What causes contact dermatitis?

Causes of Contact Dermatitis

Contact dermatitis is caused by external agents acting either as irritants (where cell-mediated immune response is not involved) or as allergens (where cell-mediated immunity is involved). 1

Types of Contact Dermatitis and Their Causes

Irritant Contact Dermatitis

• Subjective irritancy: Idiosyncratic stinging and smarting reactions occurring within minutes of contact, usually on the face, without visible changes. Common precipitants include cosmetics and sunscreen constituents. 1
• Acute irritant contact dermatitis: Results from a single overwhelming exposure or a few brief exposures to strong irritants or caustic agents. 1
• Chronic (cumulative) irritant contact dermatitis: Occurs following repetitive exposure to weaker irritants that may be either:
  • "Wet" irritants: Detergents, organic solvents, soaps, weak acids and alkalis 1
  • "Dry" irritants: Low-humidity air, heat, powders, paper, cardboard, and dusts 1

Allergic Contact Dermatitis

• Involves sensitization of the immune system to a specific allergen or allergens with resulting dermatitis or exacerbation of pre-existing dermatitis 1
• Common allergens include nickel (14.5%), fragrance compounds (37%), cobalt (22%), hydroxyisohexyl cyclohexene carboxaldehyde (14%) and p-phenylenediamine (PPD) (1%) 1
• Requires previous exposure and sensitization to the allergen, followed by re-exposure that triggers the delayed hypersensitivity reaction 2

Other Types of Contact Dermatitis

• Phototoxic, photoallergic and photoaggravated contact dermatitis: Some allergens are also photoallergens, requiring light exposure to trigger the reaction 1
• Systemic contact dermatitis: Seen after systemic administration of a substance, usually a drug, to which topical sensitization has previously occurred 1
• Protein contact dermatitis: Repetitive handling of proteins (usually foods) results initially in immediate urticarial symptoms and signs, later progressing to a dermatitic reaction. Common proteins include:
  • Vegetables (potato, garlic)
  • Meats
  • Fish (in food handlers)
  • Flour, enzymes (in bakers and pharmaceutical manufacture)
  • Animal dander and fluids (in veterinarians and abattoir workers) 1

Specific Irritants and Allergens

Common Irritants

• Soaps and detergents that damage the skin by causing denaturation of stratum corneum proteins 1
• Changes in intercellular lipids (either depletion or reorganization) 1
• Decreased corneocyte cohesion and decreased stratum corneum water-binding capacity 1
• Hot water used for handwashing 1
• Low relative humidity (most common in winter months) 1

Common Allergens

• Metals: Nickel is the most common contact allergen in the general population 1, 3
• Fragrances: One of the most common causes of allergic contact dermatitis 1
• Preservatives: Including methylisothiazolinone, which has been regulated in the European Union due to high rates of sensitization 4
• Topical antibiotics: Particularly in children 1
• Rubber accelerators: Found in various rubber products 1
• Acrylates: Found in medical devices, particularly those for diabetes control 4

Epidemiology and Risk Factors

• Contact dermatitis accounts for 4-7% of dermatological consultations 1
• Irritant contact dermatitis is more common than allergic contact dermatitis 1
• Allergic contact dermatitis carries a worse prognosis unless the allergen is identified and avoided 1
• The point prevalence of hand dermatitis is approximately 2%, with a lifetime risk of developing hand dermatitis of 20% 1
• Occupational factors play a significant role, with contact dermatitis making up approximately 70% of occupational skin disease 1
• The prevalence of contact allergy to specific allergens in the general population in Europe is estimated at between 10% and 27% 1

Pathomechanisms

• Contact allergens are small reactive chemicals with built-in autoadjuvanticity that allows them to trigger sterile inflammation following skin penetration 5
• The innate inflammatory response involves triggering pattern recognition receptors either by direct chemical interaction or by induction of endogenous activators 5
• Repeated exposure to irritants damages the skin barrier, changes skin flora, and results in more frequent colonization by staphylococci and gram-negative bacilli 1

Clinical Presentation

• In the acute phase: Characterized by erythema and vesiculation 1, 2
• In the chronic phase: Characterized by dryness, lichenification, and fissuring 1, 2
• Affected persons often complain of a feeling of dryness or burning, skin that feels "rough," and erythema, scaling, or fissures 1

Understanding these causes is essential for proper diagnosis, management, and prevention of contact dermatitis, which significantly impacts quality of life and can lead to chronic skin conditions if not properly addressed 6, 3.