Allergic contact dermatitis
Contact dermatitis presents as either allergic or irritant in etiology. While irritant contact dermatitis represents the direct toxic effect of an offending agent on the skin, allergic contact dermatitis (ACD) represents a delayed-type (type IV) hypersensitivity reaction that occurs when allergens activate antigen-specific T cells in a sensitized individual. Consequently, whereas irritant contact dermatitis can occur after one exposure to the offending agent, ACD typically requires repeat exposures before an allergic response is noted. ACD can occur 24-48 hours after exposure to the offending agent.
Contact dermatitis can demonstrate well-demarcated borders, suggestive of an "outside job" or external contact. The most common contact allergens are urishiol (poison ivy, oak, or sumac), nickel, fragrance, cobalt (a metal), chromates (leather products), neomycin, thimerosal (ophthalmic preparations and vaccines), adhesives, and oxybenzone (sunscreens). Formaldehyde-releasing preservatives in polypropylene surgical masks have been reported to cause ACD.
The distribution and geometry of lesions are important clues to diagnosis. It can also present as a systemic contact reaction with widespread lesions when the offending agent is ingested or present in an implanted device. ACD can occur in reaction to topical agents, ingested agents, implanted biomedical devices, and airborne materials.
Per February 2019, isobornyl acrylate was named the "contact allergen of the year" by the American Contact Dermatitis Society. It is an acrylic monomer often used as an adhesive in medical devices, and there have been multiple case reports of diabetes patients developing contact allergies to their insulin pumps. Acrylic nails are also a potential source. Clinician awareness is important, because testing using routine panels does not identify isobornyl acrylate.
Related topic: Hand dermatitis
L23.9 – Allergic contact dermatitis, unspecified cause
40275004 – Contact dermatitis
- Atopic dermatitis
- Irritant contact dermatitis
- Nummular dermatitis (nummular eczema)
- Dyshidrotic dermatitis (dyshidrotic eczema)
- Insect bite reaction
- Stasis dermatitis
- Tinea corporis
- Psoriasis – Cases of ACD to topical nail agents mimicking psoriasis have been reported.
- Palmoplantar psoriasis
- Pityriasis rosea
- Photoallergic dermatitis – Can be distinguished from airborne ACD by the absence of dermatitis on the upper eyelid crease and on the central neck under the chin.
- Seborrheic dermatitis – The distribution of lesions is often a helpful clue in distinguishing this entity from atopic dermatitis.
- Lichen simplex chronicus
- Ichthyosis vulgaris
- Pityriasis rubra pilaris
- Secondary syphilis
- Glucagonoma syndrome
- Cutaneous T-cell lymphoma / mycosis fungoides – If an adult patient has persistent "eczema" that is not adequately responding to therapy, this entity should be ruled out with skin biopsies.
- Tinea pedis
- Herpes simplex virus infection
- Erythema multiforme