Polymorphic light eruption

PLE · PMLE · polymorphous light eruption · prickly heat (incorrectly) · sun allergy

An itchy papular eruption on newly sun-exposed skin, commoner in brown and black skin than the textbooks imply, and with a variant that is almost peculiar to it.

DISCHARGEBand D

Red flags

A photosensitive eruption with systemic features, a malar distribution, mouth ulcers, joint pain or hair loss - consider lupus erythematosus and investigate rather than labelling it sun allergy
A new photosensitive eruption after starting a drug - a phototoxic or photoallergic drug reaction is a different diagnosis with a different action
Severe, blistering or scarring photosensitivity, or onset in early childhood - refer, because the inherited photodermatoses and porphyrias present this way
An eruption on chronically exposed skin including the face and hands, in an older patient, that does not settle - chronic actinic dermatitis, which was also commoner in dark-skinned patients in the same series

Recognise

On light skin

Itchy pink or red papules, papulovesicles or plaques appearing hours to a couple of days after sun exposure, on skin that has been covered over winter, settling over days if the sun is avoided.

On brown and black skin

This is a condition the textbooks under-attribute to brown and black skin, and the evidence points the other way. In a 23-year single-centre series of 844 patients, polymorphic light eruption was significantly commoner in those with dark skin types than light (74.0% versus 25.9%) - a clinic-based comparison rather than a population prevalence - and just over half that clinic’s general population was White, which is what makes the imbalance striking - and the same centre found the pattern for chronic actinic dermatitis. The pinpoint papular variant was predominantly seen in dark-skinned individuals, and a separate review describes that variant as peculiar to darker skin. It presents as innumerable tiny monomorphic papules rather than the plaques of the classic description, the erythema is usually not visible, and it is characteristically mistaken for lichen nitidus. Feel the skin: the eruption is palpably rough and papular where it may be nearly invisible. Post-inflammatory hyperpigmentation afterwards is prominent and outlasts the eruption.

In any skin tone

The timing is the diagnosis: hours to two days after sun exposure, on skin newly uncovered - the first hot weekend, a holiday, the start of spring
Sparing of the face and the backs of the hands is the classic and useful clue, because those sites are exposed all year and have hardened
Itch is prominent, and the eruption is monomorphic within a given patient even though it is polymorphic between patients
Settles over days to a couple of weeks if further sun is avoided, without scarring
The pinpoint papular variant - innumerable tiny papules - is reported predominantly in darker skin and is easily mistaken for lichen nitidus
Recurs each spring or on each holiday, and often improves through the summer as the skin hardens
Not the same as prickly heat, though the two are routinely conflated - miliaria arises in covered, occluded, sweaty skin rather than in newly sun-exposed skin, and the distribution separates them

Distribution

Sun-exposed skin that has recently been uncovered - the V of the neck, the outer arms and forearms, the upper chest and the lower legs. Classically spares the face and the backs of the hands, which are exposed year-round and have hardened - long-standing teaching rather than a finding from either paper cited here.

Photographs

Polymorphic light eruption in five patients, all on the elbows and extensor forearms. Grouped itchy erythematous papules coalescing into rai — Polymorphic light eruption in five patients, all on the elbows and extensor forearms. Grouped itchy erythematous **papules coalescing into raised plaques**, appearing on sun-exposed skin in spring and early summer and settling over days without scarring. In these five the diagnosis was confirmed by photoprovocation and biopsy. **Note the site: this is the spring and summer elbow variant**, not the commonest pattern. Classic polymorphic light eruption favours the V of the neck, the upper chest and the outer forearms, spares the face and the backs of the hands - skin that is exposed all year and has adapted - and comes on hours to days after the first strong sun of the year rather than during it, which is what separates it from sunburn and from a drug photosensitivity. On brown and black skin the eruption is often papular or papulovesicular with little visible redness, and commonly leaves hyperpigmentation that outlasts the rash by months.
Fitzpatrick III (Fitzpatrick type estimated from the photograph, not stated by the source) · de Galvez MV et al., Frontiers in Medicine · de Galvez MV, Castillo-Munoz RM, Lopez-Navarro N et al. Summer and spring elbow rashes is a variant of polymorphous light eruption. Front Med 2023;10:1260514, Figure 1 · CC BY

Provenance for every photograph in this library: image licences. How much of the corpus still has no darker-skin photograph: skin tone coverage.

Mimics

Lichen planus — The pinpoint papular variant is characteristically mistaken for lichen nitidus, a close relative of lichen planus - the sun-exposed distribution and the seasonal timing are what separate them
Contact dermatitis - allergic and irritant — A photoallergic contact reaction to sunscreen or a fragrance sits in the same distribution - ask what was applied before the sun exposure
Drug eruptions - morbilliform and fixed — Phototoxic drug reactions - doxycycline among the commonest - are exaggerated sunburn in a sun-exposed distribution rather than itchy papules
Acute urticaria — Solar urticaria appears within minutes of exposure and each weal resolves within an hour, rather than appearing after hours and lasting days

What to do in the ED

Take the exposure history precisely: how long after the sun, what was uncovered, and whether the face and hands are spared. That sequence makes the diagnosis and nothing else is needed
Examine by palpation in brown and black skin, and look specifically for the pinpoint papular pattern rather than the plaques of the textbook description
Treat symptomatically - emollient, a topical corticosteroid appropriate to the site for a short course, and an oral antihistamine for itch, recognising that antihistamines help the itch rather than the eruption
Give the prevention advice, because that is the treatment that matters: high-factor broad-spectrum sunscreen applied properly and reapplied, clothing cover, and graded re-exposure rather than a sudden first long exposure
Say explicitly that this is not an allergy to sunlight in the anaphylactic sense and that it is not dangerous - patients are frequently frightened by the phrase sun allergy
Warn about post-inflammatory pigment change before the patient discovers it, particularly in brown and black skin
Refer to dermatology for severe, disabling or atypical disease, or where prophylactic phototherapy before summer is being considered

Disposition

DISCHARGE

Reassure, treat symptomatically and discharge. Dermatology referral for severe or atypical disease, where the diagnosis is uncertain, or where a photodermatosis other than polymorphic light eruption is suspected.

Safety-netting

This is a reaction to sunlight on skin that has been covered up, not a dangerous allergy. It should settle within days to a couple of weeks if you keep the area out of strong sun. Use a high-factor sunscreen, reapply it, and build up sun exposure gradually rather than all at once - it often improves as the summer goes on. See your GP if it comes back every year, as there is treatment that can be given before summer. Come back sooner if you become unwell, if you develop mouth ulcers, joint pains or a rash across your cheeks and nose, or if the skin blisters or scars. Darker marks left behind will fade slowly.

Sources

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