Dermatitis Numularis

01Pathophysiology

Nummular dermatitis is a form of eczematous dermatitis characterized by its distinctive coin-shaped morphology ("nummular" derives from the Latin nummulus, meaning "little coin"). The fundamental defect lies in epidermal barrier dysfunction. In susceptible individuals, the stratum corneum fails to retain adequate moisture, leading to increased transepidermal water loss (TEWL). This barrier breakdown is why xerosis (dry skin) is both a risk factor and a perpetuating mechanism: as the skin dries, it cracks and allows environmental irritants and allergens to penetrate, triggering an inflammatory cascade.

At the histologic level, the hallmark is spongiosis, which refers to intercellular edema within the epidermis. Fluid accumulates between keratinocytes, weakening desmosomal attachments and eventually forming intraepidermal vesicles. This explains the clinical progression from early weeping, vesicular lesions to later dry, scaly, and lichenified plaques as the process becomes chronic. The localized, round morphology is believed to arise from discrete foci of barrier disruption that expand centrifugally.

Bacterial colonization, particularly by Staphylococcus aureus, plays a significant role. The disrupted barrier provides a favorable environment for bacterial adherence and biofilm formation. Staphylococcal superantigens can then amplify the T-cell-mediated inflammatory response, creating a cycle of inflammation, barrier damage, and secondary infection that sustains the disease. This is why treatment of bacterial superinfection often leads to notable clinical improvement.

The intense pruritus is mediated by a combination of inflammatory cytokines (IL-4, IL-13, IL-31) and histamine release from dermal mast cells. Scratching further damages the barrier, establishing an itch-scratch cycle that perpetuates the condition.

02Classification and Clinical Manifestation

Distribution patterns by demographics:

A clinically important variant is the dry (xerotic) form, which lacks vesiculation entirely and presents as round, scaly, erythematous patches on dry skin. This form is common in winter months and can be misidentified as psoriasis or tinea.

03Diagnostic Workup

The diagnosis of nummular dermatitis is predominantly clinical, based on the characteristic coin-shaped morphology and distribution pattern. However, the single most important step in the workup is a KOH preparation of skin scrapings. This test exists to answer one question: is this tinea corporis or nummular dermatitis? Tinea corporis (ringworm) is the most common diagnostic pitfall because both conditions present as round, scaly, erythematous plaques. A positive KOH showing branching hyphae confirms tinea. A negative KOH supports nummular dermatitis. On an exam, expect a vignette that describes a coin-shaped plaque and then tells you the KOH is negative. That negative KOH is your signal.

If the KOH result is ambiguous or if there is strong clinical suspicion for fungal infection despite a negative prep, a fungal culture can be sent. This takes days to weeks to result but offers definitive exclusion.

Skin biopsy is reserved for atypical presentations or cases that fail standard therapy. Histopathology will show spongiotic dermatitis: intercellular edema in the epidermis with a perivascular lymphocytic and sometimes eosinophilic infiltrate in the dermis. In chronic lesions, you will see acanthosis (epidermal thickening) and reduced spongiosis. This finding is not unique to nummular dermatitis (it is the general pattern of eczematous dermatitis), so biopsy confirms the category of disease rather than the exact diagnosis.

Patch testing should be considered in cases that are resistant to treatment, as an underlying contact allergy (commonly nickel, cobalt, fragrance, or balsam of Peru) may be driving the condition. This is particularly relevant in women with upper-extremity involvement.

04Management and Treatment

Step 1: Barrier Repair and Emollient Therapy.
Before any pharmacologic intervention, every patient must receive education about skin hydration. The "soak-and-seal" method is central: a lukewarm bath for 10-15 minutes followed by immediate application of a thick emollient (preferably petrolatum or a ceramide-containing ointment) within 3 minutes of patting the skin dry. This should be performed at least once daily and ideally twice daily. Patients should avoid hot water, harsh soaps, and wool clothing. This step alone can reduce flare frequency and severity.

Step 2: Topical Corticosteroids.
These are the first-line pharmacologic therapy. The potency selected should match the severity and location:

• Body and extremities: Start with triamcinolone acetonide 0.1% ointment (medium potency), applied twice daily for 2 to 4 weeks. If response is inadequate, escalate to betamethasone dipropionate 0.05% ointment (high potency) for up to 2 weeks.

• Face, intertriginous areas, and thin-skinned regions: Use low-potency agents (hydrocortisone 2.5%) or topical calcineurin inhibitors (tacrolimus 0.1%) to avoid steroid-induced skin atrophy.

• For thick, lichenified chronic plaques, occlusion with plastic wrap over the topical steroid overnight for 1-2 weeks enhances penetration.

Step 3: Treat Secondary Infection.
If there is honey-colored crusting, purulent drainage, or sudden worsening, suspect S. aureus superinfection. Obtain a wound culture and start empiric oral antibiotics covering S. aureus: cephalexin 500 mg four times daily or dicloxacillin 500 mg four times daily for 7-10 days. For localized, mild superinfection, mupirocin 2% ointment three times daily for 5-10 days is a reasonable alternative. Treating bacterial colonization often produces a dramatic improvement in lesions that seemed refractory to steroids alone.

Step 4: Manage Pruritus.
Itch control is essential to break the itch-scratch cycle. For daytime pruritus, non-sedating antihistamines (cetirizine 10 mg daily) provide mild relief, though their efficacy in eczematous itch is limited. Hydroxyzine 25-50 mg at bedtime is preferred for nocturnal pruritus because its sedating properties also improve sleep disrupted by scratching.

Step 5: Refractory Disease.
For patients who fail topical therapy after 4-6 weeks, narrowband UVB phototherapy is the next step. Sessions are performed 2-3 times per week for 8-12 weeks. This approach is particularly useful for widespread disease where applying topical steroids is impractical. If phototherapy is unavailable or ineffective, systemic agents such as methotrexate (10-15 mg weekly), azathioprine (1-3 mg/kg/day), or dupilumab (for cases with significant atopic overlap) may be considered under specialist supervision.

Contraindications and cautions:

• Avoid prolonged high-potency topical steroids on the face, groin, and axillae (risk of striae, atrophy, telangiectasias).

• Oral corticosteroids are generally avoided because nummular dermatitis rebounds aggressively upon discontinuation.

• In pregnant patients, low- to medium-potency topical corticosteroids are considered safe; calcineurin inhibitors and systemic agents require careful risk-benefit assessment.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common trap with nummular dermatitis on an exam is confusing it with tinea corporis. Both present as round, erythematous, scaly plaques. The vignette will describe a patient with a "coin-shaped" or "round" plaque on the leg and then give you a KOH result. If the KOH is positive, the answer is tinea. If the KOH is negative, the answer is nummular dermatitis. Students who do not read the KOH result carefully will pick tinea every time because the morphology looks identical. The exam is testing whether you understand that morphologic overlap demands a laboratory discriminator before committing to a diagnosis.

A second common error is treating nummular dermatitis with topical antifungals based on appearance alone without performing a KOH preparation first. The vignette may describe a patient who was started on clotrimazole by an outside provider and shows no improvement. The "failure to improve on antifungals" is the clue that this is not a fungal process.

Another testable point is the role of secondary bacterial infection. A vignette may describe a patient with worsening, crusted, weeping lesions that are unresponsive to topical steroids. The next best step in management is to obtain a culture and start oral antibiotics targeting S. aureus, not to escalate the steroid potency.

Finally, be aware that systemic corticosteroids are a distractor answer for severe nummular dermatitis. While they will transiently improve the condition, the disease rebounds upon discontinuation, often worse than baseline. The correct approach for refractory disease is phototherapy or steroid-sparing systemic agents, not a prednisone taper.

The core competency being tested is the ability to differentiate morphologically similar dermatologic conditions using the correct diagnostic test, and to recognize that treatment failure is a clinical clue pointing toward either a wrong diagnosis or an unaddressed complicating factor like superinfection.