- Contact dermatitis can have either an irritant or an allergic origin. Distinguishing between these entities can be informed by a careful history, including timing of onset of symptoms.
- Allergic contact dermatitis is caused by a delayed-type hypersensitivity (DTH), or Type IV hypersensitivity, reaction to an allergen.
- Treatment necessitates avoidance of the allergen, and often requires topical corticosteroids to reduce inflammation.
- Identifying allergens underlying a contact dermatitis can be difficult. A careful history, including occupational and intermittent exposures (i.e., hobbies, cosmetic practices), is critical. Allergen testing, such as a “use” test or standardized patch testing, can be very helpful to identify potential contactants, but clinical correlation is necessary in the interpretation.
Contact dermatitis is a common cutaneous condition, with an estimated incidence of 43 cases per 100,000 persons. It can have either an irritant or an allergic origin. Irritant reactions can present with similar features to contact dermatitis, but tend to appear less than 12 hours after exposure, and are strictly limited to exposed sites. Allergic reactions are delayed (12-72 hours, except for primary sensitization, where an induction phase of 7-10 days is typical), and exhibit a tendency to spread as the immunologic reaction reaches sites where less antigen was deposited initially.
Whereas irritant dermatitis may stem from non-specific inflammation mediated by the innate immune system, allergic contact dermatitis results from hapten-specific T cell mediated delayed-type hypersensitivity (DTH), or Type IV hypersensitivity, reaction to an allergen. Potential allergens arise from almost anything in the environment, ranging from plant-derived chemicals, preservatives, fragrances, or chemicals used in the processing of products and fabrics.
The top 10 contact allergens (North American Contact Dermatitis Group):
- nickel (nickel sulfate)
- gold (sodium gold thiosulfate)
- cobalt (cobalt chloride)
- Topical antibiotics:
- neomycin sulfate
- Fragrance elements
- fragrance mix
- balsam of peru (myroxylon perirae)
Identifying relevant contact allergens can be challenging. A careful history, including occupational and intermittent exposures due to hobbies or cosmetic practices, is critical. Allergen testing may occur by a “use” test or standardized patch testing. A “use” test involves putting a small amount of suspected allergen (such as a cosmetic product) occluded under a bandage for 48-72 hours and assessing the development of inflammation. Standardized patch testing typically involves the application of commercially available patches (such as TRUE test) impregnated with allergen onto the skin with clinical evaluation at 48, 72, and 96 hours for evidence of inflammation. Patch testing should be done on non-inflamed skin, such as on the back, and performed in the absence of systemic antihistamines or corticosteroids, when possible. Skin erythema, vesiculation, or eczematous changes indicates a positive result, and all positive results should be carefully considered for their clinical relevance.