Photocontact Dermatitis (Photoallergic or Phototoxic Contact Dermatitis)
Dermatitis affecting skin due to sun exposure is called photocontact dermatitis, which encompasses both phototoxic and photoallergic contact dermatitis. 1, 2, 1, 2
Understanding the Terminology
The British Association of Dermatologists guidelines clearly classify this condition within the spectrum of contact dermatitis reactions. Specifically, the guidelines define:
- Phototoxic contact dermatitis: A direct toxic reaction when certain substances on the skin are activated by UV radiation (predominantly UVA), causing immediate skin damage without immune system involvement
- Photoallergic contact dermatitis: An immune-mediated reaction where UV light activates a photosensitizer that binds to skin proteins, creating complete antigens that trigger a delayed-type hypersensitivity response 1, 2
- Photoaggravated contact dermatitis: Pre-existing dermatitis that worsens with sun exposure 1, 2
Clinical Characteristics
The hallmark feature is localization to sun-exposed areas such as the face, neck, arms, dorsal hands, and legs 3, 4. The guidelines note that distinguishing between phototoxic and photoallergic reactions is not always clinically straightforward 1, 2.
Common Causative Agents
According to the guidelines, the most frequent triggers include:
- Sunscreen constituents (most common cause of photoallergic reactions) 5
- Cosmetic ingredients 1, 2
- Medications: Sulfonamide antibiotics, phenothiazines, NSAIDs, thiazide diuretics, amiodarone 6, 7, 4
- Plant furocoumarins (limes, celery) causing phototoxic reactions 5
- Fragrances and antibacterial agents 5
Diagnostic Approach
Photo-patch testing is the definitive diagnostic method for photoallergic contact dermatitis, though most dermatologists do not routinely perform this specialized testing 5. The procedure involves:
- Application of suspected photoallergens
- Exposure to controlled UVA radiation
- Assessment at 48-72 hours for delayed reactions
Management Strategy
The guidelines recommend a hierarchical approach:
First-line measures:
- Strict avoidance of identified photosensitizers 1, 2
- Rigorous photoprotection: Seek shade during peak sun hours (midday), wear protective clothing, broad-brimmed hats, and sunglasses 6, 8
- Broad-spectrum sunscreens with SPF ≥30 (though ironically, sunscreens themselves can be photoallergens) 6, 8
Important caveat:
When sunscreen is the suspected photoallergen, mineral-based (physical) sunblocks containing zinc oxide or titanium dioxide are preferred over chemical sunscreens 6.
For persistent cases:
- Topical corticosteroids for acute inflammation 9
- Prophylactic phototherapy (narrowband UVB or PUVA) may be considered for desensitization in chronic photodermatoses, though this requires specialized expertise 10, 11, 8
Critical Clinical Pitfall
The most common error is failing to recognize that photocontact dermatitis requires both the photosensitizing agent AND UV exposure. Simply avoiding sun while continuing to use the offending product, or vice versa, will not resolve the condition. Both elements must be addressed simultaneously 1, 2, 4.