Abnormal Responses to Ultraviolet Radiation at a Glance
- Photosensitivity is broadly divided into phototoxicity and photoallergy, caused by topical or systemic agents that absorb ultraviolet A (UVA) energy.
- Phototoxicity occurs in anyone exposed to sufficient phototoxic agent and UV radiation and usually manifests as an exaggerated sunburn reaction.
- Photoallergy is an immune reaction to a UVA-modified chemical, commonly topical sunscreen agents and antimicrobials in the United States and the United Kingdom and topical nonsteroidal anti-inflammatory agents in Europe. It presents as eczematous eruption on sun-exposed areas.
- History taking is an important part of the evaluation; phototesting and photopatch testing are sometimes helpful.
- Differential diagnosis includes contact allergic or contact irritant dermatitis, airborne contact dermatitis, and other photodermatoses.
- Management consists of identification and avoidance of the precipitating agent, photoprotection, and symptomatic therapy.
Photosensitivity may be caused by exogenous or endogenous agents. It occurs when a compound, classically one with unsaturated double bonds in a six-carbon ring, absorbs radiation energy in its action spectrum, usually ultraviolet A (UVA) wavelengths. Exogenous photosensitizers can be agents administered systemically or applied topically. Well-characterized examples of photosensitivity induced by endogenous photosensitizers are the cutaneous porphyrias, which are associated with enzymatic defects in heme biosynthetic pathways that result in elevated levels of porphyrins, known phototoxic agents (see Chapter 132).
Photosensitivity induced by exogenous agents can be divided into phototoxicity and photoallergy. Phototoxicity is the result of direct tissue injury caused by the phototoxic agent and radiation. It can occur in all individuals exposed to adequate doses of the agent and the activating wavelengths of radiation (Table 92-1). In contrast, photoallergy is a type IV delayed hypersensitivity response to a molecule that has been modified by absorption of photons. It has a sensitization phase, occurs only in sensitized individuals, and requires only a minimal concentration of the photoallergen (see Table 92-1).
Table 92-1 Characteristics of Phototoxicity and Photoallergy |Favorite Table|Download (.pdf)
Table 92-1 Characteristics of Phototoxicity and Photoallergy
Exaggerated sunburn reaction: erythema, edema, vesicles, and bullae; burning, stinging; frequently resolves with hyperpigmentation
Eczematous lesions; usually pruritic
Eosinophilic keratinocytes, epidermal necrosis, dermal edema, sparse dermal infiltrate of lymphocytes, macrophages, and neutrophils
Spongiotic dermatitis, dermal lymphohistiocytic infiltrate
Direct tissue injury
Type IV delayed hypersensitivity response
Occurrence after first exposure
Onset after exposure
Minutes to hours
24 to 48 hours
Dose of agent needed for reaction
Cross-reactivity with other agents
Clinical + phototests
Clinical + phototests; possibly photopatch tests
Over 350 medications in the United States have been reported to cause photosensitivity.1 Only a small number of them, however, induce reactions frequently or have been well studied (Tables 92-2, 92-3, 92-4, and 92-5). In evaluations performed at photodermatology centers in New York City, Melbourne, ...