Polymorphous Light Eruption: Your Guide to Sun Poisoning Rash

Medically Reviewed By Raechele Cochran Gathers, MD
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Polymorphous light eruption is a form of sun allergy. It is a seasonal rash that appears in the spring or summer. People may call it a sun poisoning rash, but it is actually an immune reaction.  This article explains polymorphous light eruption symptoms, causes, diagnosis, and treatment.

What is polymorphous light eruption?

Woman wearing a sunhat and sunglasses
Alexey Kuzma/Stocksy United

Polymorphous light eruption (PMLE) is a rash that appears on newly sun-exposed areas. It is an immune system reaction to sunlight that occurs with the seasons. The rash typically begins in springtime or summertime and goes away during the fall and winter months. However, indoor tanning methods can also trigger it.

PMLE usually starts within a few hours to 2 days after sun exposure. The amount of exposure it takes to set off the reaction varies from person to person. 

The rash can last days to several weeks before it heals without scarring. Avoiding sun exposure will help it resolve faster.

PMLE can recur with repeated sunlight exposure throughout the summer. However, the skin’s sensitivity will decrease in a process doctors call “hardening.” The skin becomes more tolerant to sun exposure in this process. Sensitivity may also lessen with time and eventually resolve.

Other names for polymorphous light eruption include:

  • polymorphic light eruption
  • prurigo aestivalis
  • summer eruption
  • eczema solare

What are the symptoms of polymorphous light eruption?

A rash is the main symptom of PMLE. The rash is most often burning, itching, and red. 

The most common sites include:

  • forearms
  • chest
  • lower legs
  • décolletage, the V of the neck and upper chest exposed with open neck shirts

The rash rarely affects the face or back of the hands. Since these areas get more chronic sun exposure, hardening has likely desensitized them.

People may also experience other symptoms within 4 hours of sun exposure, lasting 1–2 hours. These include:

What does polymorphous light eruption look like?

The appearance of PMLE varies widely from person to person. Hence, the name poly, meaning many, and morphous, meaning forms.

Its various appearances can include:

  • dry, scaly plaques
  • hives and weals
  • lesions resembling bullseyes
  • papules, which are small bumps that are the most common form of PMLE, for all skin colors
  • small blisters

What causes polymorphous light eruption?

PMLE is idiopathic, meaning there is no known cause. The most likely theory is that it is a delayed hypersensitivity reaction to an endogenous antigen after UV light. This allergic or immune reaction includes exposure to sunlight and artificial UV light sources.

Experts also lack a clear understanding of the skin hardening effect. It could result from developing an immune system tolerance to sun exposure. However, it could also occur as the skin tans and thickens with sun exposure, providing more of a barrier to sunlight.

What are the risk factors for a polymorphous light eruption?

PMLE can affect all skin types and develop at any age. However, PMLE is more common in certain areas and in people with certain traits, including:

  • It affects females four times as often as males.
  • It is more prevalent in fair-skinned people who usually burn with sun exposure.
  • It is more common at high altitudes compared to sea level.
  • It is more common in northern latitudes and temperate climates.
  • It usually starts between the ages 20–40 years.

People with year-round sun exposure rarely get PMLE.

How do you diagnose polymorphous light eruption?

In general, PMLE is a clinical diagnosis. This means doctors mainly rely on your medical history and symptoms to make the diagnosis. Your description of when and how the rash appears will be very helpful.

In some cases, doctors may order tests to rule out other diagnoses, including:

  • blood tests to look for antigens and antibodies
  • skin biopsy to confirm polymorphous light eruption skin changes or rule out other possible skin conditions
  • phototesting, which uses light to test small areas of skin for UV and visible light sensitivity

How do you treat polymorphous light eruption?

Sun protection is part of the treatment for PMLE, including:

  • applying a broad-spectrum sunscreen with UV-A and UV-B coverage
  • avoiding sun exposure and seeking shade whenever possible
  • wearing clothing that covers and shields the skin

To treat an outbreak, doctors may recommend topical corticosteroids. Doctors may also prescribe:

  • topical steroids
  • short-course oral corticosteroids
  • hydroxychloroquine
  • calcipotriene cream (Donovex)
  • antioxidants, such as Polypodium leucotomes extract

Strong immunosuppressants are reserved for severe disease. These include azathioprine (Imuran) or cyclosporine (Gengraf, Neoral, Sandimmune).

Desensitization is another treatment option. Doctors can use artificial UV light in controlled doses to speed the hardening process.

How can you prevent polymorphous light eruption?

In sunny weather, sun protection can help prevent PMLE outbreaks. 

Doctors may also recommend phototherapy in the winter or early spring. Exposing the skin to UV light in small doses may help prevent PMLE outbreaks or reduce their severity. This usually involves 2–3 treatments each week over 4–6 weeks.

Frequently asked questions

Here are some other questions people often ask about polymorphous light eruption.

How common is polymorphous light eruption? 

PMLE is the most common form of photosensitivity or skin disease that is sunlight-related. It affects up to 20% of the population in the United States.

What is the difference between polymorphous light eruption and solar urticaria?

Solar urticaria is another form of photosensitivity. Unlike PMLE, which is a delayed reaction, solar urticaria happens immediately, within a few minutes of sun exposure. Solar urticaria appears as red bumps on sun-exposed areas, with burning and itching. Symptoms usually resolve within a day of stopping sun exposure.

Summary

PMLE is a rash that appears within hours to days of sun exposure. It tends to occur in the spring and summer in people who do not get year-round sun exposure. As the summer progresses, the skin can become less sensitive to sunlight. PMLE recurs seasonally, although it may lessen with time.

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Medical Reviewer: Raechele Cochran Gathers, MD
Last Review Date: 2022 Sep 29
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THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
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