The shape is the giveaway: round, well-defined, coin-sized patches on a background of dry skin, with an itch that is out of proportion to how they look.
On brown and Black skin the patches are usually dark brown, purple or grey rather than red, so the inflammation is easy to underestimate, and small bumps around hair follicles are more common. Because the patches are round and the borders are sharp, they can look even more like ringworm on darker skin. After a patch settles it commonly leaves a dark or pale mark for months, sometimes longer than the eczema itself lasted. That is pigment change, not scarring.
The round shape sends most people down the wrong path. See Lookalikes near the bottom of this page.
Nummular eczema comes from the same underlying problem as other eczema: a weak skin barrier and an immune system that overreacts. The barrier is meant to hold water in and keep irritants out. When it leaks, the skin dries out, small irritants get through, and the immune system responds with inflammation and itch.
What makes this type different is how strongly it is tied to dryness. Most people who get it have very dry skin to begin with, which is why it peaks in winter, in older men whose skin naturally produces less oil, and in centrally heated homes. It also often starts at a specific point of injury — an insect bite, a scratch, a graze, a burn — and then spreads outward into a coin-shaped patch, so a single incident can begin a flare that lasts months. Contact allergies are found more often in nummular eczema than in other types, particularly to nickel, rubber and formaldehyde, which is why patch testing is sometimes worth doing. Poor circulation in the lower legs contributes in older adults. Unlike childhood eczema, it is much less associated with asthma and hay fever, and it is not caused by anything you ate or by being unclean.
The everyday basics matter more here than in almost any other kind of eczema, because very dry skin is what this condition sits on. Do these whether you are flaring or not — they are also what keeps next winter quieter.
This is one type of eczema where seeing someone early is usually worth it, because over-the-counter treatment is often not strong enough. Book an appointment if:
What the visit gets you: a scraping to settle the ringworm question in minutes. A steroid strong enough to actually work, which for the shins usually means something well above hydrocortisone. A swab if it looks infected. Patch testing if it keeps returning, since contact allergies are found more often in this type. And a clear instruction on how long to keep treating, which is the single thing that decides whether it comes back.
Nummular eczema is diagnosed by looking. There is no test that confirms it, but a couple of tests genuinely change what happens next.
A KOH scraping. This is the important one. Because nummular eczema is round and often clears in the centre, it is mistaken for ringworm constantly. A few flakes are scraped from the edge of a patch and looked at under the microscope for fungus, in the room, in a few minutes. It matters because the treatments are opposite: a steroid cream on ringworm makes it spread, and an antifungal cream on nummular eczema does nothing at all.
A skin swab. Taken when a patch is weeping, yellow-crusted or suddenly worse. Nummular eczema is one of the types most likely to get a bacterial infection on top, and the swab shows which antibiotic will clear it.
Patch testing. Worth doing when patches keep coming back or will not settle. Contact allergies are found more often in nummular eczema than in other types — nickel, rubber chemicals and formaldehyde among them. Small patches of common allergens are taped to the back and read over about a week. It is the only way to find a substance you are touching that is keeping this going.
A biopsy is occasionally done when the diagnosis does not fit, but it is not part of routine care.
Nummular eczema usually needs a stronger steroid and a longer course than people expect, especially on the shins. If two weeks of over-the-counter treatment has not shifted it, this is what a dermatologist will reach for.
Reserved for widespread or stubborn nummular eczema that has not responded to creams.
The itch settles first, usually within a few days of starting a proper strength steroid. The patches will still look obvious. Judge early progress by the itch.
Weeping and crusting stop, and the patches flatten and lose their angry colour. This is where most people assume it is finished and stop the cream. It is not finished.
This is the realistic time frame for a nummular patch to fully clear. It is slower than other eczema, and knowing that in advance prevents the stop-start cycle that keeps it going for months.
Dark or pale patches where the eczema was, most noticeable on brown and Black skin, lasting months. They are not scars and they do fade. Moisturizer and sun protection help them even out.
Nummular eczema comes and goes, and it favours the same spots each time. Winter is the usual trigger. The people who do best are the ones who keep moisturizing heavily through the cold months when their skin looks completely fine — that is what decides whether next winter is a bad one.
Nummular eczema is not dangerous, but broken, scratched skin creates a few problems worth knowing about.
The round shape is what causes the confusion. One lookalike accounts for most of it.