Nummular Eczema: Causes, Triggers, and Treatment for Coin-Shaped Patches

What Is Nummular Eczema?

Nummular eczema (also called nummular dermatitis or discoid eczema) is a type of eczema characterized by well-defined, coin-shaped (nummular comes from the Latin “nummulus,” meaning coin) patches on the skin. These round or oval plaques can range from 1 to 10 centimeters in diameter and tend to appear on the arms, legs, and trunk [1].

Unlike the poorly defined, widespread patches typical of atopic dermatitis, nummular eczema produces discrete, sharply bordered lesions. This distinctive shape often leads to confusion with tinea corporis (ringworm) or psoriasis, and misdiagnosis is common. If a “ringworm” patch does not respond to antifungal cream after 2-4 weeks, nummular eczema should be strongly considered [2].

Who Gets Nummular Eczema?

Nummular eczema has two peak age groups with a notable sex difference [1]:

• Men: Most commonly affected between ages 50-65. This is the larger demographic group.
• Women: A smaller peak occurs between ages 15-25, often in association with atopic dermatitis.

The overall prevalence is estimated at about 2 per 1,000 people. It accounts for roughly 10% of dermatitis cases seen in dermatology clinics [3]. While nummular eczema can occur alongside atopic dermatitis, many patients have no personal or family history of atopy. This sets it apart from classic atopic dermatitis, which almost always has an atopic background.

What Causes Nummular Eczema?

The exact cause is not fully understood, but several contributing factors have been identified.

Dry Skin (Xerosis)

Severely dry skin is the most consistent underlying factor. The condition is far more common in winter months when low humidity and indoor heating strip moisture from the skin. Excessive bathing with hot water and harsh soaps compounds the problem by disrupting the skin’s lipid barrier [4]. Older adults are particularly susceptible because skin barrier function naturally declines with age, resulting in increased transepidermal water loss.

Bacterial Colonization

Staphylococcus aureus plays a more prominent role in nummular eczema than in most other eczema subtypes. Studies have found S. aureus colonization in up to 90% of nummular eczema lesions, and bacterial superantigens are thought to amplify the local inflammatory response [5]. This bacterial involvement helps explain why nummular eczema patches often have a weeping, crusted surface and why antibacterial treatments can be particularly helpful.

Skin Injury and Irritants

Nummular eczema frequently develops at sites of prior skin trauma, including insect bites, abrasions, chemical burns, and surgical scars. This phenomenon (a type of Koebner response) is well documented and is one reason patches sometimes appear in unusual locations [2]. Contact irritants such as formaldehyde, metals (particularly nickel and cobalt), and chemical solvents can trigger or worsen lesions.

Other Contributing Factors

• Venous insufficiency: In older adults, nummular eczema on the lower legs often coexists with venous stasis, and poor circulation contributes to skin breakdown [6].
• Medications: Certain drugs have been associated with nummular eczema, including isotretinoin, interferon, ribavirin, and some targeted cancer therapies [2].
• Stress: Emotional stress is reported as a triggering factor by many patients, consistent with the neuroimmune connection seen across eczema subtypes [3].
• Alcohol use: Chronic heavy alcohol use is associated with increased incidence, possibly through dehydration and nutritional deficiency effects on skin barrier function.

What Nummular Eczema Looks Like

The clinical presentation follows a characteristic pattern [1]:

Acute Lesions

• Round or oval plaques with well-defined borders
• Erythematous (red) base covered with tiny vesicles (blisters), papules, and sometimes crusting
• Weeping or oozing surface, especially when S. aureus colonization is heavy
• Intensely pruritic (itchy)

Chronic Lesions

• Dry, scaly, lichenified (thickened) plaques
• Hyperpigmented or hypopigmented post-inflammatory changes
• Less itchy than acute lesions but still symptomatic

Common Locations

• Extensor surfaces of arms and legs (especially the shins and forearms)
• Dorsal hands
• Trunk
• Less commonly: face and scalp (more typical in children)

Lesions can be single or multiple and may appear in clusters. New patches can appear while older ones are healing, giving the impression that the condition is spreading. A key diagnostic clue: unlike ringworm, which typically clears centrally to form a ring, nummular eczema remains uniformly involved throughout the patch [7].

Diagnosis

Diagnosis is primarily clinical, based on the distinctive coin-shaped morphology. However, several conditions mimic nummular eczema:

• Tinea corporis (ringworm): KOH preparation or fungal culture will be positive for dermatophytes. Ringworm tends to have central clearing, while nummular eczema does not.
• Psoriasis: Typically has thicker, silvery-white scale and involves characteristic sites (elbows, knees, scalp, nails). Biopsy can differentiate if needed.
• Contact dermatitis: Distribution follows exposure patterns rather than the random coin-shaped distribution of nummular eczema.
• Cutaneous T-cell lymphoma (mycosis fungoides): Should be considered when patches are persistent, treatment-resistant, and located in non-sun-exposed areas. Biopsy is indicated [8].

When the diagnosis is uncertain, patch testing for contact allergens and skin biopsy are the most useful next steps. A bacterial culture can help guide antibiotic selection when secondary infection is suspected.

Treatment

Aggressive Moisturizing

Restoring the skin barrier is foundational. Patients should apply a thick, fragrance-free ointment or cream at least twice daily and immediately after bathing. Petroleum jelly and ceramide-containing creams (such as CeraVe Moisturizing Cream) are top choices. Lotions are insufficient for nummular eczema because they evaporate too quickly to provide adequate barrier repair [4].

Bathing should be brief (5-10 minutes), with lukewarm water and a gentle, soap-free cleanser. Pat skin dry and apply moisturizer within 3 minutes while skin is still damp.

Topical Corticosteroids

Mid-to-high potency topical corticosteroids are the first-line anti-inflammatory treatment. Nummular eczema patches are often thick and stubborn, requiring stronger steroids than typical atopic dermatitis [9]:

• For trunk and limbs: Triamcinolone acetonide 0.1% ointment or betamethasone valerate 0.1% ointment, applied twice daily
• For thick, resistant plaques: Clobetasol propionate 0.05% ointment for 2-4 week courses
• Occlusion technique: Applying the steroid under plastic wrap or a hydrocolloid dressing overnight dramatically increases penetration into thick plaques

Treatment typically needs to continue for 3-4 weeks or longer. Stopping too early is a common reason for relapse. Once the patch clears, proactive maintenance with a mid-potency steroid 2-3 times per week can prevent recurrence at the same site [9].

Addressing Bacterial Colonization

Given the high rates of S. aureus involvement in nummular eczema, antibacterial strategies are often necessary [5]:

• Bleach baths: One-quarter cup of regular household bleach in a full bathtub, soaking for 10-15 minutes, 2-3 times per week. This reduces staphylococcal colonization without antibiotic resistance concerns.
• Topical antibiotics: Mupirocin 2% ointment applied to crusted or weeping patches for 5-7 days.
• Oral antibiotics: Cephalexin or dicloxacillin for 7-10 days when there is clear clinical infection (increased erythema, warmth, purulent drainage, or crusting spreading beyond the eczema patch).

Topical Calcineurin Inhibitors

Tacrolimus 0.1% ointment and pimecrolimus 1% cream are steroid-sparing alternatives that can be used for maintenance therapy once corticosteroids have brought the flare under control. They are particularly useful for patches in areas where prolonged steroid use is a concern, such as the face or skin folds [10].

Phototherapy

Narrowband UVB phototherapy is effective for widespread or resistant nummular eczema. Treatment is given 2-3 times per week for 8-12 weeks. It works by suppressing the T-cell-mediated inflammation driving the patches and has the added benefit of reducing bacterial colonization on the skin [11]. PUVA (psoralen plus UVA) is an alternative for very resistant cases.

Systemic Therapy

For severe, widespread nummular eczema that does not respond to topicals and phototherapy:

• Short course of oral corticosteroids: Prednisone 0.5 mg/kg for 1-2 weeks with a taper can break a severe flare cycle. Rebound is common, so a maintenance plan with topicals should already be in place.
• Dupilumab: Case reports and small series suggest benefit for recalcitrant nummular eczema, given its broad anti-type 2 inflammatory action [12].
• Methotrexate or cyclosporine: Reserved for severe, refractory cases. Cyclosporine offers rapid onset but is limited to short-term use.

How Nummular Eczema Differs from Other Eczema Types

Understanding these differences helps guide accurate diagnosis and appropriate treatment:

• Atopic dermatitis: Poorly defined patches in flexural areas (elbow creases, behind knees). Strong atopic background. Nummular eczema has sharply defined, coin-shaped patches in non-flexural areas, and many patients have no atopic history.
• Dyshidrotic eczema: Tiny deep blisters on palms, fingers, and soles. Nummular eczema rarely involves the palms or soles.
• Contact dermatitis: Distribution matches exposure patterns. Nummular eczema appears in a scattered, seemingly random pattern.
• Seborrheic dermatitis: Affects sebaceous gland-rich areas (scalp, eyebrows, nasolabial folds). Greasy, yellowish scale. Nummular eczema avoids these areas.

Prognosis and Long-Term Outlook

Nummular eczema can be frustrating because patches are slow to resolve (weeks to months) and tend to recur. However, the long-term prognosis is generally good [1]:

• Most patches clear with consistent treatment over 4-8 weeks
• Recurrence is common, particularly in winter months or when moisturizing habits slip
• Post-inflammatory hyperpigmentation or hypopigmentation may persist for months after the eczema itself resolves (this is cosmetic and not a sign of ongoing disease)
• With good skin care habits and trigger avoidance, many patients can reduce flare frequency significantly over time

The most important factor in long-term control is maintaining an aggressive moisturizing routine even between flares. Patients who treat their skin as “cured” and stop moisturizing when patches resolve are far more likely to experience recurrence.

When to See a Dermatologist

• Patches that do not respond to OTC moisturizers and hydrocortisone within 2-3 weeks
• Multiple or widespread coin-shaped lesions
• Signs of infection (increasing redness, warmth, pus, honey-colored crusting)
• Uncertainty about whether the rash is nummular eczema, ringworm, or psoriasis
• Persistent patches in a single location that resist treatment (biopsy may be needed to rule out other conditions)

Related Reading

• Eczema: Complete Guide
• Eczema Treatment: Steroids, Biologics, Natural Options
• Dyshidrotic Eczema: Hand and Foot Blistering Eczema Guide

References

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2. Silverberg JI. Practice gaps in the management of nummular eczema. J Am Acad Dermatol. 2021;85(3):743-744. doi:10.1016/j.jaad.2019.06.024
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