Make the Diagnosis

October 2018

A 30-year-old female presented with 2 days of intensely pruritic erythematous papules and vesicles on her bilateral arms and hands. The lesions began appearing 1 day after a camping trip. Her neck, chest, and upper back were clear.

What's your diagnosis?


Allergic contact dermatitis

Irritant contact dermatitis

Polymorphous light eruption

Allergic contact dermatitis (ACD) can affect individuals regardless of age, race, or sex, but ACD accounts for 20% of all contact dermatitis reactions. ACD results in an inflammatory reaction in those who have been previously sensitized to an allergen. This type of delayed hypersensitivity reaction is known as cell-mediated hypersensitivity. Generally, no reaction is elicited upon the first exposure to the allergen. In fact, it may take years of exposure to allergens for someone to develop an allergic contact dermatitis.

Courtesy Dr. Donna Bilu-Martin

Once sensitized, epidermal antigen-presenting cells (APCs) called Langerhans cells process the allergen and present it in a complex on the surface of the cell to a CD4+ T cell. Subsequently, inflammatory cytokines and mediators are released, resulting in an allergic cutaneous (eczematous) reaction. Lesions may appear to be vesicular or bullous. Occasionally, a generalized eruption may occur. With repeated exposure, reactions may be acute or chronic.

Common causes of allergic contact dermatitis include toxicodendron plants (poison ivy, oak, and sumac; cashew nut tree; and mango), metals (nickel and gold), topical antibiotics (neomycin and bacitracin), fragrance and Balsam of Peru, deodorant, preservatives (formaldehyde), and rubber (elastic and gloves).

Patch testing is the standard means of detecting which allergen is causing the sensitization in an individual. The Thin-Layer Rapid Use Epicutaneous (TRUE) test or individually prepared aluminum (Finn) chambers containing the most common allergens are applied to the patient’s upper back. The patches are removed after 48 hours and read, and then reevaluated at day 4 or 5. Positive reactions appear as eczematous or vesicular papules or plaques.

Treatment includes avoidance of the allergens. Topical corticosteroid creams are helpful. For severe or generalized reactions, oral prednisone may be used. It is important to note that patient may be allergic to topical steroids. Patch testing can be performed to elucidate such allergens.

Courtesy Dr. Donna Bilu-Martin

In contrast, 80% of contact dermatitis reactions are irritant, not allergic. Irritant contact dermatitis results is a local inflammatory reaction in people who have come into contact with a substance. Previous sensitization is not required. The reaction usually occurs immediately after exposure. Common causes include alkalis (detergents, soaps), acids (often found as an industrial work exposure), metals, solvents (occupational dermatitis), hydrocarbons, and chlorinated compounds.

This case and photo were submitted by Dr. Bilu Martin.

Dr. Bilu Martin is a board-certified dermatologist in private practice at Premier Dermatology, MD, in Aventura, Fla. More diagnostic cases are available at To submit a case for possible publication, send an email to

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