Polymorphic Light Eruption

The commonest photodermatosis. It is a delayed type hypersensitivity reaction to an ultraviolet induced skin antigen, mainly provoked by UVA (320 to 400 nm), and also UVB, and in some patients visible light.

• Prevalence is about 10 to 20% in temperate climates and lower in the tropics. It affects women more than men (about 2 to 3 to 1), usually with onset in the second to fourth decades, and is more common in fair skin but also occurs in dark skin (where it may present as a papular or pinpoint form).
• Onset is in spring or early summer, or on the first holiday in a sunny region, because the skin is not yet "hardened" (tolerance develops with repeated exposure through the summer). Glass transmits UVA, so lesions occur through car and office windows.
• Familial cases and a link with HLA type occur. Associations include prior sunburn and the use of photosensitising drugs.
• The condition recurs each year and is chronic, but may improve over a decade.

• Itchy (sometimes burning) eruption on sun exposed skin, starting 30 minutes to several days (usually 2 to 24 hours) after exposure, and lasting 2 days to 2 weeks. The pattern in each patient is monomorphic (the same lesion type each time) although "polymorphic" between patients.
• Types: papular (the commonest), papulovesicular, plaque, urticarial, erythema multiforme like, and pinpoint papular in dark skin.
• Distribution: V of the neck, dorsa of forearms and hands, upper arms, shins and upper chest. The face is spared in many patients, and covered skin is unaffected.
• No systemic symptoms and no scarring.
• Differential diagnosis: sunburn, photoallergic and phototoxic drug eruption, lupus erythematosus (acute and subacute), solar urticaria, actinic prurigo, chronic actinic dermatitis, porphyria cutanea tarda, erythropoietic protoporphyria, airborne contact dermatitis, atopic dermatitis and dermatomyositis.
• Red flags: scarring, persistent lesions beyond 2 to 3 weeks, involvement of covered skin, systemic features, blistering with fragility, or dark urine (porphyria).
• Complications: itch and sleep disturbance, post-inflammatory pigmentation, impaired quality of life, and avoidance of sun with vitamin D deficiency.

• Mainly clinical, from the history of delayed, recurrent eruption after sun exposure, with spared covered skin.
• Investigations are needed when the diagnosis is uncertain or the pattern is atypical:
  • Photo provocation (repeated UVA irradiation of the skin for 3 to 5 days) reproduces the eruption in about 60 to 70%. Phototesting with minimal erythema doses for UVB and UVA is usually normal.
  • Antinuclear antibodies, anti-Ro (SSA) and anti-La (SSB) antibodies to exclude lupus erythematosus.
  • Plasma, urine and faecal porphyrins if blistering, fragility or pain with exposure is present.
  • Patch and photopatch testing for photoallergic contact dermatitis.
  • Skin biopsy shows a superficial and deep perivascular lymphocytic infiltrate with papillary dermal oedema, which differs from lupus (interface change and mucin).
• Check serum 25-hydroxyvitamin D in those with strict sun avoidance.

• Photoprotection (the foundation):
  • Seek shade, avoid peak sun (10 a.m. to 4 p.m.), wear tightly woven long sleeved clothing, a wide brimmed hat and sunglasses.
  • Broad spectrum, high UVA protection sunscreen, SPF 50 or more, applied thickly (2 mg/cm2) 20 minutes before exposure and reapplied every 2 hours. Choose products with stabilised UVA filters (such as avobenzone with stabilisers, ecamsule, or tinosorb filters) or tinted mineral products when visible light plays a role.
  • Window film or tinted glass for vehicles and offices.
  • Gradual sun exposure to build tolerance (hardening) in spring.
• Treatment of an attack:
  • Potent topical corticosteroid (betamethasone valerate 0.1% or mometasone furoate 0.1% cream or ointment) once or twice daily for 5 to 7 days (clobetasol 0.05% for the trunk and limbs for 3 to 5 days).
  • Cetirizine 10 mg or fexofenadine 180 mg orally once daily for itch (sedating hydroxyzine 10 to 25 mg at night for sleep).
  • Severe attacks: oral prednisolone 0.5 mg/kg per day (30 to 40 mg) for 3 to 5 days.
• Prophylactic phototherapy (first line for preventing attacks in people with moderate to severe disease or those who cannot avoid the sun), done in early spring before exposure:
  • Narrowband UVB, 3 times weekly for 4 to 6 weeks (about 15 sessions), starting at 50 to 70% of the minimal erythema dose and increasing by 20% per treatment.
  • PUVA (oral or bath psoralen with UVA), 2 to 3 times weekly for 4 to 6 weeks, which is more effective in severe disease but carries long term skin cancer risk. Use is limited by the cumulative dose.
  • Hardening protects for about 4 to 8 weeks, and treatment is repeated each year.
• Systemic therapy for refractory disease (specialist care):
  • Hydroxychloroquine 200 mg orally 12 hourly (maximum 5 mg/kg per day of real body weight) for the high risk months, with baseline and annual eye examination after 5 years (earlier with risk factors) and caution in renal impairment.
  • Azathioprine 1 to 2.5 mg/kg per day, with thiopurine methyltransferase testing before starting and monitoring of blood count and liver tests.
  • Ciclosporin 3 to 5 mg/kg per day for short periods with blood pressure and creatinine monitoring.
  • Beta carotene, nicotinamide (500 mg orally 12 hourly), and antioxidant supplements have limited and inconsistent evidence.
• Vitamin D: supplement with 800 to 2000 IU (20 to 50 micrograms) daily if levels are low because of sun avoidance.
• Special groups:
  • Pregnancy: photoprotection, topical steroids, and narrowband UVB. Avoid PUVA, azathioprine (unless essential), ciclosporin and hydroxychloroquine dose changes without advice.
  • Children: protective clothing, sunscreen, and short courses of mild to moderate steroid, with narrowband UVB if essential.
  • Dark skin: use tinted sunscreens (visible light protection) and avoid heavy treatment that increases pigmentation.
  • Renal or hepatic impairment: adjust or avoid systemic agents.
• Follow up: review at 4 to 6 weeks of phototherapy, then annually before spring. Refer to a dermatology photobiology unit if the diagnosis is unclear, the response is poor, or systemic therapy is needed.