Photoallergic Contact Dermatitis

Photoallergic contact dermatitis is a delayed hypersensitivity eczema to a topically applied agent that requires ultraviolet light, usually ultraviolet A, for activation. It affects light exposed skin and may spread to covered areas.

It is less common than phototoxic reactions. Sensitisation takes 7 to 10 days after first exposure, and elicitation follows within 24 to 72 hours of a subsequent exposure to the agent and light.

Causes include organic ultraviolet filters in sunscreens (benzophenone-3, octocrylene, avobenzone), topical ketoprofen and other topical anti-inflammatory drugs, fragrance (musk ambrette), phenothiazines, halogenated salicylanilides and antiseptics. Cross-reactions are important: ketoprofen cross-reacts with benzophenones, octocrylene and the oral drug fenofibrate.

Pathophysiology.

Light converts the agent to a photoproduct that binds skin proteins and forms a complete antigen. This activates a T-cell mediated type IV reaction. Some patients develop persistent light reactivity, with sensitivity continuing for years after contact has stopped.

• Itchy, eczematous eruption (erythema, papules, vesicles, scale) on the face, V of the neck, dorsal hands and forearms.
• Delayed onset, 1 to 3 days after exposure, with spread beyond exposed skin in severe cases.
• Chronic lichenification with long standing exposure, and sensitivity to sunlight after the allergen is removed.
• Sparing of shielded areas such as under the chin and behind the ears.

Red flags.

• Widespread eczema or erythroderma after sun exposure.
• Progression to persistent light reactivity or chronic actinic dermatitis.
• Reaction to an essential long term drug such as fenofibrate.

• Careful history of topical products, sunscreens, fragrances and plants, with time to onset and relation to sun.
• Photopatch testing: duplicate series applied to the back for 24 hours, with one set irradiated with ultraviolet A (5 J/cm2) and read at 48 and 96 hours. A reaction only in the irradiated set indicates photoallergy, and a reaction in both indicates contact allergy.
• Phototesting to determine the minimal erythema dose to ultraviolet A, ultraviolet B and visible light.
• Biopsy is rarely needed.

Differential diagnosis. Phototoxic dermatitis, allergic contact dermatitis, airborne contact dermatitis, polymorphic light eruption, chronic actinic dermatitis, cutaneous lupus and atopic dermatitis.

• Identify and avoid the photoallergen and cross-reacting substances, with a written list of agents to avoid. Replace organic filters with sunscreens containing zinc oxide or titanium dioxide.
• Strict photoprotection with clothing and shade, and avoidance of peak ultraviolet exposure.
• Potent corticosteroid (betamethasone valerate 0.1%) once daily for 1 to 2 weeks on the body, and moderate potency (clobetasone butyrate 0.05%) on the face, then taper.
• Tacrolimus 0.1% ointment twice daily on the body, or 0.03% on the face, for steroid sparing control.
• Severe flares: prednisolone 0.5 mg/kg/day (usually 30 to 40 mg) for 5 to 7 days.
• Persistent light reactivity: narrowband ultraviolet B hardening, azathioprine 1 to 3 mg/kg/day after thiopurine methyltransferase testing, or ciclosporin 2.5 to 5 mg/kg/day in two divided doses.

Monitoring. Review at 2 to 4 weeks. Recheck photopatch tests if the cause remains unclear. Monitor blood pressure and creatinine on ciclosporin.

Referral and escalation. Refer to a photodermatology or contact dermatitis service for testing and for persistent light reactivity.