced by tightening the adjustable heart bar shoe. One hour later, capsaicin ointment was applied over the medial and lateral palmar digital nerves 3 cm proximal to the coronary band, or horses were left untreated. One week later, treatment assignments were reversed, and the experiment was repeated. The heart rate was markedly lower in treated than in untreated trials at 20 and 40 minutes after capsaicin and between 1.6 and 3.6 hours after capsaicin (P < .05). The lameness score was markedly decreased in capsaicin-treated horses at 40 minutes and from 1.3 to 4 hours after treatment (P < .05). We conclude that the topical application of capsaicin ointment over the palmar digital nerves provided measurable pain relief for up to 4 hours after treatment (P < .05). The clinical application of this analgesic technique in horses with spontaneous clinical or induced laminitis or other sources of foot pain remains to be shown.