Courtesy of Dr D. Tennstedt and Prof. J.-M. Lachapelle, Louvain, Belgium.
The eruption runs a remitting and relapsing course and is worse during the winter months. The exacerbations that occur in winter have been associated with increased dryness of the skin due to central heating .
In most patients, the condition persists for many months. Parents should be warned that the disease may be expected to improve only slowly. Cowan  followed a series of patients for up to 2 years and found that 53% were never without lesions, except when using local therapy.
Nummular dermatitis must be differentiated from two common paediatric conditions: impetigo and tinea corporis. The wet form, with oozing and crusting, is frequently confused with impetigo. The dry form, with erythema and scaling, may resemble tinea corporis. Direct microscopic examination, fungal cultures and sometimes biopsies are decisive. Nummular lesions may also be found in children with typical atopic dermatitis. A history of atopy, IgE and radio-allergosorbent test (RAST) determination, and skin tests may be diagnostically helpful. Psoriasis should also be considered and is characterized by drier and scalier lesions, without vesicles. Contact dermatitis should be suspected if the lesions are asymmetrical and localized to the dorsal aspects of hands or feet. Patch tests can be helpful in differentiating these two conditions. Pityriasis alba, pityriasis rosea, halo dermatitis, lichen simplex chronicus, seborrhoeic dermatitis  and other conditions, which, on occasion, exhibit a nummular pattern must also be considered in the differential diagnosis (Box 40.1).
Box 40.1 Differential Diagnosis of Discoid Lesions
- Nummular atopic dermatitis
- Tinea corporis
- Allergic contact dermatitis
- Pityriasis alba
- Pityriasis rosea
- Halo dermatitis
- Seborrhoeic dermatitis
- Lichen simplex chronicus