Photodermatoses

Photodermatoses are skin disorders caused or aggravated by ultraviolet (UVB 290 to 320 nm, UVA 320 to 400 nm) and sometimes visible light exposure. They are divided into immunologically mediated, DNA-repair defects, drug- and chemical-induced (photoallergic, phototoxic), and photoaggravated dermatoses.

Immunologically mediated (idiopathic)

• Polymorphic light eruption (PLE): the commonest (10 to 20% of people in temperate regions), delayed type IV hypersensitivity to an unidentified UV-induced antigen. Affects young women, onset in spring after first sun exposure, with "hardening" through the summer.
• Actinic prurigo: chronic, familial, HLA-DRB1*0407 and *0401 (Native American and Latin American populations), starting in childhood.
• Chronic actinic dermatitis: older men with severe photosensitivity to UVA, UVB and visible light, associated with contact allergens (sesquiterpene lactones, fragrances).
• Solar urticaria: wheals within minutes of sun exposure (type I), triggered by UVA, visible light or UVB.
• Hydroa vacciniforme: rare childhood vesicular scarring eruption associated with EBV (hydroa vacciniforme lymphoproliferative disorder).

Genetic and metabolic

• Porphyrias (erythropoietic protoporphyria, congenital erythropoietic porphyria, porphyria cutanea tarda, variegate porphyria, hereditary coproporphyria): photosensitivity from porphyrin accumulation (see Porphyrias).
• DNA-repair disorders: xeroderma pigmentosum, Cockayne syndrome, trichothiodystrophy.
• Others: Smith-Lemli-Opitz, Hartnup disease, pellagra (niacin deficiency), Bloom syndrome, Rothmund-Thomson, and Kindler syndrome.

Exogenous photosensitivity

• Phototoxic reactions (non-immune, dose-dependent, exaggerated sunburn): tetracyclines (doxycycline), fluoroquinolones, amiodarone, thiazides, furosemide, NSAIDs (naproxen, piroxicam), voriconazole, psoralens, phenothiazines, vemurafenib, and plant furocoumarins (phytophotodermatitis from lime, celery, parsnip, giant hogweed).
• Photoallergic reactions (type IV): topical sunscreens (oxybenzone), NSAIDs (ketoprofen), fragrances (musk ambrette), chlorpromazine.
• Photoaggravated dermatoses: lupus erythematosus, dermatomyositis, atopic dermatitis, rosacea, psoriasis, pemphigus, Darier disease, herpes simplex, vitiligo, and pellagra.
• Epidemiology and impact: PLE affects up to 18% in northern Europe, and photodermatoses cause avoidance behaviour, vitamin D deficiency, and reduced quality of life.

• PLE: itchy or burning papules, papulovesicles, plaques (most common) on sun-exposed sites (V of neck, forearms, dorsal hands, shins, but face often spared), appearing hours to 2 days after exposure and resolving over 1 to 14 days without scarring.
• Actinic prurigo: intensely pruritic papules and nodules on face, with excoriation, cheilitis, conjunctivitis, scarring, and persistence in winter.
• Chronic actinic dermatitis: eczematous, lichenified plaques on exposed skin, extension to covered sites, persistent in winter, possible erythroderma.
• Solar urticaria: pruritic wheals within 5 to 10 minutes of exposure lasting less than 24 hours, confined to exposed skin.
• Hydroa vacciniforme: vesicles with umbilication and varioliform scars on the face and hands, fever and systemic EBV disease in the lymphoproliferative variant.
• Phototoxic: exaggerated sunburn with erythema, oedema, blistering, hyperpigmentation, photo-onycholysis (tetracyclines, psoralens), pseudoporphyria (naproxen, voriconazole, dialysis) with skin fragility and blisters on dorsal hands. Amiodarone causes a slate-grey (blue-grey) pigmentation.
• Photoallergic: eczematous eruption, delayed 24 to 72 hours, spreading beyond exposed skin.
• Phytophotodermatitis: linear streaky erythema and blistering with hyperpigmentation (margarita burn).
• Porphyrias: erythropoietic protoporphyria shows immediate burning pain within minutes of sunlight, with oedema, erythema and little blistering. Porphyria cutanea tarda shows skin fragility, blisters, milia, hypertrichosis, and scarring on the hands.
• Xeroderma pigmentosum: severe sunburn on minimal exposure in infancy, freckling, early skin cancers, ocular damage, and neurological degeneration.
• Red flags: onset in infancy with severe burn on minimal exposure (xeroderma pigmentosum), immediate burning pain (protoporphyria), photosensitivity with systemic features (lupus), neurological decline, skin fragility (porphyria cutanea tarda), and new photosensitivity in older adult on multiple drugs.

• Approach: history of timing, drug and contact exposures, family history, and morphology. Investigations depend on the suspected disorder.
• Photoprovocation testing: monochromator phototesting (minimal erythema dose to UVB, UVA, and visible light) distinguishes solar urticaria (wheals), chronic actinic dermatitis (low erythema thresholds), and normal. Repeated broad-spectrum UVA/UVB exposure over 3 to 5 days reproduces PLE.
• Photopatch testing with UVA irradiation (5 J/cm2) of duplicate allergens, for photoallergy.
• Standard patch tests for chronic actinic dermatitis.
• Laboratory: ANA, anti-Ro/SSA and anti-La/SSB (lupus), CBC, ferritin, liver tests. Plasma, urine, and faecal porphyrins, with plasma fluorescence scanning (peak at 620 nm for variegate), erythrocyte protoporphyrin (erythropoietic protoporphyria), and genetic testing. Niacin and tryptophan metabolites for pellagra. Hepatitis C, HFE mutations, iron studies, and alcohol screening in porphyria cutanea tarda.
• Skin biopsy: supports PLE (perivascular lymphocytic infiltrate), lupus (interface dermatitis, DIF lupus band), and rules out other diseases. Direct immunofluorescence for pseudoporphyria (blisters).
• Xeroderma pigmentosum: DNA repair assays (unscheduled DNA synthesis), genetic testing (XPA to XPG, POLH). Dermatology and ophthalmology surveillance.
• EBV PCR for hydroa vacciniforme-like lymphoma.
• Review of all drugs and topical products for photosensitisers, and withdrawal in a test period.

Photoprotection (essential for all)

• Broad-spectrum sunscreen SPF 50+ with high UVA protection (PPD or critical wavelength over 370 nm), applied 2 mg/cm2 (about 2 mg per cm2, one teaspoon for the face and neck), 20 minutes before exposure, reapplied every 2 hours. Physical filters (titanium dioxide, zinc oxide, iron oxide tinted preparations for visible light protection in solar urticaria, melasma, and porphyria).
• Sun avoidance during peak hours (10 am to 4 pm), shade, tightly woven UPF 50+ clothing, wide-brimmed hats, UV-filtering window films, and sunglasses.
• Vitamin D supplementation (800 to 2000 IU daily) because of sun avoidance, and monitor serum 25-hydroxyvitamin D.

Polymorphic light eruption

• Prophylactic narrowband UVB or PUVA phototherapy in spring (3 times weekly for 5 to 6 weeks, hardening), for severe disease, repeated annually.
• Acute treatment: potent topical corticosteroids (betamethasone valerate 0.1% or clobetasol 0.05%) for 3 to 7 days, short course oral prednisolone 0.5 mg/kg for 3 to 5 days in severe flares, antihistamines for itch.
• Hydroxychloroquine 200 to 400 mg daily in resistant cases, Polypodium leucotomos extract 240 mg twice daily, nicotinamide 500 mg twice daily (prevention), beta-carotene (limited), and afamelanotide in selected cases.

Actinic prurigo and chronic actinic dermatitis

Strict photoprotection, topical corticosteroids and tacrolimus 0.1%, narrowband UVB hardening (after UVA tolerance improves), PUVA, thalidomide 50 to 100 mg daily for actinic prurigo (teratogenic), and immunosuppressants in chronic actinic dermatitis: azathioprine 1 to 2.5 mg/kg/day, ciclosporin 3 to 5 mg/kg/day, mycophenolate mofetil 1 to 2 g daily, or short oral prednisolone. Avoid sensitisers and patch-test-positive allergens.

Solar urticaria

• Non-sedating antihistamines at up to 4 times the standard dose (cetirizine 10 mg up to 40 mg, fexofenadine up to 720 mg daily), omalizumab 150 to 300 mg SC every 4 weeks (effective in many), UV hardening (rush phototherapy), plasmapheresis, IVIG, and ciclosporin for severe disease.
• Hydroa vacciniforme: photoprotection, EBV monitoring, and treatment of lymphoproliferative disease as lymphoma (chemotherapy, haematopoietic stem cell transplant).

Drug-induced photosensitivity

• Withdraw or substitute the culprit drug (for example, replace doxycycline with minocycline or azithromycin, replace thiazide, change NSAID). When the drug is essential (amiodarone, voriconazole), maximal photoprotection. Voriconazole carries increased risk of cutaneous squamous cell carcinoma (switch to alternatives in long-term use). Treat acute phototoxic burn with cool compresses, topical corticosteroids, and analgesia. Photoallergy: avoid the allergen and treat as eczema.
• Phytophotodermatitis: wash exposed areas, topical corticosteroids, sunscreen, and pigmentation improves in months.
• Pseudoporphyria: stop causative drug, sunscreen, N-acetylcysteine 600 mg twice daily in dialysis-related disease.

Porphyrias

• Erythropoietic protoporphyria: afamelanotide 16 mg subcutaneous implant every 2 months (Scenesse), dersimelagon (oral MC1R agonist), beta-carotene 60 to 180 mg daily (limited benefit), cysteine 500 mg twice daily, vitamin D supplementation, and liver monitoring (cholestasis risk). Haem arginate and hepatic care for liver crises.
• Porphyria cutanea tarda: stop alcohol, oestrogens, and iron exposure, venesection (500 mL every 2 weeks to ferritin of 20 to 50 ng/mL) or low-dose hydroxychloroquine 100 mg twice weekly (200 mg twice weekly), and treatment of hepatitis C with direct-acting antivirals.
• Acute porphyrias:.

Xeroderma pigmentosum and other DNA-repair disorders

• Lifelong rigorous UV protection (UV-blocking clothing, hats, gloves, masks, window films, avoiding daytime outings), ophthalmology care, monitoring and early treatment of skin cancers (excision, Mohs), topical 5-fluorouracil, imiquimod, oral isotretinoin 0.5 mg/kg or acitretin for chemoprevention (with supervision), T4N5 liposome lotion, neurological and audiological monitoring, genetic counselling, and prenatal diagnosis.
• Photoaggravated diseases: treat the underlying condition and add photoprotection. Lupus: hydroxychloroquine 200 to 400 mg daily, sunscreen, and sun avoidance.

Follow-up

Annual review and full-body skin examination for high-risk groups (xeroderma pigmentosum every 3 to 6 months). Educate on early symptoms, travel precautions to high UV-index regions, and vitamin D monitoring.