Section 10

Rare
Not so common
Common
Low morbidity
Considerable morbidity
Serious

ICD-9 : 692.70 • ICD-10 : L56.8

The term photosensitivity describes an abnormal response to light, usually sunlight, occurring within minutes, hours, or days of exposure and lasting up to weeks, months, and even longer. Cutaneous photosensitivity reactions require absorption of photon energy by appropriately shaped molecules leading to molecular deformity. Energy is either dispersed harmlessly or is directed to chemical reactions that lead to molecular, cellular, and tissue damage resulting in 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 have acquired antigenic properties and thus elicit a photoradiation-driven immune reaction. Photosensitivity disorders occur only in body regions exposed to solar radiation (5185971).

###### Image 10-1

Variations in solar exposure on different body areas.

There are three broad types of acute photosensitivity:

1. A sunburn-type response with the development of morphologic skin changes simulating a normal sunburn with erythema, edema, and bullae, such as in phototoxic reactions to drugs or phytophotodermatitis.

2. A rash response to light exposure with development of varied morphologic expressions: macules, papules, or plaques, as in eczematous dermatitis. These are usually photoallergic in nature or belong to the so-called idiopathic photodermatoses such as polymorphous light eruption.

3. Urticarial responses are typical for solar urticaria; 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, or photoaging. A classification of skin reactions to sunlight is shown in Table 10-1.

Table 10-1 Simplified Classification of Skin Reactions to Sunlight

### Basics of Clinical Photomedicine

The main culprit of solar radiation–induced skin pathology is the ultraviolet portion of the solar spectrum. Ultraviolet radiation (UVR) in photomedicine is divided into two principal types: UVB (290–320 nm), the “sunburn spectrum,” and UVA (320–400 nm). UVA is subdivided into UVA-1 (340–400 nm) and UVA-2 (320–340 nm). The unit of measurement of sunburn is the minimum erythema dose (MED), which ...

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