(See also Overview of Dermatitis.)
The cause of nummular dermatitis is unknown. Nummular dermatitis usually affects middle-aged and older people and occurs along with dry skin, especially in winter. Dermatophytid (id) reactions may manifest as nummular dermatitis. However, the rash may come and go without any apparent reason.
The round spots start as itchy patches of pimples and blisters that later ooze and form crusts. The rash may be widespread. Often, spots are more obvious on the backs of the arms or legs and on the buttocks, but they also appear on the torso. New spots tend to appear in areas where old spots have healed.
No treatment is always effective. People may benefit from skin moisturizers. Other treatments of nummular dermatitis include antibiotics taken by mouth, tap-water compresses, and corticosteroid creams or skin injections. Phototherapy (exposure to ultraviolet light—see Phototherapy) is used for severe or treatment-resistant disease. Immunosuppressive drugs taken by mouth such as cyclosporine or methotrexate may be prescribed when topical treatments do not work.