INTRODUCTION
Sensitive skin, or increased irritation or sensorial response to skin care products, environmental factors, and/or stressors compared to population norms, has been increasingly studied over the past 30 years. The face has been shown to be more sensitive than other parts of the body1,2; however, skin sensitivity can also vary significantly between individuals due to a number of factors such as ethnicity, gender, age, product irritancy, etc.3-7 Rising prevalence of sensitive skin and research on previously unknown factors that increase the symptoms of sensitive skin has further fueled the need to better understand the condition, as well as research into different skin care technologies to address the symptoms and minimize the onset of future sensitive skin events. The intensity of the symptoms varies widely and can also change throughout an individual’s lifetime. Although erythema and dryness are typically involved, sensations such as stinging, burning, tingling, tightness, itching, pain, etc. can persist with no clinical signs of dryness or erythema. Sensitive skin sufferers have lower Quality of Life (QoL) scores compared to normal-skin individuals, and impaired barrier function is the main biophysical finding across the different subtypes of sensitive skin individuals.8-11 Research in this area has resulted in more comprehensive subtyping of sensitive skin, to better understand the underlying causes and, consequently, potential skin care regimens to alleviate symptoms for a particular sub-type.Kligman et al12 described the characteristics of sensitive skin by different symptoms:
Pons-Giraud 14 proposed three clinical forms or subtypes of sensitive skin individuals:
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The methods for positively identifying sensitive skin individuals or subtypes have evolved over time. One of the first methods used was the lactic sting assay, 15 to identify sting potential individuals for facial skin. One limitation to this method is that an individual’s response to sting can vary over time and in addition this can be a very small subset of the population, whereas the prevalence for individuals with sensitive skin is much higher. In addition, a negative response in the lactic acid sting test is not a predictor for a sensitive skin response to other ingredients. Retinoid intolerance, for example, is fairly common in sensitive-skin individuals and will not necessarily be identified in this test. The time course for the dryness, irritation, and other symptoms in retinoid-intolerant populations follow
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ing topical application also varies widely across individuals. Consequently, subjective surveys and QoL questionnaires have been validated to confirm the sensitive skin subtypes and/or better understand the impact of products designed specifically for sensitive skin.Despite the prevalence of sensitive skin, and possibly because the symptoms are not always visible, many individuals will not seek help of a physician to alleviate the symptoms. 13,15 Characteristics of Sensitive Skin Subject Self-Assessment Surveys and Quality of Life Measures Querleux et al showed a strong prevalence in individuals with self-perceived sensitive skin prone to the following symptoms or triggers: irritation, redness, reactivity to products, temperature (eg, hot and cold environment or rapid change in environment), wind, sun, and pollution. 17 These questions, particularly when an individual responds to several factors, are a valuable way to confirm if there is general facial skin sensitivity. In this study, functional MRI (fMRI) was used to evaluate subjects’ responses to a lactic acid sting test in both sensitive skin and normal subjects. Application of lactic acid increased activity in the primary sensorimotor cortex contralateral to the application site with greater intensity in sensitive skin individuals compared to the control group.The Bauman Skin Type Indicator (BSTI) describes 16 different skin facial sub-types using the axes of dry or oily; sensitive or resistant; pigmented or nonpigmented; and wrinkled or unwrinkled (tight). 18 Within the sensitive skin category, there are four discrete subtypes described: acne, rosacea, stinging, and allergic.The links between sensitive skin and oily and dry facial skin types has been investigated in a prevalence study in a population of 1000 subjects, representative of US demographics. 19 Approximately half of the population was sensitive-skinned. The authors compared the prevalence in sensitive to non-sensitive skin individuals with dry, normal, oily, or combination facial skin, and saw that for dry and combination skin there was greater prevalence of sensitive skin individuals compared to normal skin individuals, and approximately equal prevalence for oily skin.Sensitive skin can alter an individual’s QoL, as has been assessed by the Dermatology Quality of Life Index. 9 Health-related quality of life (HRQOL), an individual’s perception of their physical, mental, and social health, is very often impaired in patients with chronic sensitive skin 14, 19-21 Misery et al. 22 assessed QoL using the short form (SF?12) questionnaire and depressive symptoms using the Hospital Anxiety and Depression (HAD) rating scale. Subjects with sensitive skin had lower QoL scores, and this worsened with increasing skin sensitivity.ing Skin Barrier and Tolerance in Sensitive Skin Compromised skin barrier is a commonality across different sensitive skin subtypes and therefore has led to study of potential morphological and biophysical differences between groups with relatively higher skin sensitivity. 9,10 Overall, sensitive skin is characterized by decreased natural moisturizing factors (NMFs), ceramides, and fatty acids, as well as an increased transepidermal water loss (TEWL) and increased permeability to exogenous environmental factors. 10,23,24 More recent studies have shown promise with other biophysical measurement evaluations of sensitive skin responses, such as fMRI17 and sensorial thresholds. 25 Sensitive Skin Populations Ethnic Skin Differences Some of the population surveys have not shown an association with sensitive skin and ethnicity .22,26 This was further studied in a series of sting tests followed by immunostaining and biophysical measures, comparing Caucasian subjects to Japanese subjects in age-matched cohorts. The goal of these studies was to see if the skin reactivity to products differed between the two populations and if there were underlying physiologic differences. Healthy female subjects provided informed consent to participate in these double-blind, Institutional Review Board-approved studies. Japanese vs Caucasian Stinger Study 27 Thirty-four Caucasian subjects were from New York, NY and the surrounding area, and thirty-one Japanese subjects were from Tokyo, Japan. Following application of glycolic acid lotion, subjects reported self-perceived unpleasant sensations (sting, itch, burn, and others). Subjects were categorized as stingers or non-stingers based on a cumulative score. Sellotapes® were collected from the sub-orbital cheek region for corneocyte structure analysis, and a board-certified dermatologist collected a punch biopsy (2 mm) from a pre-selected site on the hairline for H&E staining and immunolocalization.A greater number of Japanese women were characterized as strong stingers (48%) compared to Caucasian women (32%). Japanese women characterized as stingers were more likely to have greater numbers of fragile corneocytes (9 of 12; Figure 1). Japanese women characterized as non-stingers were more likely to have greater numbers of rigid corneocytes (12 of 16). A similar trend was observed for Caucasian women, with stingers having greater numbers of fragile corneocytes (9 of 11) and non-stingers having more rigid corneocytes (16 of 20; Figure 1).No difference in staining patterns between non-stingers and stingers was observed, or between Japanese and Caucasian. This suggested that there was no change in underlying markers of epidermal proliferation and differentiation that may contrib
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ute to increased sensitivity to sting. There were significantly fewer cells per unit area of epidermis in stingers compared to non-stingers (15%, P=0.02; Figure 2).The results of this study confirmed previous reports that Japanese women are more sensitive to irritants than their Caucasian counterparts. 7,28 The observation of greater numbers of fragile corneocytes in Japanese compared to Caucasians provided evidence that the stratum corneum of Japanese women may not be as structurally resilient as Caucasian women. Further, the increased amount of fragile corneocytes found in ‘stingers’ of both populations provided evidence that the stratum corneum of sensitive individuals may have a structural weakness. This offers a potential explanation for these individuals' increased sensitivity as topically applied irritants may be able to penetrate a weakened skin barrier. Differences in Sensory Nerve Distribution on the Cheek Between Japanese and Caucasian Stingers 29 Study populations (twelve Caucasian and twenty-three Japanese) were segmented into stingers (S) and non-stingers (NS) based on their response to glycolic acid-induced sting. Biopsies were obtained with punch biopsies (2mm) from the face (cheek site), followed by immunostaining to evaluate epidermal nerve amount and density.Hot pain (HP), cold pain (CP), warm sensation (WS), and cool sensation (CS) detection thresholds were measured by a thermal probe applied to skin using the Quantitative Sensory Testing (QST) device. Tactile thresholds were determined by touching the skin gently with Von Frey hairs.Fibers in the epidermis were clearly seen as well as nerve bundles in the dermis. The nerve terminals terminated at different levels in the epidermis, with the highest at the stratum corneum/granulosum interface (see Figures 3 and 4). Epidermal nerve fiber quantity (PGP9.5 staining) in Japanese subjects was not significantly different from Caucasian subjects.Japanese stingers had less overall nerve fiber quantity than the non-stingers (P less than 0.05), whereas there was no statistically significant difference in the Caucasian subjects. Innervation density (fiber quantity/distribution area) was five-fold less in Japanese
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