]. Cosmet. Sci., 56, 153-166 (May/June 2005) Detection thresholds of capsaicin: A ��w'lest to assess facial skin neurosensitivity ROLAND JOURDAIN, PHILIPPE BASTIEN, ." OLIVIER DE LACHARRIERE, and GILLES RUBINST�NN, / L'Oreal Recherche, 90 Rue du General Roguet, 92�3 Clic1!z.._ (R/::.'./ O.d.L., G.R.), and L'O � eal Recherche, 1 �ve , Euge17,_Schue;jet:�, 93601 Au/nay sous Bots (P.B.), France. · ··.. ' · � '·", ., '· Accepted for publication February 15, 200 5. ', �· -, .·� ' '· /�' �: ) . I Synopsis ·,� / The goal of this study was to assess the accuracy/reliability of a new test li.!.!n�ea$tlre cutaneous neurosensitivity. The test was carried out on a random population of 150 h a adult�omen and was based on the determination of individual detection thresholds of topically applied c psaJfn. Five capsaicin concentrations were used in 10% ethanol aqueous solution (3.16 x 10- 5 % 1 x 10- 4 %\ 3.16 x 10- 4 % 1 x 10- 3 % 3.16 x 10-'%). The methodology used to attain the detection threshold was capsaicin application in increasing concentration on the nasolabial folds. The vehicle was simultaneously applied following a split-face, single-blind plan. The test was stopped as soon as the subject reported a specific sensation lasting more than 30 seconds on the capsaicin side. The safety of the test was judged as excellent by the panelists since all the reported sensations were considered as slightly or moderately perceptible. The test allowed the classification of the test population according to six threshold levels corresponding to the sensitive reaction to one of the five capsaicin concentrations and to the absence of sensitivity to the highest concentration. Surprisingly, the distribution of the population was not unimodal and seemed to reveal the existence of two different sub-groups: individuals with a low capsaicin detection threshold and those with a high threshold. These two sub-populations strongly differed in their respective self-perception of sensitive skin. The higher the self-declared sensitive skin incidence was, the lower the detection threshold was. This new test of skin neurosensitivity is easy, quick, and truly painless. It appears to be a promising tool for the cosmetic diagnosis of sensitive skin. INTRODUCTION Some individuals have a skin that reacts more easily than that of others to certain environmental factors (wind, cold, fast changes in temperature) or to certain topically applied products. This skin reactivity expresses itself, especially on the face, in a variety of discomfort signs (stinging, burning, itching, tingling) associated or not with erythe- ma and scaling (1). This phenomenon is called "sensitive skin." The unpleasant sensa- tions are at least partly due to the stimulation of cutaneous nerve endings specialized in pain transmission, called nociceptors. Different epidemiological surveys have shown that sensitive skin is a common and widespread phenomenon that concerns about 50% of the adult female population in industrialized countries (2-4). There are a lot of similarities 153

154 JOURNAL OF COSMETIC SCIENCE in the perception of sensitive skin by women from different geographical areas (4) and ethnic origins (3 ). Incidence of self-perceived sensitive skin is lower in the male popu- lation (30%) (2,4). It tends to decrease with age (4) and in summer (5 ). Although these unpleasant cutaneous signs tend to disappear quickly, this condition can render the use of certain kinds of cosmetic product very problematic (2,3). The uncomfortable sensa- tions with no visible signs represent 50% of the reported adverse reactions to cosmetics and toiletries (6) and could be the very first symptoms of an irritant contact dermatitis (7). Consequently, because of the awareness of that problem, cosmetic manufacturers have tried to develop formulations without ingredients likely to induce uncomfortable sensations or containing soothing ingredients intended to decrease skin over-reactivity (8). Products labeled for sensitive skins have met with a growing success: they are purchased by 80% of self-assessed sensitive skin subjects and by 25% of those with non-sensitive skin (9). Sensitive skin is not a pathological disorder (1). This condition is not easy to assess because it lacks both a consensual definition (9) and visible, physical, or histologically measurable signs. Consequently, there is neither a simple nor pertinent method available to assess sensitive skin (10), unlike the dry or greasy character of skin. The absence of any measurable physical features of self-declared sensitive skin by classic methods has even led some authors to question the reality of this skin condition (11). Owing to the difficulty in exploring this subjective disorder, psychophysical tests based on the report of sensations induced by topically applied chemical probes have been proposed to identify individuals with sensitive skin and subsequently to test products for "sensitive skin" in this population. The approach was recently validated by functional magnetic resonance imaging (£MRI). The neurophysiological reality of sensitive skin was illustrated by a specific pattern of brain activation in self-assessed sensitive-skin subjects when a chemical probe was applied to their face (12). Moreover, self-reported unpleasant sensations have been reported as a useful tool for irritancy assessment of detergents and soaps (7). In this study, panelists differentiated products in terms of sensation of dryness several washings before observable differences were detected. Nevertheless, much greater attention was given to the visually observable or instrumentally measurable signs of irritation such as redness and inflammation (13). The lactic acid stinging test, proposed in 1977 ( 14), relies on the intensity of stinging sensations induced by a lactic acid solution applied on the nasolabial folds. The subjects who report stinging sensations are called "stingers." Using a slightly modified procedure (1), this test is currently performed by cosmetic manufacturers for the selection of "stingers." This selected population prone to experience neurosensory problems with topical products is asked to test new products to substantiate claims indicating that they are appropriate for sensitive skin. Although very useful for product safety, the lactic acid stinging test does not fully render the complexity of self-assessed sensitive skin, as illustrated by the discrepancy between acid lactic response and self-perception of sen- sitive skin (5,9,10). In 2000, this difference was taken into account for the recommen- dation to include "stingers" with a concomitant self-declared sensitive skin as panelists for safety testing (9). Owing to the great similarity of symptoms induced by topically applied capsaicin to those associated with sensitive skin (13), a new elicitation test using a 0.075% emulsion of a pungent compound extracted from chili peppers was proposed in the 1990s to