(See also Overview of Dermatitis.)
The cause of nummular dermatitis is unclear. It often develops in people who have hay fever or asthma. Sometimes certain bacteria that inhabit the skin (called bacterial colonization) appear to cause nummular dermatitis. An allergic reaction to a substance a person has come in contact with is also a possible cause. Sometimes spots appear on an area of the body that is not the area where the infection or reaction first began.
Nummular dermatitis usually affects middle-aged and older people.
Doctors base the diagnosis of nummular dermatitis on the characteristic appearance of the spots. They sometimes take samples of skin and send them to a laboratory (biopsy).
If people do not have atopy (a combination of atopic dermatitis, hay fever, and asthma), they sometimes undergo patch testing. In patch testing, small patches containing standard contact allergens are placed on the skin of the upper back and left on for 48 hours to see whether a rash develops beneath one of them. After 48 hours, they are removed, and the doctor evaluates the skin underneath. The skin is evaluated again a day or two later.
Treatment of nummular dermatitis is similar to that of atopic dermatitis and includes counseling about general skin care, anti-itch drugs, corticosteroids, and sometimes phototherapy.
Dupilumab injections, creams that contain tacrolimus or pimecrolimus, crisaborole ointment, or a combination may be given to some people.
Rarely, immunosuppressive drugs taken by mouth are needed.