Differential diagnosis of nummular eczema
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"nummular eczema" OR "discoid eczema" AND "differential diagnosis"
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I now have comprehensive data from all the major dermatology references. Here is the full answer:
Differential Diagnosis of Nummular Eczema
Nummular eczema (discoid eczema) presents as coin-shaped, erythematous, vesicular, crusted plaques - typically 2-4 cm in diameter - on the extremities and trunk. Because the circular morphology is shared by several other dermatoses, a structured differential is important.
Most Likely (Common Mimics)
| Condition | Key Distinguishing Features |
|---|---|
| Allergic contact dermatitis | Geometric distribution matching allergen exposure; history of contactant (nickel, fragrance, rubber, neomycin); positive patch testing (32-50% of NE patients have relevant positive tests - these may overlap or be primary cause) |
| Atopic dermatitis (nummular type) | More chronic/lichenified lesions; personal/family history of atopy; begins in childhood; flexural predilection; elevated IgE common; nummular morphology more typical in adolescents with AD |
| Stasis dermatitis | Restricted to lower legs with dependent edema; associated with varicose veins and hemosiderin deposition; can mimic or disseminate as nummular plaques |
| Tinea corporis | Must always be ruled out - KOH scraping shows hyphae; active scaly border with central clearing (central sparing); single or few lesions common; responds to antifungals; NE lacks central clearing |
Consider
| Condition | Key Distinguishing Features |
|---|---|
| Psoriasis (longstanding plaques) | Silvery micaceous scale; Auspitz sign; nail pitting/onycholysis; well-defined plaques at extensor elbows, knees, scalp; Koebner phenomenon; family history; biopsy shows acanthosis, dilated capillaries, neutrophilic parakeratosis without spongiosis |
| Impetigo | Honey-colored crusts; rapidly spreading; fever; positive bacterial culture; common in children; responds to antibiotics |
| Mycosis fungoides (CTCL) | Longstanding, recalcitrant plaques; may be misdiagnosed for years; photodistributed early lesions; skin biopsy with atypical lymphocytes in epidermis (epidermotropism); CD4+ T-cell dominance; clonal TCR rearrangement |
| Paget disease | Unilateral involvement of nipple/areola only; persistent, resistant to steroids; biopsy shows large pale Paget cells |
| Bullous pemphigoid | Tense blisters, not vesicles; urticarial prodrome; elderly; direct immunofluorescence shows IgG + C3 at dermoepidermal junction |
| Pemphigus vulgaris | Flaccid blisters; mucosal involvement; Nikolsky sign positive; acantholysis on biopsy; IgG against desmoglein |
| Bowen disease (SCC in situ) | Solitary, slowly enlarging, sharply defined plaque; hyperkeratotic/scaly surface; sun-exposed or anogenital sites; biopsy shows full-thickness epidermal dysplasia |
Other Nummular Dermatoses to Consider
| Condition | Key Distinguishing Features |
|---|---|
| Fixed drug eruption | Recurs at same site(s) with drug re-exposure; often hyperpigmented; medication history |
| Pityriasis rotunda | Circular, scaly, hyperpigmented or hypopigmented patches; often in Asian or African patients; associated with systemic disease (malignancy, liver disease) |
| Erythema migrans (Lyme disease) | Expanding annular erythema; central clearing; tick exposure; flu-like illness; southern/mid-Atlantic endemic area; not vesicular or crusted |
Always Rule Out
- Tinea corporis - KOH preparation and/or fungal culture is mandatory in all cases before initiating steroid therapy, as steroids will worsen tinea (tinea incognito).
Diagnostic Approach
- KOH preparation - first step to exclude tinea corporis
- Patch testing - indicated in chronic/recalcitrant cases; clinically relevant allergens found in 12-67% of positive testers
- Skin biopsy - if psoriasis, CTCL, bullous pemphigoid, or Bowen disease is suspected; histology shows spongiotic dermatitis (acute), or parakeratosis, scale crust, psoriasiform hyperplasia (subacute)
- Bacterial culture - if impetigo or secondary infection suspected
- Direct immunofluorescence - if autoimmune blistering disorder is in the differential
Sources: Fitzpatrick's Dermatology, 9e (Table 23-1); Andrews' Diseases of the Skin, p. 98; Dermatology 2-Vol Set 5e, p. 993; Textbook of Family Medicine 9e, p. 934
Create a diagnostic algorithm or decision tree for nummular eczema vs. mimics
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Here is the diagnostic algorithm. The flowchart works through a sequential rule-out strategy:
Logic of the algorithm:
- KOH scrape first - tinea corporis must be excluded before any steroid is applied (steroids worsen tinea, causing "tinea incognito")
- Blistering disorders next - bullous pemphigoid and pemphigus present urgently and require DIF biopsy
- Impetigo - quick to diagnose clinically, especially in children
- Psoriasis - distinguished by thick silvery scale, Auspitz sign, nail changes, extensor predominance
- Paget disease - rare but important; always consider if a unilateral nipple/areola plaque fails to respond
- Mycosis fungoides - flagged for any chronic, steroid-resistant plaque over 6 months; biopsy with TCR rearrangement testing
- Allergic contact dermatitis - patch testing is indicated here; up to 50% of NE patients have a relevant positive allergen
- Stasis dermatitis - lower-leg distribution with venous signs
- Atopic dermatitis (nummular type) - more lichenified, chronic, flexural, with atopic history
- Nummular eczema - diagnosis of exclusion at the bottom, confirmed by spongiotic biopsy if needed
The color coding reflects clinical urgency: green nodes should be ruled out early, orange signals biopsy-urgent conditions, and teal marks diagnoses needing additional workup.
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