Polymorphous light eruption (PMLE or PLE) is the most common form of photodermatosis. Photodermatoses are skin diseases which are caused by exposure to sunlight; the term “sun poisoning” is sometimes used to describe this family of conditions. Although doctors have researched polymorphous light eruption fairly extensively, the condition appears to be idiopathic in origin, meaning that no cause has been determined.
Typically, polymorphous light eruption starts under the age of 30, and it is more common in women and residents of the Northern Hemisphere. After an exposure to sunlight, the skin develops an itchy red rash, which often starts in the form of spots which eventually merge together. Within one day to a week, the rash disappears, but it reappears if the patient is exposed to sun again.
The mechanics of polymorphous light eruption are not fully understood, but doctors suspect that it is an acquired reaction to ultraviolet light. It has been theorized that the condition is caused by compounds in the skin which change in reaction to UV exposure, triggering an immune system response which in turn creates the rash. As the patient grows older, the rash can grow much more severe with each exposure.
Rashes caused by polymorphous light eruption can be treated with the use of topical corticosteroids and ingested antihistamines, drugs which inhibit the immune system reaction. These drugs can also sometimes be used to manage the skin's response to sun exposure. The best treatment for polymorphous light eruption is desensitization therapy, in which the patient is exposed to controlled amounts of UV light for set periods in the hopes of desensitizing the body so that the reaction will not occur again.
This sun sensitivity is not believed to be harmful, but it can be irritating or embarrassing, as the rash favors the upper chest, arms, and face, all areas which are highly visible. To avoid the recurrence of the rash, some people like to seek treatment, although this is generally not required. When it recurs, the rash usually resolves on its own. However, if it does not disappear within a week, it is a good idea to consult a dermatologist, and to mention previous episodes of PMLE. Other forms of photodermatosis can sometimes resemble PMLE, so if the diagnosis has not been confirmed, it may be wise to seek the advice of a doctor.