Polymorphous light eruption !
An itchy rash on sun-exposed skin that appears hours to days after sun exposure; it is the most common sun allergy and fades without scarring.
Updated 2026-11-01
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What it is
The most common idiopathic photodermatosis: a delayed immune reaction to ultraviolet (UVA and/or UVB) light that produces itchy papules, plaques or vesicles on sun-exposed skin hours to a few days after exposure. The pattern is the same in each patient but differs between patients, hence 'polymorphous'.
Who gets it
Very common; it mainly starts in young adults (20–40 years) and is several times more frequent in women. It is classically seasonal in temperate countries (spring and early summer). In Colombia's equatorial climate, sun exposure is high all year, so episodes relate more to sudden intense exposure (holidays at the coast, high altitude) than to seasons. Actinic prurigo, more common in mestizo and Indigenous populations of Latin America, is an important differential.
Causes
- Delayed-type hypersensitivity to UV-induced skin antigens not yet identified
- Failure of the normal UV-induced immune suppression in the skin
- Genetic predisposition (family history in a large minority)
Risk factors
- Female sex
- Young adulthood
- Lighter skin, although it occurs in all skin types
- Family history
- Sudden intense sun exposure after a period with little sun
Pathophysiology
In healthy skin, UV light suppresses local immune responses. In polymorphous light eruption this suppression fails, so UV-modified skin proteins trigger a T-cell mediated reaction with perivascular lymphocytic infiltrates and dermal oedema. Repeated exposure thickens the epidermis and increases pigment, which explains the 'hardening' that makes the rash fade as exposure continues and why chronically exposed skin (face, backs of hands) is often less affected.
Classification
- By morphology: papular (most common), papulovesicular, plaque, vesiculobullous, urticarial and erythema multiforme-like
- Juvenile spring eruption (variant on the ears of boys)
Symptoms and signs
- Intensely itchy red papules, plaques or vesicles
- Appear 30 minutes to a few days after sun exposure
- Sites: V of the neck, upper chest, outer arms and forearms, lower legs
- Resolve in days to a week without scarring if further exposure is avoided
- Recur with each new intense exposure; improve with repeated moderate exposure
Diagnosis
- Clinical
- Clinical: an itchy, non-scarring eruption on exposed skin that follows sun exposure by hours to days and repeats in the same pattern.
- Tests
- Antinuclear and anti-Ro/SSA antibodies to exclude cutaneous lupus; porphyrin studies if blisters or fragility. Biopsy (nonspecific, negative immunofluorescence) when uncertain. Phototesting and photoprovocation in specialized centres.
- Imaging
- Not needed.
Differential diagnosis
- Subacute cutaneous and systemic lupus erythematosus
- Actinic prurigo
- Drug-induced photosensitivity and photoallergic contact dermatitis
- Solar urticaria (appears within minutes, resolves within hours)
- Erythropoietic protoporphyria and porphyria cutanea tarda
- Pellagra
Goal of treatment
Prevent outbreaks with photoprotection and gradual 'hardening', and control itching and inflammation during flares.
Treatment
- Lifestyle
- Broad-spectrum high-protection sunscreen covering UVA, protective clothing and hats, and gradual increase of sun exposure; avoid sudden intense exposure.
- Medical
- Flares: potent topical corticosteroids and oral antihistamines for itch; a short course of systemic corticosteroids for severe flares.
- Procedure
- Prophylactic narrowband UVB or PUVA phototherapy in early season or before travel to induce tolerance in frequently affected patients.
- Medical
- Severe or resistant cases: hydroxychloroquine or nicotinamide; other immunosuppressants rarely.
Complications
- Severe itch and sleep disturbance
- Secondary excoriation and infection
- Avoidance of outdoor activities, impact on quality of life
- A small proportion later develop autoimmune disease (thyroid disease, lupus)
Prognosis
Benign and non-scarring. Many patients improve gradually over years, and a large proportion resolve or improve over decades.
Prevention
- Daily photoprotection with UVA cover
- Gradual sun exposure at the start of holidays or the sunny season
Test yourself
Think of the answer, then tap the question to see it.
What is the most common idiopathic photodermatosis?
Polymorphous light eruption.
When does the rash appear after sun exposure?
Hours to a few days later, and it fades in days without scarring.
What is 'hardening'?
Gradual tolerance with repeated moderate sun exposure, so the rash lessens as exposure continues.
Which disease must be excluded with antinuclear and anti-Ro antibodies?
Cutaneous (especially subacute) and systemic lupus erythematosus.
Related conditions
- Systemic lupus erythematosus
- Discoid lupus erythematosus
- Erythema multiforme
- Contact dermatitis (irritant and allergic)
- Urticaria and angioedema
- Erythropoietic protoporphyria
- Porphyria cutanea tarda
- Pellagra (niacin deficiency)
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Polymorphic light eruption (PMLE) (DermNet, 2026).
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