Photosensitivity and other reactions to light

Vitamin D Photochemistry Cutaneous exposure to UV-B causes photolysis of epidermal 7dehydrocholesterol converting it to pre-vitamin D3, which then undergoes a temperature-dependent isomerization to form the stable hormone vitamin D3. This compound then diffuses to the dermal vasculature and circulates systemically where it is converted to the functional hormone 1,25-dihydroxyvitamin D3[1,25(OH)2D3]. Vitamin D metabolites from the circulation or those produced in the skin itself can augment epidermal differentiation signaling.

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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.

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Porphyria cutanea tarda is the most common type of human porphyria and is associated with decreased activity of the enzyme uroporphyrinogen decarboxylase associated with a number of gene mutations. There are two basic types of PCT: (1) the sporadic or acquired type, generally seen in individuals ingesting ethanol or receiving estrogens; and (2) the inherited type, in which there is autosomal dominant transmission of deficient enzyme activity. Both forms are associated with increased hepatic iron stores.

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Topical Systemic 6-Methylcoumarin + Aminobenzoic acid and esters + Bithionol + Chlorpromazine + Diclofenac + Fluoroquinolones + Halogenated salicylanilides + Hypericin (St John's Wort) + + Musk ambrette + Piroxicam + Promethazine + Sulfonamides + Sulfonylureas + A very uncommon type of persistent photosensitivity is known as chronic actinic dermatitis. These patients are typically elderly men with a long history of preexisting allergic contact dermatitis or photosensitivity. They are usually exquisitely sensitive to UV-B, UV-A, and visible wavelengths.

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Polymorphous Light Eruption After sunburn, the most common type of photosensitivity disease is polymorphous light eruption (PLE), the mechanism of which is unknown. Many affected individuals never seek medical attention because the condition is often transient, becoming manifest each spring with initial sun exposure but then subsiding spontaneously with continuing exposure, a phenomenon known as "hardening.

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Immunologic Effects Exposure to solar radiation causes local (inhibition of immune responses to antigens applied at the irradiated site) and systemic (inhibition of immune responses to antigens applied at remote unirradiated sites) immunosuppression. The action spectrum for UV-induced immunosuppression closely mimics the absorption spectrum of DNA. Pyrimidine dimers in LCs may inhibit antigen presentation. The absorption spectrum of epidermal urocanic acid closely mimics the action spectrum for UV-B-induced immunosuppression.

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Molecular Targets for UVR-Induced Skin Effects Epidermal DNA, predominantly in keratinocytes and in Langerhans cells (LCs), which are dendritic antigen-presenting cells, absorbs UV-B and undergoes structural changes including the formation of cyclobutane dimers and 6,4photoproducts. These structural changes are potentially mutagenic and can be repaired by mechanisms that result in their recognition and excision and the reestablishment of normal base sequences.

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Harrison's Internal Medicine Chapter 57. Photosensitivity and Other Reactions to Light Solar Radiation Sunlight is the most visible and obvious source of comfort in the environment. The sun provides the beneficial effects of warmth and vitamin D synthesis; however, acute and chronic sun exposure also have pathologic consequences. Few effects of sun exposure beyond those affecting the skin have been identified, but cutaneous exposure to sunlight is the major cause of human skin cancer and can exert immunosuppressive effects as well.

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