Polymorphous light eruption, also known as polymorphic light eruption, is a rash caused by sun exposure in people who have developed sensitivity to sunlight. The rash usually appears as red, tiny bumps or slightly raised patches of skin.
Polymorphous light eruption occurs most often during spring and early summer when a person's exposure to sunlight increases. Repeat episodes are less likely as the summer progresses. But the rash often recurs each year after the first incident.
Polymorphous light eruption usually goes away on its own without scarring within 10 days. People with severe or persistent rashes may need treatment with medication.
The term "eruption" refers to the rash, which usually appears 30 minutes to several hours after exposure to sunlight. The rash typically appears on areas of the body that tend to be covered during winter but exposed in summer: the upper chest, front of the neck and arms.
Characteristics of the rash may include:
- Dense clusters of small bumps and blisters
- Red, raised rough patches
- Itching or burning
Rarely people may have other signs or symptoms, such as fever, chills, headache or nausea. These conditions may be the result of an associated sunburn rather than polymorphous light eruption.
When to see a doctor
See your doctor if you have any rash with no obvious cause, such as a known allergy or recent contact with poison ivy.
Polymorphous light eruption rashes look similar to rashes caused by other diseases, some of which are serious. So it's important to get a prompt diagnosis and appropriate treatment.
Seek immediate medical care if your rash is:
- Accompanied by fever
The exact cause of polymorphous light eruption isn't well-understood. The rash appears in people who have developed sensitivity to components of sunlight, and in particular ultraviolet (UV) radiation from the sun or other sources, such as tanning beds or tanning lamps. This sensitivity is called photosensitivity. It results in immune system activity that causes a rash.
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. UVA may even penetrate through most sunscreens. So exposure to sunlight through windows or even sunscreen-protected skin may cause a reaction in some people with photosensitivity.
Sensitivity to sunlight lessens with repeated exposure in polymorphous light eruption. Features of polymorphous light eruption are somewhat predictable:
- 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 each spring or early summer.
- Some people gradually become less sensitive over several years and eventually no longer experience the annual rash.
Anyone can develop polymorphous light eruption, but several factors are associated with an increased risk of the condition:
- Being female
- Experiencing the first episode during the teenage years or 20s
- Having light skin and living in northern regions
- Having a family history of the condition