Contact Dermatitis: Definition and Classification
Contact dermatitis is a polymorphic pattern of cutaneous inflammation caused by external agents (exogenous substances) that act either as irritants (non-immune mediated) or allergens (cell-mediated immune response). 1
Clinical Presentation
The disease manifests differently depending on its phase:
- Acute phase: Characterized by erythema and vesiculation
- Chronic phase: Characterized by dryness, lichenification, and fissuring 1
Major Types and Subtypes
Contact dermatitis encompasses two primary forms with several distinct subtypes 1:
Irritant Contact Dermatitis (more common than allergic form)
Subjective irritancy: Idiosyncratic stinging/smarting reactions occurring within minutes of contact, typically on the face, without visible changes. Common triggers include cosmetics and sunscreen constituents.
Acute irritant contact dermatitis: Results from single overwhelming exposure or brief exposures to strong irritants or caustic agents.
Chronic (cumulative) irritant contact dermatitis: Develops following repetitive exposure to weaker irritants:
- "Wet" irritants: detergents, organic solvents, soaps, weak acids/alkalis
- "Dry" irritants: low-humidity air, heat, powders, paper, cardboard, dusts
Allergic Contact Dermatitis (carries worse prognosis unless allergen identified and avoided)
Involves immune system sensitization to specific allergen(s), resulting in dermatitis or exacerbation of pre-existing dermatitis. 1
Specialized Forms
Phototoxic, photoallergic, and photoaggravated contact dermatitis: Some allergens function as photoallergens.
Systemic contact dermatitis: Occurs after systemic administration of a chemical (usually a drug) to which topical sensitization previously occurred.
Protein contact dermatitis: Repetitive protein handling (usually foods) initially causes immediate urticarial symptoms, later progressing to dermatitic reaction. Prick tests and radioallergosorbent tests are positive; patch tests are negative. Common in food handlers, bakers, veterinarians, and abattoir workers handling vegetables, meats, fish, flour, enzymes, and animal products.
Epidemiology
- Point prevalence of dermatitis in the U.K.: approximately 20%
- Point prevalence of hand dermatitis: 2%
- Lifetime risk of developing hand dermatitis: 20%
- Contact dermatitis accounts for 4-7% of dermatological consultations
- Most common allergens in Europe: nickel (14.5%), fragrance (37%), cobalt (22%), hydroxyisohexyl cyclohexene carboxaldehyde (14%), and p-phenylenediamine (1%) 1
Critical Clinical Pitfall
Clinical features alone are unreliable in distinguishing allergic contact from irritant and endogenous dermatitis, particularly with hand and facial dermatitis. 1 This applies equally to children with atopic dermatitis. Therefore, patch testing should be offered to any patient with chronic or persistent dermatitis, as visual assessment cannot reliably differentiate between these conditions. 1