Nummular dermatitis is a condition confined to the skin. Nearly all acutely oozing lesions will reveal S. aureus by culture, however, the majority simply reflect superficial colonization. Such non-invasive colonization does not require antibiotic therapy but will be eradicated by relieving the dermatitis with topical steroids. Spreading dermal erythema, tenderness and frank pus are indicators of invasive infection and the need for antibiotic therapy. No racial predilection has been observed.
Nummular dermatitis is more common in males.
The incidence peaks in the sixth and seventh decades. Another common population is atopics in the second and third decades. Nummular dermatitis is rare in children. Lesions begin as papules or vesicles that coalesce to form confluent plaques with an erythematous base. Early lesions may be exudative and crusted, and due to the moist environment, often become colonized by staphylococci, which increases the severity and duration of the lesion.