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Causes
By Mayo Clinic staffThe exact cause of polymorphous light eruption isn't well understood. The rash appears in people who have developed a sensitivity to ultraviolet (UV) radiation from the sun or other sources, such as tanning beds or tanning lamps. This sensitivity results in sunlight-induced immune system activity that produces inflammation and a rash.
UV radiation
UV radiation is a wavelength of sunlight in a range too short for the human eye to see. UV light that reaches the earth is divided into two wavelength bands — ultraviolet A (UVA) and ultraviolet B (UVB).
A person with photosensitivity can react to both types of UV radiation. Although UVB doesn't penetrate glass, UVA does. Therefore, exposure to sunlight through windows may cause a reaction in some people with photosensitivity.
Photosensitivity
Sensitivity to sunlight lessens with repeated exposure. Therefore, there are somewhat predictable features of polymorphous light eruption:
- An episode is most likely to occur after the first one or two exposures to sunlight after a long period of no exposure. This usually means that an episode occurs during the spring or early summer or during a winter vacation in a sunnier location.
- Episodes are less likely to occur as the summer progresses.
- After the first episode of polymorphous light eruption, additional episodes are likely to recur on an annual basis each spring or early summer.
- Some people gradually become less sensitive over several years and no longer experience recurring episodes.
- Habif T. Clinical Dermatology: A Color Guide to Diagnosis and Therapy. 5th ed. Philadelphia, Pa.: Mosby Elsevier; 2009. http://www.mdconsult.com/books/page.do?eid=4-u1.0-B978-0-7234-3541-9..00028-6--s0355&isbn=978-0-7234-3541-9&type=bookPage§ionEid=4-u1.0-B978-0-7234-3541-9..00028-6--s0355&uniqId=227127777-3#4-u1.0-B978-0-7234-3541-9..00028-6--s0355. Accessed Nov. 18, 2010.
- Honigsmann H. Polymorphous light eruption. Photodermatology, photoimmunology & photomedicine 2008;24:155.
- Facts about sunscreens. American Academy of Dermatology. http://www.aad.org/media/background/factsheets/fact_sunscreen.htm. Accessed Nov. 18, 2010.
- Bylaite M, et al. Photodermatoses: Classification, evaluation and management. The British Journal of Dermatology 2009;161(suppl 3):61.
- Systemic lupus erythematosus (SLE). The Merck Manuals: The Merck Manual for Healthcare Professionals. http://www.merckmanuals.com/professional/sec04/ch032/ch032g.html. Accessed Nov. 19, 2010.
- Photosensitivity. The Merck Manuals: The Merck Manual for Healthcare Professionals. http://www.merckmanuals.com/professional/sec10/ch115/ch115c.html. Accessed Nov. 19, 2010.
- Sun-protective clothing: Wear it well. Federal Trade Commission. http://www.ftc.gov/bcp/edu/pubs/consumer/alerts/alt094.shtm. Accessed Nov. 18, 2010.
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