Differential Diagnosis of Nummular Dermatitis
When evaluating coin-shaped eczematous lesions, you must systematically exclude tinea corporis, contact dermatitis (both allergic and irritant), psoriasis, atopic dermatitis, and less commonly pityriasis rosea, lichen planus, and granuloma annulare 1, 2.
Primary Differentials to Rule Out
Tinea Corporis (Most Critical to Exclude)
- Perform potassium hydroxide (KOH) examination of skin scrapings immediately - this is the single most important diagnostic step 1, 2
- Look for: centrifugally expanding annular patches with raised, scaly borders and central clearing
- Unlike nummular dermatitis, tinea typically shows more prominent scale at the advancing edge
Contact Dermatitis
Patch testing should be performed in chronic, recalcitrant cases 2, as contact allergy is common with nummular eczema:
- Allergic contact dermatitis: Affects only susceptible individuals; common triggers include nickel (10% of women with pierced ears), neomycin (5-15% of chronic external otitis patients), cosmetics, soaps, and topical medications 3
- Irritant contact dermatitis: Dose-dependent reaction affecting all individuals; caused by direct chemical damage from acids or alkalis 3
- Key distinguishing feature: maculopapular or eczematous eruption in areas of contact with specific agents
Atopic Dermatitis
Distinguish by these specific features 4:
- Age-specific distribution: flexural surfaces in children/adolescents vs. extensor surfaces in infancy
- Symmetrical distribution (nummular eczema can be asymmetric)
- Personal or family history of atopy (allergic rhinitis, asthma)
- Chronic pruritus typically starting in childhood (80% develop symptoms within first 5 years)
- Multiple body areas involved, not just discrete coin-shaped lesions
Psoriasis
- Sharply demarcated, erythematous plaques with silvery scale 1
- Typically involves extensor surfaces, scalp, and nails
- Less pruritic than nummular eczema
- Plaques are more uniform in appearance
Secondary Differentials
Seborrheic Dermatitis
- Greasy yellowish scaling affecting ears, scalp, central face, and sebaceous areas 3
- Associated with Malassezia yeast
- More pronounced in Down syndrome, HIV, and Parkinson's disease
Pityriasis Rosea
- Multiple erythematous lesions with raised, scaly borders 1
- Herald patch typically precedes generalized eruption
- Self-limited course (usually resolves in 6-8 weeks)
Lichen Planus
- Planar, purple, polygonal, pruritic papules and plaques 1
- Wickham's striae visible on surface
- Often involves oral mucosa
Less Common Considerations
- Erythema multiforme: Target lesions with central clearing, often triggered by infections 1
- Granuloma annulare: Non-scaly annular plaques (key differentiator from nummular eczema)
- Discoid lupus erythematosus: Scarring, follicular plugging, characteristic skin lesions 3
- Secondary syphilis: Consider in appropriate clinical context with systemic symptoms 1
Diagnostic Approach Algorithm
Examine lesion morphology: Coin-shaped (1-10cm), erythematous, eczematous plaques with intense pruritus on lower/upper limbs 2
Assess distribution pattern:
- Symmetrical + flexural + childhood onset → consider atopic dermatitis
- Asymmetrical + coin-shaped + adult onset → favors nummular eczema
Evaluate for contact allergens: If chronic/recalcitrant, perform patch testing 2
Look for associated conditions: Nummular eczema patients have increased risk of lichen simplex chronicus (RR 9.71), irritant contact dermatitis (RR 9.60), and pruritus (RR 5.71) 5
Critical Pitfalls to Avoid
- Never assume nummular eczema without KOH examination - missing tinea corporis leads to treatment failure and spread
- Don't overlook contact dermatitis - 18% of nummular eczema cases have coexisting atopic dermatitis 5, and contact allergy is common
- Beware of neomycin sensitization - 13% of normal volunteers are hypersensitive; avoid neomycin-containing topicals in chronic cases 3
- In immunodeficient patients with persistent eczema-like lesions following minor trauma, consider underlying immune dysregulation 6