Itchy, red, inflamed skin can have many causes, and two of the most common are contact dermatitis and atopic dermatitis (eczema). These conditions look similar, can occur simultaneously in the same patient, and are frequently confused.
What Is Atopic Dermatitis (Eczema)?
Atopic dermatitis is a chronic, systemic inflammatory condition driven by an overactive immune response, specifically Th2 inflammation, that compromises the skin barrier. It is genetic in origin, often begins in childhood, and is associated with asthma, hay fever, and food allergies.
The rash tends to appear in characteristic locations: the insides of the elbows and knees, the back of the neck, and the wrists.
What Is Contact Dermatitis?
Contact dermatitis is inflammation caused by direct skin contact with an external substance. It comes in two forms:
Irritant contact dermatitis, direct chemical damage from soaps, detergents, acids, solvents, or repeated water exposure. No immune sensitization required.
Allergic contact dermatitis, a true Type IV hypersensitivity reaction to a specific substance. Common allergens include nickel (jewelry), fragrances, preservatives, latex, hair dye (PPD), and poison ivy.
Key Differences
- Cause: Genetic/immune (atopic) vs. external substance (contact)
- Location: Flexural areas (atopic) vs. where contact occurred (contact)
- Age of onset: Usually childhood (atopic) vs. any age (contact)
- Family history: Common (atopic) vs. not typical (contact)
- Testing: Clinical diagnosis (atopic) vs. patch testing for allergic type (contact)
Treatment Implications
For contact dermatitis, identifying and eliminating the triggering substance is the primary treatment. For atopic dermatitis, systemic treatment with topical steroids, calcineurin inhibitors, or biologic therapies targets the underlying immune dysfunction.