Polymorphic Light Eruption – PMLE
Polymorphic light eruption (PMLE) is the most common idiopathic photodermatosis seen in clinical practice, affecting 10–20% of people worldwide and especially common in young women aged 20–40 with fair skin (types I–III). It is marked by the sudden appearance of intensely itchy lesions on sun-exposed skin after a long break from the sun, typically in spring or on the first beach trip of summer.
PMLE is thought to be a T cell–mediated delayed-type (type IV) hypersensitivity reaction against a newly formed photoantigen created by ultraviolet light (mainly UVA, though UVB or visible light can also be responsible).
- Failure of UV-Induced Immunosuppression:
- In most people, sunlight produces a mild local immunosuppressive effect (through IL-10 and prostaglandins) that stops the skin reacting to proteins altered by light.
- In PMLE, this protective mechanism breaks down. Antigen-presenting Langerhans cells carry the photoantigen to the regional lymph nodes, where CD4+ and CD8+ T cells multiply and then migrate back into the skin.
- Natural Light Tolerance (Hardening Effect):
- A distinctive feature of PMLE is "hardening": as the skin receives gradually increasing UV exposure through the summer, the immune system adapts (more melanin, a thicker stratum corneum and restored regulatory T cell suppression), so the rash settles and flares less often by late summer.
- Histopathology:
- The epidermis shows mild to moderate spongiosis, sometimes with superficial vesicles.
- The dermis shows marked papillary dermal edema with a dense perivascular lymphocytic infiltrate, made up mostly of CD4+ T cells. Neutrophils and eosinophils are seen only rarely.
PMLE is often misdiagnosed as allergic contact dermatitis, a seafood allergy or insect bites, solar urticaria, or, most importantly, the rash of lupus erythematosus (LE). At Haute-Derma, threshold phototesting combined with a narrowband UVB prophylactic hardening protocol helps patients keep their symptoms well controlled ahead of each sunny season.
Clinical Features and Lesion Types (Polymorphism)
PMLE symptoms usually appear a few hours to 48 hours after sun exposure, not within minutes as in solar urticaria. Patients feel intense itching, stinging or burning.
Despite the name polymorphic, any one patient tends to get the same type of lesion (monomorphic) with each flare:
- Papular and Papulovesicular Type: The most common form, with crops of pinkish-red papules 1–3 mm across, scattered or clustered, often topped by tiny blisters.
- Plaque Type: Slightly raised, swollen red plaques ranging from a few millimeters to several centimeters, which can look very much like subacute cutaneous lupus.
- Erythema Multiforme-Like and Prurigo-Like Types: Lesions look faintly target-like or form itchy, prurigo-like bumps, mainly in skin folds, the V of the chest and the backs of the hands.
- Typical Distribution:
- Lesions are worst on skin that stays covered all winter and is suddenly exposed in summer: the V-neck area, arms, lower legs and tops of the feet.
- The face and backs of the hands are often spared because year-round exposure has already produced natural hardening.
How PMLE Differs From Other Photosensitivity Disorders
| Polymorphic Light Eruption (PMLE) | Solar Urticaria | Photocontact Dermatitis |
|---|---|---|
| A few hours to 48 hours after UV exposure | Within 1–10 minutes of going into the sun | A few hours to 24 hours after contact |
| 7–14 days (clears on its own if UV is avoided) | 1–2 hours once in the shade | Fades slowly, with marked post-inflammatory hyperpigmentation (PIH) |
| Delayed type IV hypersensitivity (T cell) | Immediate type I hypersensitivity (IgE) | Phototoxic / photoallergic reaction |
| ANA (−), Ro/SSA (−), La/SSB (−) | Normal IgE antibodies | Negative |
| Hardening PRESENT (characteristic) | Little or no hardening | No hardening |
Diagnosis, Flare Treatment and Preventive Phototherapy at Haute-Derma
At Haute-Derma, patients with PMLE are managed under our phototesting and NB-UVB hardening pathway (Haute-Derma PMLE Protocol):
Immunology Tests and Threshold Phototesting
- Antinuclear antibodies (ANA) and anti-Ro/SSA and anti-La/SSB tests are mandatory to rule out subacute cutaneous lupus erythematosus (SCLE).
- Provocative phototesting: Increasing doses of UVA or narrowband UVB are applied to normally covered skin on 3 consecutive days to reproduce the rash for diagnosis.
Treating Acute Flares
- Topical corticosteroids: Potent steroids (mometasone furoate 0.1%, clobetasol propionate 0.05%) applied once or twice daily for 5–7 days quickly calm the lymphocytic inflammation.
- Second-generation H1 antihistamines: Desloratadine 5 mg or levocetirizine 5 mg, 1–2 tablets a day, to relieve the intense itch.
- Short course of oral steroids: For widespread flares (more than 30% of body surface area), prednisolone 0.5 mg/kg/day for 3–5 days.
Preventive Hardening Phototherapy (Prophylactic NB-UVB Hardening Protocol)
- Timing: Start about 4–6 weeks before a beach holiday or the start of summer.
- Procedure: Narrowband UVB (NB-UVB) 2–3 times a week, starting at 70% of the minimal erythema dose (MED) and increasing by 10–15% each session.
- How it works: Sub-irritant UV doses stimulate melanin production, thicken the stratum corneum and re-establish local immune tolerance, substantially lowering the risk of a PMLE flare once the patient is out in real sunlight.
Photoprotection and Oral Supplements
- High-performance sunscreen: Use a medical-grade broad-spectrum sunscreen with a UVA-PF/PPD above 30 (containing photostable filters such as Tinosorb S, Tinosorb M, Mexoryl SX/XL or Uvinul A Plus), applied 30 minutes before going out.
- Oral antioxidants:
- Fern extract (Polypodium leucotomos): 480–960 mg/day helps reduce the effect of UV on Langerhans cells.
- Nicotinamide (vitamin B3): 500 mg helps restore ATP in skin cells damaged by ultraviolet light.
