The systematic evaluation of photosensitive patients involves a comprehensive history, physical examination, phototesting, and, if necessary, photopatch testing and laboratory evaluation. Polymorphous light eruption, chronic actinic dermatitis, solar urticaria, and photosensitivity secondary to systemic medications are the most commonly encountered photodermatoses in dermatology clinics worldwide.
Key points
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There are 4 general categories of photodermatoses: (1) immunologically mediated photodermatoses, (2) drug-induced and chemical-induced photosensitivity, (3) defective DNA repair disorders, and (4) photoaggravated conditions.
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Significant components of the history include age of onset, exposure to photosensitizers, interval between sun exposure and development of eruptions, season and duration of eruption, effect of window glass, and family history.
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Careful evaluation of the distribution of lesions on sun-exposed, relatively sun-exposed, and sun-protected areas should be done during the physical examination.
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Phototesting can be used to confirm the presence of a photosensitivity disorder, and photopatch testing is used to evaluate patients with photoallergic contact dermatitis.
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For most photodermatoses, strict photoprotection is first-line therapy.
Overview
Only a limited portion of the solar spectrum reaches the Earth’s surface, and this includes 2% of ultraviolet radiation (UVR), 32% of visible light, and 66% of infrared light. UVR is divided into ultraviolet B (UVB; 290–320 nm, the sunburn spectrum) and ultraviolet A (UVA; 320–400 nm). UVA is further subdivided into UVA-1 (340–400 nm) and UVA-2 (320–340 nm). UVB, and to a lesser extent UVA-2, is mainly responsible for erythema, whereas UVA is predominantly responsible for tanning, photoaging, and drug-induced photosensitivity.
Cutaneous photosensitivity is due to the existence of molecules called chromophores that absorb UVR during exposure to the sun. DNA is the most abundant chromophore, triggering UVR-related changes such as tanning, sun burning, hyperplasia, aging, and carcinogenesis. Only certain individuals, however, develop aberrant reactions to UVR, also known as photodermatoses.
Photodermatoses are disorders that are caused or exacerbated by exposure to UVR or visible light, and can be classified into 4 broad categories: (1) immunologically mediated photodermatoses, previously referred to as idiopathic; (2) drug-induced and chemical-induced photosensitivity (either exogenous from ingested or externally applied drugs or chemicals, or endogenous, as in cutaneous porphyrias); (3) photoaggravated dermatoses, including autoimmune diseases, infectious conditions, and nutritional deficiencies; and (4) defective DNA repair disorders. Box 1 lists the photodermatoses by these categories. Furthermore, photodermatoses can range from extremely rare disorders such as hydroa vacciniforme (prevalence of 0.34 per 100,000) to common disorders such as polymorphous light eruption (PMLE; prevalence 10%–20% of the general population).
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