Chapter 057. Photosensitivity and Other Reactions to Light (Part 8)

Natural photoprotection is provided by structural proteins in the epidermis, particularly keratins and melanin. The amount of melanin and its distribution in cells is genetically regulated, and individuals of darker complexion (skin types IV–VI) are at decreased risk for the development of acute sunburn and cutaneous malignancy.Other forms of photoprotection include clothing and sunscreens. Clothing constructed of tightly woven sun-protective fabrics, irrespective of color, affords substantial protection. Wide-brimmed hats, long sleeves, and trousers all reduce direct exposure. Sunscreens are now considered to be over-the-counter drugs and category I ingredients are recognized by the U.S. Food and Drug Administration (FDA)...
Nội dung trích xuất từ tài liệu:
Chapter 057. Photosensitivity and Other Reactions to Light (Part 8) Chapter 057. Photosensitivity and Other Reactions to Light (Part 8) Natural photoprotection is provided by structural proteins in the epidermis,particularly keratins and melanin. The amount of melanin and its distribution incells is genetically regulated, and individuals of darker complexion (skin typesIV–VI) are at decreased risk for the development of acute sunburn and cutaneousmalignancy. Other forms of photoprotection include clothing and sunscreens. Clothingconstructed of tightly woven sun-protective fabrics, irrespective of color, affordssubstantial protection. Wide-brimmed hats, long sleeves, and trousers all reducedirect exposure. Sunscreens are now considered to be over-the-counter drugs andcategory I ingredients are recognized by the U.S. Food and Drug Administration(FDA) as monographed and safe and effective. These are listed in Table 57-5.Sunscreens are rated for their photoprotective effect by their SPF. The SPF issimply a ratio of the time required to produce sunburn erythema with and withoutsunscreen application. The monograph stipulates that sunscreens must be rated ona scale ranging from minimal (SPF ≤2 and ≥12) to moderate (SPF ≤12 and ≥30) tohigh (SPF ≥30, labeled as 30+). No SPF number >30 can be placed on the label. Table 57-5 FDA Category 1 Monographed Sunscreen Ingredientsa Ingredients Maximum Concentration, % p-Aminobenzoic acid (PABA) 15 Avobenzone 3 Cinoxate 3 Dioxybenzone (benzophenone-8) 3 Ecamsuleb 15Homosalate 15Menthyl anthranilate 5Octocrylene 10Octyl methoxycinnamate 7.5Octyl salicylate 5Oxybenzone (benzophenone-3) 6Padimate O (octyl dimethyl PABA) 8Phenylbenzimidazole sulfonic acid 4Sulisobenzone (benzophenone-4) 10Titanium dioxide 25 Trolamine salicylate 12 Zinc oxide 25 a FDA, U.S. Food and Drug Administration. b Recently approved by the FDA. In addition to light absorption, a critical determinant of the sustainedphotoprotective effect of sunscreens is their water-resistance. The FDAmonograph has also defined strict testing criteria for sunscreens making this claim. Some degree of photoprotection can also be achieved by limiting the timeof exposure during the day. Since the majority of an individuals total lifetime sunexposure may occur by the age of 18, it is important to educate parents and youngchildren about the hazards of sunlight. Simply eliminating exposure at midday willsubstantially reduce lifetime UV-B exposure. Phototherapy and Photochemotherapy UV can also be used therapeutically. The administration of UV-B alone orin combination with topically applied agents can induce remissions of psoriasisand atopic dermatitis. Photochemotherapy in which topically applied or systemically administeredpsoralens are combined with UV-A (PUVA) is also effective in treating psoriasisand in the early stages of cutaneous T cell lymphoma and vitiligo. Psoralens aretricyclic furocoumarins that, when intercalated into DNA and exposed to UV-A,form adducts with pyrimidine bases and eventually form DNA cross-links. Thesestructural changes are thought to decrease DNA synthesis and relate to theimprovement that occurs in psoriasis. The reason that PUVA photochemotherapyis effective in cutaneous T cell lymphoma is not clear. In addition to its effects on DNA, PUVA photochemotherapy alsostimulates epidermal thickening and melanin synthesis; the latter provides therationale for its use in the depigmenting disease vitiligo. Oral 8-methoxypsoralenand UV-A appear to be most effective in this regard, but as many as 100treatments extending over 12–18 months may be required to promote satisfactoryrepigmentation. Not surprisingly the major side effects of long-term UV-B phototherapyand PUVA photochemotherapy mimic those seen in individuals with chronic sunexposure and include skin dryness, actinic keratoses, and an increased risk of skincancer. Despite these risks, the therapeutic index of these modalities continues tobe excellent. Further Readings Lim HW et al: Sunlight, tanning booths, and vitamin D. J Am AcadDermatol 52:868, 2005 [PMID: 15858480] Miller AJ, Mihm MC Jr: Melanoma. N Engl J Med 355:51, 2006 [PMID:16822996] Morison WL: Photosensitiv ...