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MEDICAL BIOLOGY: ON PHOTOSENSITIVITY

The following points are made by Warwick L. Morison (New Engl. J. Med. 2004 350:1111):

1) Photosensitivity can be defined by the development of a rash, an exacerbation of an existing rash, an exaggerated sunburn, or symptoms such as pruritus or paresthesias after exposure to sunlight or an artificial source of light. Photosensitivity is common and probably affects members of all racial and ethnic groups, although there have been no studies of its prevalence in the general population. Polymorphous light eruption is the most common form of photosensitivity and probably accounts for more than 90 percent of all cases. This condition was found to have a prevalence of 10 percent in a survey of 271 apparently healthy people in Boston(1) and in other studies was diagnosed in 21 percent of 397 people in Sweden,(2) 14 percent of 182 in London,(3) and 5 percent of 172 in Perth, Australia.(3)

2) There is a broad spectrum of degrees of photosensitivity among people in whom polymorphous light eruption develops. Those who have a low threshold for the condition can tolerate only minutes of exposure, whereas many people have a high threshold and require prolonged exposure to sunlight before the reaction develops. People who have a mild form of the disease often do not consult a physician or see a primary care physician rather than a dermatologist.(3) Polymorphous light eruption is a recurrent condition that persists for many years in most patients.(4) Other types of photosensitivity are much less common, but most of them are associated with serious disability either because of their chronic nature or because of their severity and are more likely than polymorphous light eruption to bring the patient to a physician.

3) Most cases of photosensitivity develop after exposure to sunlight. Because exposure to sunlight is typically associated with exposure to other agents capable of causing a rash, other causes of what appears to be photosensitivity must be considered. Insect bites may be confined to exposed skin, but the central punctum suggests the diagnosis. Plants may cause a contact dermatitis, but the eruption is asymmetric and localized. Sunscreens and airborne allergens may be a cause of eczema and should therefore be considered in the diagnosis of photosensitive eczema.

4) Polymorphous light eruption is the most common cause of acute or episodic photosensitivity. Lupus erythematosus and drug reactions are less common but important causes. Polymorphous light eruption usually begins in the first three decades of life and occurs more commonly in women than in men. The rash develops within hours to a day or more after a specific exposure to sunlight and lasts for several days to a week or more; in some cases, multiple exposures are required to elicit the rash. Frequently, the rash appears in the spring, and episodes become less marked or cease altogether toward the end of summer. The exceptions to this are severe cases, in which episodes may occur at any time of the year, and cases in people who vacation in a sunny climate during the winter. The rash typically involves the V of the neck and the arms, legs, or both. The hands and face, which are exposed to sunlight in both summer and winter, tend to be spared. Ultraviolet A (UVA) radiation is the most common cause of the reaction, although in a minority of cases the reaction is induced by ultraviolet B (UVB) radiation or by a combination of UVA and UVB radiation.(4,5)

References (abridged):

1. Morison WL, Stern RS. Polymorphous light eruption: a common reaction uncommonly recognized. Acta Derm Venereol 1982;62:237-240

2. Ros AM, Wennersten G. Current aspects of polymorphous light eruptions in Sweden. Photodermatology 1986;3:298-302

3. Pao C, Norris PG, Corbett M, Hawk JLM. Polymorphic light eruption: prevalence in Australia and England. Br J Dermatol 1994;130:62-64

4. Hasan T, Ranki A, Jansen CT, Karvonen J. Disease associations in polymorphous light eruption: a long-term follow-up study of 94 patients. Arch Dermatol 1998;134:1081-1085

5. Elpern DJ, Morison WL, Hood AF. Papulovesicular light eruption: a defined subset of polymorphous light eruption. Arch Dermatol 1985;121:1286-1288

New Engl. J. Med. http://www.nejm.org

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