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Nerve endings shrivel away from topical capsaicin

PainSci » bibliography » Kennedy et al 2010
Tags: chronic pain, good news, pain problems

Three articles on PainSci cite Kennedy 2010: 1. Heat for Pain and Rehab2. Voltaren Gel & Friends, A Review and Guide3. Complete Guide to Frozen Shoulder

PainSci summary of Kennedy 2010: ?This page is one of thousands in the bibliography. It is not a general article: it is focused on a single scientific paper, and it may provide only just enough context for the summary to make sense. Links to other papers and more general information are provided wherever possible.

Ever wish you could get rid of some nerve endings? It turns out that you can — just apply chili peppers! This experiment showed that nerve endings shrivel away from an application of capsaicin, the active ingredient in peppers. The effect was quite dramatic.

Healthy volunteers took one for the team: a single application of highly concentrated capsaicin on their thighs, for just one hour. The density of nerve endings and sensitivity to various stimuli was recorded before and after, and then checked again after 1 and 12 and 24 weeks. The results were amazing: nerve ending density was down 80% after a week, and pain sensitivity was also reduced (though much less). Touch sensitivity reduced slightly, and heat and cold sensation remained normal.

Over the next several weeks, the nerves regenerated and sensation returned to normal. Given this surprisingly potent effect on nerve endings, capsaicin may be an effective and safe way to treat some pain problems.

~ Paul Ingraham

original abstract Abstracts here may not perfectly match originals, for a variety of technical and practical reasons. Some abstacts are truncated for my purposes here, if they are particularly long-winded and unhelpful. I occasionally add clarifying notes. And I make some minor corrections.

Desensitization of nociceptive sensory nerve endings is the basis for the therapeutic use of capsaicin in neuropathic pain syndromes. This study evaluated the pharmacodynamic effects of a single 60-minute application of NGX-4010, a high-concentration (8% w/w) capsaicin patch, on both thighs of healthy volunteers.

Epidermal nerve fiber (ENF) density and quantitative sensory testing (QST) using thermal, tactile, and sharp mechanical-pain (pinprick) stimuli were evaluated 1, 12 and 24 weeks after capsaicin exposure. After 1 week, there was about an 80% reduction of ENF density compared to unexposed sites. In addition, there was about an 8% increase in tactile thresholds compared to baseline and the proportion of stimuli reported as sharp mechanical pain decreased by about 15 percentage points.

Twelve weeks after exposure to capsaicin, ENF regeneration was evident, but not complete, and sharp mechanical-pain sensation and tactile thresholds did not differ from unexposed sites. Nearly full (93%) ENF recovery was observed at 24 weeks. No statistically significant changes in heat- or cold-detection thresholds were observed at any time point. NGX-4010 was generally well tolerated. Transient, mild warming or burning sensations at the site of application were common adverse effects.

PERSPECTIVE: This article evaluates the effect of a single 60-minute NGX-4010 application on ENF density and QST in healthy volunteers followed for 24 weeks. The results help predict the long-term safety of NGX-4010 applications in patients.

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