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).
Three types of acute photosensitivity:
Chronic photosensitivity: Chronic repeated sun exposures over time result in polymorphic skin changes that have been termed dermatoheliosis or photoaging. A classification of skin reactions to sunlight is shown in Table 10-1.
Basics of Clinical Photomedicine
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 hours after a single exposure. UVB erythema develops in 6 to 24 hours and fades within 72 to 120 hours. UVA erythema develops in 4 to 16 hours and fades within 48 to 120 hours.
VARIATIONS IN SUN REACTIVITY IN NORMAL PERSONS Sunburn is seen most frequently in individuals who have light skin with limited capacity to develop inducible, melanin pigmentation (tanning) after exposure to UVR. The SPT (skin phototype), which is used to estimate phototherapy dosing, is based on skin color and on a person's own estimate of sunburning and tanning (Table 10-2).
SPT I persons usually have very light skin, blond or red hair, and blue eyes. They sunburn easily with short exposures, and do not tan. SPT II persons sunburn easily, but tan with difficulty, whereas SPT III persons may have some sunburn with short exposures, but can develop marked tanning. SPT IV persons tan with ease, and do not sunburn with short exposures. Persons with constitutive brown skin are termed SPT V and with dark brown-colored skin SPT VI.