Photosensitivity to exogenous agents is broadly divided into phototoxicity and photoallergy; it is caused by topical or systemic agents that absorb ultraviolet A (UVA) radiation.
Phototoxicity occurs in anyone exposed to sufficient doses of phototoxic agent and UV radiation; it 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 antiinflammatory agents in Europe. It presents as eczematous eruption on sun-exposed areas.
Phototoxicity and photoallergy do occur in patients with skin of color, frequently resulting in postinflammatory hyperpigmentation.
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 6-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. This is covered in Chap. 124.
This chapter focuses on photosensitivity induced by exogenous agents, which 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 97-1). 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 (Table 97-1).
++ Table Graphic Jump Location TABLE 97-1Characteristics of Phototoxicity and Photoallergy ||Download (.pdf) TABLE 97-1 Characteristics of Phototoxicity and Photoallergy
| ||PHOTOTOXICITY ||PHOTOALLERGY |
|Clinical presentation ||Exaggerated sunburn reaction: erythema, edema, vesicles, and bullae; burning, stinging; frequently resolves with hyperpigmentation ||Eczematous lesions; usually pruritic |
|Histologic features ||Eosinophilic keratinocytes, epidermal necrosis, dermal edema, sparse dermal infiltrate of lymphocytes, macrophages, and neutrophils ||Spongiotic dermatitis, dermal lymphohistiocytic infiltrate |
|Pathophysiology ||Direct tissue injury ||Type IV delayed hypersensitivity response |
|Occurrence after first exposure ||Yes ||No |
|Onset after exposure ||Minutes to hours ||24-48 hours |
|Dose of agent needed for reaction ||Large ||Small |
|Cross-reactivity with other agents ||None ||Common |
|Diagnosis || || |
|Topical agent ||Clinical ||Photopatch tests |
|Systemic agent ||Clinical + phototests ||Clinical + phototests; possibly photopatch tests |
Even though hundreds of medications in the United States are reported to cause photosensitivity, only a small number of them induce ...