ANTIPRURITIC EFFECT OF PRETREATMENT WITH TOPICAL CAPSAICIN 8% ON HISTAMINE- AND COWHAGE-EVOKED ITCH IN HEALTHY VOLUNTEERS

FCAAIA Notes: Itch is sometimes but not always from allergy. Histamine released from certain cells in the skin is one cause of itch.  In those cases, antihistamines might be useful symptoms relievers.  When itch is from allergy, topical or oral steroids might be helpful.

But what about itch from other causes?  For instance, molecules released from nerve endings (“neurotransmitters”) can also trigger “neurogenic” itch that will not respond to antihistamines or steroids.  In those cases, topical capsaicin might help. Capsaicin is the molecule in chili peppers that gives them their heat. When used on the skin it acts as an analgesic and as an anti-pruritic (anti-itch) by decreasing release of cutaneous neurotransmitters.

Capsaicin is available over the counter in numerous products.  It’s worth a try if nothing else helps, isn’t it?

(Source: https://www.medscape.com/viewarticle/888090?nlid=119132_3821&src=WNL_mdplsfeat_171121_mscpedit_aimm&uac=112079PK&spon=38&impID=1489162&faf=1, Nov.21, 2017. From The British Journal of Dermatology. 2017;177(1):107-116. For Medscape articles: User name: FCAAIA, Password: Allergies)

Abstract

Background: Chronic itch is difficult to treat. Low-concentration topical capsaicin (0·006–0·05%) has previously been applied in itch therapy but evidence on its efficacy is contradictory.

Objectives: This vehicle-controlled, double-blinded study investigated the effect of topical capsaicin 8% after 1- and 24-h application on evoked itch, neurogenic inflammation and itch-associated dysaesthesia.

Methods: Sixteen healthy volunteers (aged 22 ± 0·5 years, nine female) were treated with capsaicin for 1 h and 24 h, and vehicle for 24 h on each volar forearm. Subsequently, histamine (1%, administered prick test lancets) and cowhage (40–45 spicules) were applied to the pretreated areas. Evoked itch and pain intensities were recorded for 10 min using a visual analogue scale (0–10 cm), while sensitivity to touch-evoked itch was evaluated using von Frey filaments before and after itch provocations. Neurogenic inflammation was assessed using perfusion imaging.

Results: In the vehicle areas peak itch responses to histamine and cowhage were 4·67 ± 0·58 and 5·15 ± 0·71, respectively. Capsaicin pretreatment reduced peak itch responses to histamine and cowhage after 24-h pretreatment to 1·41 ± 0·58 (P = 0·003) and 0·81 ± 0·18, (P < 0·001), respectively. Capsaicin pretreatment for 1 h reduced only cowhage-induced itch (P = 0·023). Furthermore, 24-h capsaicin pretreatment abolished punctuate hyperknesis and lowered histamine-induced neurogenic inflammation but did not affect weal reactions.

Conclusions: Topical capsaicin 8% pretreatment for 24 h reduced histaminergic and nonhistaminergic itch by about 75%, while a significant reduction (≈60%) was achieved for only nonhistaminergic itch in a standard 1-h treatment. Further investigations are needed to elucidate the clinical potential of high-concentration capsaicin as an antipruritic.

Introduction

Itch is an unpleasant sensation evoking a desire to scratch and it frequently manifests as a chronic nuisance in diseases such as urticaria, atopic dermatitis, uraemia and psoriasis. Chronic itch is often refractory and is associated with a decreased quality of life. It is relatively common; prevalence estimates are between 8·2 and 15·4% of the general population.

Topical low-concentration capsaicin (0·006–0·05%) has previously been investigated and applied in the treatment of itch in patients with conditions such as atopic dermatitis, psoriasis, pruritus ani and prurigo nodularis. Low-concentration topical capsaicin has to be applied frequently for several weeks, often resulting in low adherence, and there is contradictory evidence on its efficacy. A systematic review from 2010 concluded that there was no convincing evidence on the use of topical low-concentration capsaicin to treat itch associated with any condition and that most trials on the subject either reported insignificant findings, low effect sizes or were inadequately controlled. Recently, capsaicin 8% patches have become available and are approved for the treatment of nondiabetic neuropathic pain conditions, for example post-herpetic neuralgia. These patches contain 320–1330 times as much capsaicin as the low-concentration capsaicin previously used in clinical trials, as well as in the treatment of pruritus (and pain), thus making it possible to conduct the treatment for only 1 h every 3 months.

While studies on the effect of low-concentration capsaicin on evoked itch and in patients with chronic itch have yielded mixed results,[8] the effect of hig  h-concentration capsaicin as an antipruritic has not yet been investigated and the only evidence stems from case studies, which indicate that capsaicin 8% has a promising effect in patients with pruritus of neuropathic origin. Itch is conveyed by at least two neuronal pathways: a subgroup of nociceptive mechanoinsensitive C-fibres (CMi) that transmit a histaminergic itch and a subgroup of polymodal nociceptive C-fibres (PmC) that transmit a nonhistaminergic itch. Recent evidence suggests that itch in clinical conditions may predominantly result from activity in one of these pathways, so that itch in chronic urticaria is thought to be largely histaminergic, while itch in atopic dermatitis is thought to be largely nonhistaminergic. This proposition also accounts for the intractability to antihistamines that many itch conditions, including atopic dermatitis, exhibit.

The aim of this vehicle-controlled, double-blinded, cross-over proof-of-concept study was to investigate the effect of 1-h and 24-h pretreatment with high-concentration topical capsaicin 8% on subsequent itch responses provoked by the application of histamine (histaminergic itch) and cowhage spicules (nonhistaminergic itch). Furthermore, the effects of capsaicin on sensitivity to touch-evoked itch (STTI) and neurogenic inflammatory responses were investigated.

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