Nummular dermatitis (nummular eczema) is a particular form of dermatitis characterized by pruritic, coin-shaped, scaly plaques. It is of uncertain etiology, but the onset is associated with triggers such as frequent bathing, low humidity, irritating and drying soaps, skin trauma, interferon therapy for hepatitis C, and exposure to irritating fabrics such as wool. Venous stasis may be a predisposing factor to developing lesions on the legs. Many authorities consider it to be a form of eczema, and some patients often have some of the signs and symptoms associated with classic atopic dermatitis. However, a majority of individuals with nummular dermatitis do not have a personal or family history of atopy. Infectious etiologies, particularly respiratory and dental infections, have been investigated but this association remains unclear. Ultimately, the etiology of nummular dermatitis remains incompletely elucidated.

Nummular dermatitis is pruritic but may be less pruritic than other common diagnoses with scaly plaques (eg, tinea). Autoeczematization (widespread eczematous eruption secondary to triggers such as infection or severe localized eczema) and impetiginization (superinfection of impaired skin barrier) may be seen. Nummular dermatitis is most common in men, and the peak age of incidence is between 50 and 65 years. The second most common age range of incidence is 15-25 years.