Differential Diagnosis of Dermatitis
The differential diagnosis for a dermatitis-type rash includes atopic dermatitis (most common, ~20% point prevalence), contact dermatitis (both irritant and allergic), seborrheic dermatitis, psoriasis, and less commonly cutaneous T-cell lymphoma, pityriasis rubra pilaris, and systemic conditions. 1, 2
Primary Categories to Distinguish
Atopic/Endogenous Dermatitis
- Most common form comprising the majority of dermatitis cases 2
- Presents with chronic pruritus starting in childhood with symmetrical, age-specific distribution 3
- Personal or family history of atopy (asthma, hay fever) 1
- Infancy: affects cheeks, neck, trunk, extensor surfaces; spares diaper area 3
- Childhood/adolescence: flexor surfaces, lichenification 3
- Approximately 80% develop symptoms within first 5 years of life 3
Contact Dermatitis (4-7% of dermatological consultations) 1, 2
Irritant Contact Dermatitis (more common than allergic)
- Direct chemical damage from acids, alkalis, or repetitive exposure to weak irritants 2, 4
- Dose-dependent; affects all individuals 4
- Can be acute (single overwhelming exposure) or chronic/cumulative (repetitive exposure to detergents, solvents, water) 2
Allergic Contact Dermatitis (worse prognosis unless allergen identified) 1, 2
- Type IV delayed hypersensitivity reaction 2
- Only affects sensitized individuals 4
- Common allergens: nickel (14.5%), fragrances (37%), cobalt (22%), PPD (1%) 1
- Critical point: Pattern and morphology alone are unreliable in predicting cause 1
Seborrheic Dermatitis
- Affects ears, scalp, central face, sebaceous areas 4
- Greasy yellowish scaling with Malassezia yeast involvement 4
- Affects groin and axillary regions (unlike atopic dermatitis) 5
- Not typically pruritic 5
Psoriasis
- Indurated plaques with silvery scale 6
- Can present as erythroderma in severe cases 6
- May have nail pitting, joint involvement 6
Less Common but Important Differentials
Cutaneous T-cell Lymphoma/Mycosis Fungoides
- Consider in adults not responding to therapy 5
- Look for atypical lymphocytes on biopsy 6
- May require flow cytometry, Sézary cell count 6
Pityriasis Rubra Pilaris
- Diffuse erythroderma with psoriatic-like scale 6
- "Skip" areas characteristic 6
- Significant palmoplantar keratoderma 6
Essential History Elements to Narrow Diagnosis
You must systematically assess: 1
- Atopic history: Personal history of childhood dermatitis, asthma, hay fever; family history of atopy
- Temporal pattern: Where symptoms began, spread pattern, chronic/relapsing course
- Product exposure: Cosmetics, personal-care products, topical medications, clothing, bandages, gloves
- Wash products: Most contain harsh emulsifiers/surfactants damaging skin barrier in predisposed individuals
- Activity relationship: Hairdressing, holidays, home improvements, recreation
- Occupational factors: Detailed work history, products handled, health/safety data sheets
- Environmental changes: Improvement on weekends/holidays, recurrence at work
- Photosensitivity: Worsening after sunlight exposure
Critical Diagnostic Pitfalls
The morphology and pattern of dermatitis, particularly on hands and face, is unreliable in distinguishing atopic from contact dermatitis 1. This is true even in children with known atopic dermatitis 1.
Coexistence is common: Endogenous, irritant, and allergic etiologies frequently coexist, particularly in hand and foot eczema 2. Allergic contact dermatitis may both mimic AD and exacerbate existing AD 5.
When to Pursue Patch Testing
Patch testing is essential when contact allergy is suspected or cannot be ruled out in persistent eczematous eruptions 2. The test has 70-80% sensitivity and specificity 2.
Specific indications:
- Hand and facial eczema (clinical features alone are unreliable) 2
- Children with hand and eyelid eczema 2
- Not responding to standard therapy 1
Other Conditions to Consider
- Protein contact dermatitis: Positive prick tests but negative patch tests; seen in food handlers, bakers, veterinarians 1
- Nutritional deficiencies, metabolic conditions, primary immunodeficiencies: Especially if not responding to therapy 3
- Subacute cutaneous lupus 7
- Scabies, tinea, non-bullous pemphigoid (in older patients) 7
No reliable biomarker exists for atopic dermatitis; elevated IgE is absent in ~20% of cases and non-specific 5.