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Reactions to ultraviolet light are varied. In many disorders, ultraviolet light aggravates but does not cause disease. Examples of this type of reaction include lupus erythematosus, dermatomyositis, porphyria cutanea tarda, dermatitis of niacin deficiency (pellagra), and recurrences of herpes simplex virus. Other disorders are caused by the sun, the most common of which is a sunburn reaction. A sunburn reaction is the inflammatory response to skin injury as a result of ultraviolet radiation. Individuals with fair skin, light eyes, and naturally light hair color are more susceptible to sunburns; however, darker pigmented skin can develop skin injury with sufficient ultraviolet light exposure.
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Exogenous photosensitivity disorders result from topical application or ingestion of an agent that causes the skin to be more sensitive to ultraviolet light. Photosensitivity disorders may be phototoxic or photoallergic. Phototoxic drug reactions occur quickly and appear as a sunburn. Photoallergic reactions occur later and exhibit eczema-like changes in the skin with vesiculation (Figure 250-12).
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Topical photosensitizers usually result in a cutaneous eruption at the sites of application. Ultraviolet exposure is necessary for the eruption to occur. Furocoumarins are the most common group of agents causing topical photoeruptions. Lime juice, fragrances, figs, celery, and parsnips contain furocoumarins. Table 250-3 lists some ingested substances that can result in a photosensitivity eruption.
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The sunburn reaction begins 2 to 6 hours after exposure and peaks in 1 to 3 days. It may be minimal with little discomfort to the patient, or it may be severe with extensive blistering. Erythema and warmth in sun-exposed areas occurs. Vesiculation is equivalent to a second-degree burn.
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The diagnosis of sunburn is clinical and typically self-reported by the patient. For phototoxic and photoallergic reactions, history and distribution of the eruption can assist in differentiation. Often a linear or spray pattern suggests that an externally applied substance is the culprit. Because photosensitizing medications are ingested and distributed throughout the body, the eruption can involve all sun-exposed areas. The characteristic distribution of a photosensitivity eruption is the face, posterior neck, dorsal hands, and extensor arms. Certain areas, including the creases of the eyelids, upper lip, submental anterior neck, and posterior auricular neck, are spared.
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The diagnosis is based on identifying the offending agent. Photopatch testing performed by a dermatologist or allergist may be helpful in identifying the photosensitizing agent. If the diagnosis is unclear, other photosensitivity disorders, such as lupus erythematosus, dermatomyositis, and polymorphous light eruption, should be excluded.
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The most important part of treatment of sunburn and photosensitivity is prevention. Avoid the midday sun, apply sunscreen liberally and frequently (ultraviolet A and B protection with sun protection factor of at least 30), wear protective clothing, and seek shade. Manufactured clothing with a sun protection factor of 50+ is available and helpful. Sunburns can be treated symptomatically with nonsteroidal anti-inflammatory drugs and tepid baths and by application of topical antibiotics to areas of vesiculation. Emollients may be soothing but will not prevent eventual exfoliation. Individuals should also be advised to avoid the sun until the eruption resolves.
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In cases of photosensitivity, discontinue the causative agent. Initial management includes topical corticosteroids and management similar to a sunburn reaction. The patient should avoid the sun until the eruption has cleared completely.