Description

A polymorphic light eruption (PMLE) occurs in response to sunlight after a winter in which the skin has been covered and scarcely exposed to the sun. It is caused by ultraviolet (UVA or UVB) or visible light¹ - so it may occur when behind glass as in a car.

The term polymorphous indicates the variable nature of the rash but for any individual, the rash tends to be fairly constant from year to year.

Epidemiology

• It is a fairly common condition, perhaps affecting15% of people in the UK.²
• It is more common in those with fair skin and tends to affect women more often than men.
• Age of onset is usually 20-40 years.

Aetiology

• It is thought to be caused by an immunological reaction but the precise nature is unknown. It has been suggested that 17beta-oestradiol prevents UV-induced suppression of the contact hypersensitivity response caused by the release of cytokines from keratinocytes. This would account for its prevalence in adult women and its tendency to wane after the menopause.²

Presentation^3,4

• The typical history is sudden onset of a rash within hours of sun exposure at the start of the sunny season. The rash subsides over about a week (unless there is continuing exposure). Symptoms can recur, but reduce as summer and sun exposure progress.
• The most common form of rash is crops of pink or red, raised spots of 2-5mm on the arms, chest or lower legs. Usually the face is spared.
• Other presentations include:
  • A dermatitis-like rash i.e. vesicles followed by dry red patches.
  • Target lesions, resembling erythema multiforme.
  • In African Americans, there may be a rash with pinpoint papules on sun-exposed areas, sparing the face.⁵
• The rash is usually itchy or 'burning'.
• There is occasionally a systemic flu-like illness.

The course of the rash also varies:

• In most, the rash settles in a few days - 2 weeks.
• It may or may not recur next time the sun shines on the skin.
• If the rash gets more sun exposure before it has cleared, the condition tends to worsen.
• In most individuals there is a 'hardening' as the summer progresses and tolerance to sun increases. However, this does not always occur. Some people can develop PMLE even in the winter.

Most people are not severely affected and may not consult about their symptoms.

Differential diagnosis

• Lupus erythematosis
• Erythropoietic porphyria
• Drug eruption
• Photosensitivity drug reaction
• Actinic reticuloid is an extremely photosensitive disorder, which may overlap with PMLE⁶

Investigations

• Usually the diagnosis is clinical, based on a careful history.⁷
• Investigations may be requested to exclude other diseases such as SLE or porphyria (e.g. antinuclear antibody (ANA), anti-Ro (SS-A), and anti-La (SS-B) tests and/or porphyrin levels).⁴
• Phototesting is sometimes used to aid diagnosis and determine which type of light the skin is sensitive to. This involves irradiating a patch of skin with UVA, UVB or visible light, repeated as necessary to produce a PMLE lesion. Phototesting results do not correlate with the severity of PMLE.⁷
• Skin biopsy is occasionally used.⁵

Associated diseases⁶

• It has been suggested that a polymorphic light eruption may, rarely, precede the development of lupus erythematosis.
• There may be an association with autoimmune thyroid disease.

Management^3,8

Prevention:

• Protection from sunlight - protective clothing and sunscreens (note these may not filter out all sunlight).
• Allow gradual exposure to sunlight, which may acclimatise the skin and prevent the rash.
• One small trial suggested effective prevention using certain antioxidants combined with sunscreen.⁹

For treating the acute condition:

• Topical steroids or a short course of oral steroids.
• Antihistamines may help pruritus (but note that phenothiazines can also cause photosensitivity).

For severe PMLE:

• Prophylactic light therapy (before sun exposure e.g. in early spring) may help. This is known as 'photohardening' and aims to induce sunlight tolerance using controlled exposure. This may use UVB, UVA/UVB^10,1 or PUVA⁸. Recently, light-emitting diode treatment has also been suggested.¹¹
• Betacarotene and hydroxychloroquine can both improve tolerance to the sun.¹²
• Immunomodulators e.g. azathioprine or thalidomide might help PMLE, but the potential side-effects generally preclude their use in PMLE.⁴

Prognosis

• Most patients can control their PMLE with simple treatments.⁸
• The condition is likely to ease as the spring and summer advance, but it will probably recur the following year unless precautions are taken.
• Long-term prognosis - PMLE often improves over time (years), and may resolve in some patients.⁶
• A natural fall in oestrogens may account for the tendency to remit after the menopause.²