Itch in Ethnic Populations

Acta Derm Venereol 2010; 90: 227–234 REVIEW ARTICLE Itch in Ethnic Populations Hong Liang TEY1 and Gil YOSIPOVITCH2 1National Skin Centre, 1, Mandalay Road, Singapore, Singapore, and 2Department of Dermatology, Neurobiology & Anatomy, and Regenerative Medicine, Wake Forest University Health Sciences, Winston-Salem, NC, USA Racial and ethnic differences in the prevalence and clini- DIFFERENCES IN SKIN BIOLOGY AND ITCH cal characteristics of itch have rarely been studied. The aim of this review is to highlight possible associations Few studies have examined the differences between between ethnicity and different forms of chronic itch. We skin types in relation to ethnicity and neurobiology of provide a current review of the prevalence of different the skin. Differences between ethnic skin types and types of itch in ethnic populations. Genetic variation may skin properties may explain racial disparities seen in significantly affect receptors for itch as well as response pruritic dermatologic disorders. to anti-pruritic therapies. Primary cutaneous amyloidosis, a type of pruritic dermatosis, is particularly common Epidermal structure and function in Asians and rare in Caucasians and African Ameri- cans, and this may relate to a genetic polymorphism in A recent study has shown that the skin surface and the Interleukin-31 receptor. Pruritus secondary to the melanocyte cytosol of darkly pigmented skin is more use of chloroquine for malaria is a common problem for acidic compared with those of type I–III skin (1). Serine African patients, but is not commonly reported in other protease enzymes, which have significant roles as pruri- ethnic groups. In patients with primary biliary cirrho- togens in atopic eczema and other chronic skin diseases, sis, pruritus is more common and more severe in African have been shown to be significantly reduced in black Americans and Hispanics compared with Caucasians. skin too, which have significant roles as pruritogens in Racial and ethnic differences in itch and its medical care atopic eczema and other chronic skin diseases (2, 3) and are poorly understood. Research is needed to examine have increased activity in alkaline skin pH (4). Taken biological, psychosocial, and lifestyle factors that may together, these epidermal functional differences could contribute to these disparities. Key words: pruritus; race; explain differences in itch and skin irritation between Black; Asian; Caucasian; Hispanics. different skin types. There have been several studies that indicate that (Accepted March 16, 2010.) Asian skin is more sensitive to exogenous chemicals, and this may be due to a thinner stratum corneum and a Acta Derm Venereol 2010; 90: 227–234. higher density of eccrine glands (5). Japanese cheek skin Gil Yosipovitch, Department of Dermatology, Neurobio- was found to have a greater number of acid-sensing ion logy & Anatomy, and Regenerative Medicine, Wake Forest channels (ASICs) and heightened sensory thresholds in University Health Sciences, Winston-Salem, NC 27157, comparison with Caucasians. This may explain why Ja- USA. E-mail: [email protected] panese women more frequently report itch and burning sensations after application of cosmetic products (6). The prevalence of itch varies significantly across diffe- Skin innervation rent ethnic populations. Several pruritic conditions occur more frequently in certain ethnic groups and in certain To the best of our knowledge, no study has examined diseases. However, the impact of ethnicity on itch has so the differences in the innervation density of the skin far received minimal attention. between African Americans, Hispanics, and other The study of chronic itch is complicated, as it is in- ethnic populations. fluenced by multiple factors, such as age, environment, Microscopic evaluation has revealed that black skin social, cultural, level of education and psychological contains larger mast cell granules compared with white factors. Knowledge of racial differences in pruritic skin, and differences in structural properties and enzy- conditions is useful to aid diagnosis and to provide mes of mast cells have been found (7). However, the appropriate management. This short review summarizes clinical relevance of these findings to itch is unclear. the differences in skin structure and function and its Recently, Wang et al. (8) compared the differences association with itch in different ethnic populations, in sensory response and neurogenic inflammation to and describes itch entities that are specific to different topical capsaicin between four different ethnic popula- ethnic populations. tions: African Americans, East Asians, Hispanics and © 2010 The Authors. doi: 10.2340/00015555-0867 Acta Derm Venereol 90 Journal Compilation © 2010 Acta Dermato-Venereologica. ISSN 0001-5555 228 H. L. Tey and G. Yosipovitch Caucasians. Capsaicin is known to induce initial burning sensation as well as itch and is used in alleviating pain and itch via activation of TRPV1 receptor. African Ame- ricans, in sharp contrast to the three other ethnic groups, demonstrated a lack of hyperalgesia and neurogenic inflammation. Hispanics were the only group to report significant itching after capsaicin application. It would be of interest to examine whether there are any functional genetic polymorphisms in receptors for TRPV1 that may explain these findings. In addition, these prelimi- nary results may suggest that response to anti-pruritic drugs may differ between ethnic populations, similar to reports of differences in response to pain-relieving medications between different races (9, 10). ITCH IN THE GENERAL POPULATION A cross-sectional study was conducted in Norway, which involved postal questionnaire response from 18,770 adults (11). Eighty-four percent of the sample population was Norwegian, 5% was immigrants from Western countries, and 3% was immigrants from the Indian subcontinent. The presence of itch over the past week in respondents was found to be significantly more in men from East Asia (18%) and Middle East/ North Africa (13%) compared with Norwegians (7%). Psychosocial factors, however, are confounding factors as immigrants have different health behaviors and a lower socio-economic status, and these are known to result in increased morbidity (12, 13). On the other Fig. 1. Brown keratotic papules on the shins and calves of a patient with hand, in a pilot study investigating chronic itch in the lichen amyloidosis. general population, the life prevalence of itch in a sample of the German population (n = 200) was found In localized cutaneous amyloidosis, the deposition of to be 22.6% (14). amyloid either occurs along reticulin fibers and the basal membrane (peri-reticulary amyloidoses) or along DERMATOLOGIC ITCH collagen fibers (peri-collagenous amyloidosis) (17). Primary cutaneous amyloidosis has been known to be Pruritus secondary to dermatologic diseases is the more common in certain populations, namely Southeast most common cause of itch, and chronic dermatologic Asians and South Americans (18–21). itch poses significant morbidity. In a German study, Itch is often present in primary localized cutaneous an underlying dermatosis was identified in 42% of amyloidosis, in both the sporadic and hereditary forms. 263 patients with chronic pruritus (15). In a study The prevalence of itch various in different studies: 62% comparing patients with pruritus in Germany and in in Singapore (22), 82% in Saudi Arabia (23), and 90% Uganda, pruritic dermatoses were responsible in 57% in India (24). The lesions of localized cutaneous amy- (75/132) of the cases in Germany (16). In contrast, in loidosis commonly occurs over the upper back, outer the Ugandan population, dermatoses were responsible arms, and the shins and in the study by Salim et al., the for causing itch in 96% (81/84) of the cases. most common site of itch was the shins (24). Several pruritic skin conditions are unique in skin A study compared the skin innervation density of 30 of specific ethnic groups, and some of these will be Hispanic patients with clinico-pathologically proven discussed in more detail below. lichen amyloidosis with 11 healthy Hispanic controls (25). The amount and area covered by nerve fibers in the Primary localized cutaneous amyloidosis epidermis and the dermoepidermal junction were found to be significantly higher in healthy skin compared with Primary localized cutaneous amyloidosis consists of skin affected by lichen amyloidosis. No difference in in- primary macular and lichenoid amyloidosis (Fig. 1). nervation density in the papillary dermis was found. An Acta Derm Venereol 90 Ethnic itch 229 explanation for these findings may be that the pruritus in lichen amyloidosis is due to damage to the nerve fibers. The cause of this damage, however, is not known. Molecular studies have recently opened new insights into the pathogenetic mechanisms of pruritus in familial primary localized cutaneous amyloidosis. It has been proposed that when mutant oncostatin M receptor β present on keratinocytes, cutaneous nerves, and specific neurons in the dorsal root ganglia are stimulated by oncostatin M (OSM) or interleukin-31 (IL-31), signa- ling abnormalities via the OSM type II receptor and/ or the IL-31 receptor leads to keratinocyte apoptosis (with subsequent amyloid deposition), reduced number of cutaneous nerves (from possible apoptosis) and, in most individuals, pruritus (26). Lin et al. (27) subsequently identified a point mutation in the IL-31 receptor A gene in a family with hereditary primary cutaneous

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