How to Tell the Difference Between Nummular Eczema and Ringworm

Nummular eczema and ringworm look very similar, and it can be very difficult for the average person to distinguish it. “Even a general doctor without an expertise in dermatology can have a hard time spotting the subtle differences,” said dermatologist Dr. Benjamin Ungar, director of the Alopecia Center of Excellence and director of the Rosacea & Seborrheic Dermatitis Clinic at Mount Sinai in New York. 

“I would suggest seeing a dermatologist — the differences in appearance [between nummular eczema and ringworm] can often be subtle and the treatments are not aligned,” Dr. Ungar said. “It’s better to see a specialist who is more likely to be able to distinguish between the two entities.”

What is nummular eczema and how is it treated?

Nummular eczema, also known as discoid eczema and nummular dermatitis, is an inflammatory condition that features scattered circular, often itchy and sometimes oozing patches. The word “nummular” comes from the Latin word for “coin,” as the spots can look coin-shaped on the skin. 

“Nummular eczema is usually treated with an anti-inflammatory treatment, often something like a topical steroid,” said Dr. Ungar. 

Nummular eczema tends to occur later in life compared to atopic dermatitis, which can start in infancy. 

What is ringworm and how is it treated?

“Ringworm is a bit of a misnomer,” Dr. Ungar said, “because it is, in fact, caused by a fungus.” Not by a worm. 

Ringworm is a superficial fungal infection, typically treated with a topical antifungal cream. 

“There can be occasional circumstances where an oral or systemic antifungal may be needed [for ringworm],” Dr. Ungar said. 

How do dermatologists diagnose you with nummular eczema or ringworm? 

Since both conditions look so alike on the skin, there aren’t any hard and fast rules to distinguish the two, said Dr. Ungar. 

“There’s no one approach [to diagnosis],” Dr. Ungar said. There are often clinical clues that may be more subtle that a dermatologist with training and experience can identify.

“One clue that can sometimes be used to distinguish the two, although should not be used as kind of a clear cut indication, is that often tinea, or ringworm, will have some clearing or more normal-appearing skin in the center of the lesion with more of the scaling and red part of it towards the periphery,” Dr. Ungar said. 

But, he added, nummular eczema can sometimes look like this, as well, so it’s not an ironclad rule. 

Additionally, nummular eczema tends to affect the limbs more than the trunk, although it can sometimes affect the trunk of the body, he said. In contrast, ringworm can affect anywhere on the body. 

“The manifestation of a similar fungal infection in areas, such as the groin, can certainly happen quite commonly, although that’s typically referred to as jock itch rather than ringworm,” he said. “But it is the same kind of process where there’s that superficial fungal infection, it just appears a little different depending on the area of the body that it affects.”

In some cases, a biopsy or scraping can help with diagnosis, although Dr. Ungar said this isn’t common. 

When should I see a doctor for my nummular eczema or ringworm? 

“I wouldn’t necessarily encourage people to try to self-diagnose because the approaches to treating it are different,” Dr. Ungar said. 

“It’s not a bad idea to make an appointment, and, in some cases, maybe it resolves before the appointment comes around,” he explained. “But it’s generally better to assume that it won’t resolve.”

There are some skin conditions where Dr. Ungar thinks it’s OK to try home diagnosis and remedies, but with nummular eczema and ringworm, “if someone’s trying to distinguish between the two, it’s just going to be too difficult to do that.”