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The term photosensitivity describes an abnormal response to sunlight. Cutaneous photosensitivity reactions require absorption of photon energy by molecules in the skin. Energy is either dispersed harmlessly or elicits chemical reactions that lead to clinical disease. Absorbing molecules can be: (1) exogenous agents applied topically or systemically, (2) endogenous molecules either usually present in skin or produced by an abnormal metabolism, or (3) a combination of exogenous and endogenous molecules that acquire antigenic properties and thus elicit a photoradiation-driven immune reaction. Photosensitivity disorders occur only in body regions exposed to solar radiation (Fig. 10-1).

Figure 10-1

Variations in solar exposure on different body areas.

Three types of acute photosensitivity:

  1. A sunburn-type response with skin changes simulating a normal sunburn such as in phototoxic reactions to drugs or phytophotodermatitis (PPD).

  2. A rash response with macules, papules, or plaques, as in eczematous dermatitis. These are usually photoallergic in nature.

  3. Urticarial responses are typical for solar urticaria; but urticarial lesions can also occur in erythropoietic porphyria.

Chronic photosensitivity : Chronic repeated sun exposures over time result in polymorphic skin changes that have been termed dermatoheliosis (DHe) or photoaging. A classification of skin reactions to sunlight is shown in Table 10-1.

TABLE 10-1Simplified Classification of Skin Reactions to Sunlight


The main culprit of solar radiation-induced skin pathology is the ultraviolet (UV) portion of the solar spectrum. Ultraviolet radiation (UVR) is divided into two principal types: UVB (290 to 320 nm), the "sunburn spectrum," and UVA (320 to 400 nm) that is subdivided into UVA-1 (340 to 400 nm) and UVA-2 (320 to 340 nm). The unit of measurement of sunburn is the minimum erythema dose (MED), which is the minimum UV exposure that produces an erythema 24 h after a single exposure. UVB erythema develops in 6 to 24 h and fades within 72 to 120 h. UVA erythema develops in 4 to 16 h and fades within 48 to 120 h.


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