Polymorphic Light Eruption - Causes and Diagnosis

What causes polymorphic light eruption?

Polymorphic light eruption (PMLE) tends to happen when you go out in the sun in the spring or early in the summer, when your skin is not used to sunshine. It may also happen if you go on holiday to a sunny place. It is thought that in people who get PMLE, there is an immune system reaction in the skin which is triggered by UVA (see below).

Sunlight contains various types of rays (radiation). The ones which can cause PMLE are ultraviolet (UV) light and visible light. UV light has two types, UVA and UVB. While sunburn is caused by UVB, it is UVA which causes PMLE. Unlike UVB, UVA and visible light can pass through glass, so it is possible to get PMLE when exposed to strong sunlight through a window. Sometimes it can even occur after exposure to fluorescent lighting.

It is not known exactly how the sunlight causes PMLE, or why some people get PMLE and others don't.

How is polymorphic light eruption diagnosed?

The diagnosis of polymorphic light eruption (PMLE) can usually be made by a doctor from a description of the appearance of the rash and the relationship of the rash to the time of exposure to sunlight.

Generally no tests are needed but sometimes blood tests or a skin sample (biopsy) are taken if the doctor wants to rule out other conditions. If a biopsy is required, a small sample of skin is removed and examined under the microscope in a laboratory. This means the cells in the skin can be seen and an accurate diagnosis can be made.

Photo-testing is sometimes used to confirm the diagnosis. This involves shining some artificial sunlight-type rays (UV or visible light) on to a small area of skin to observe how the skin reacts.

Rarely, PMLE may be an early feature of a condition called systemic lupus erythematosus (SLE).

See separate leaflet called Systemic Lupus Erythematosus for more details.

Did you find this information useful?

Thanks for your feedback!

Why not subcribe to the newsletter?

We would love to hear your feedback!

Dr Jan Sambrook
Peer Reviewer:
Dr John Cox
Document ID:
9374 (v4)
Last Checked:
02 April 2015
Next Review:
01 April 2018

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. Patient Platform Limited has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.