Nummular Dermatitis

Nummular dermatitis (discoid eczema) is a chronic, relapsing eczema that produces coin shaped, well demarcated, itchy plaques, most often on the lower limbs and extensor surfaces of the arms. Plaques usually measure 1 to 5 cm and can reach 10 cm.

It has two age peaks: older men, and younger women in late adolescence and early adult life. It is more frequent in winter and in dry climates, and it is uncommon in young children. Exact prevalence is uncertain.

Triggers and associations include dry skin, venous insufficiency of the legs, secondary colonisation with Staphylococcus aureus, contact allergy (nickel, other metals, fragrance and topical medicaments), skin injury such as insect bites or burns, and drugs such as isotretinoin and the combination of interferon with ribavirin. Atopy is present in a minority.

Pathophysiology.

Barrier failure and xerosis allow penetration of irritants, allergens and bacteria. This drives a spongiotic, T-cell mediated dermatitis with a mixed helper T-cell response. Staphylococcus aureus colonisation increases inflammation and the risk of crusting and spread.

• Round or oval plaques of grouped papules and vesicles that coalesce, ooze and crust in the acute phase, then become scaly, dry and lichenified when chronic.
• Symmetrical distribution over the shins, dorsal hands and feet, forearms and, less often, the trunk. The face is usually spared.
• Intense itch, often worse at night, and a burning sensation when lesions are weeping.
• Secondary impetiginisation (honey coloured crusts, pustules) and postinflammatory pigment change after healing. Relapse at the same sites is typical.

Red flags.

• A single plaque that does not respond to a potent topical corticosteroid after 4 to 6 weeks: biopsy to exclude Bowen disease, early mycosis fungoides and psoriasis.
• Fever, spreading erythema or pain, suggesting cellulitis or eczema herpeticum.
• Progression to generalised dermatitis or erythroderma.

• Clinical diagnosis from the morphology and distribution.
• Skin scraping for potassium hydroxide microscopy and fungal culture to exclude tinea, and a swab of crusted or weeping lesions.
• Patch testing for chronic, treatment resistant disease or when contact allergy is suspected, including the baseline series and a metal series.
• Assessment of venous disease in leg lesions: palpation of pulses, ankle brachial pressure index, and duplex ultrasound when varicosities or oedema are present.
• Skin biopsy for atypical, asymmetric or non-responding plaques: spongiotic dermatitis with parakeratosis.

Differential diagnosis. Tinea corporis, plaque psoriasis, atopic dermatitis, allergic or irritant contact dermatitis, stasis dermatitis, lichen simplex chronicus, discoid lupus erythematosus, Bowen disease, mycosis fungoides, scabies and impetigo.

Skin care and triggers.

• Liberal emollients several times daily, soap substitutes, short lukewarm washes, and avoidance of irritants and identified allergens. Treat venous insufficiency in leg disease.

Topical treatment.

• Potent corticosteroid (betamethasone valerate 0.1% or mometasone furoate 0.1%) once daily for 1 to 2 weeks, extended to 4 weeks if needed, then step down to a moderate potency (clobetasone butyrate 0.05%) and stop gradually.
• Very potent corticosteroid (clobetasol propionate 0.05%) once daily for up to 2 weeks for thick lichenified plaques on the limbs, avoiding the face and flexures.
• Occlusive medicated bandages (zinc paste with ichthammol) for resistant limb plaques.
• Tacrolimus 0.1% ointment twice daily as a steroid sparing agent (off label outside atopic dermatitis).
• Infected lesions: flucloxacillin 500 mg orally four times daily for 7 days, or clarithromycin 500 mg twice daily if penicillin allergic.
• Itch at night: a sedating antihistamine such as chlorphenamine 4 mg at night. Cetirizine 10 mg daily is non-sedating but has less effect on itch.

Phototherapy and systemic treatment.

• Narrowband ultraviolet B (311 nm) two to three times weekly for widespread or resistant disease.
• Prednisolone 0.5 mg/kg/day (usually 30 to 40 mg) for 5 to 7 days for severe flares only, because relapse after stopping is common.
• Steroid sparing drugs for chronic severe disease: methotrexate 7.5 to 15 mg once weekly (maximum 25 mg weekly) with folic acid 5 mg once weekly on a different day; ciclosporin 2.5 to 5 mg/kg/day in two divided doses, with blood pressure and creatinine monitoring; or azathioprine 1 to 3 mg/kg/day after thiopurine methyltransferase activity is measured.

Monitoring. Review at 2 to 4 weeks for response and signs of steroid atrophy. Continue emollients after clearance, since relapse is common.

Referral and escalation. Refer to dermatology for diagnostic doubt, failure of potent topical treatment after 4 to 6 weeks, or a need for phototherapy or systemic treatment.

Special groups. In children use a lower potency corticosteroid and tacrolimus 0.03% from 2 years of age. Methotrexate is contraindicated in pregnancy and requires reliable contraception in both sexes.