Nummular dermatitis — also known as discoid eczema or nummular eczema — is a chronic skin condition that causes itchy, swollen, and cracked patches of skin. The word “nummular” is derived from the Latin word for “coin,” as the circular or oval-shaped patches that form on the skin resemble the size and shape of a coin.
Nummular dermatitis often affects the limbs — especially the legs — although the rash may be generalized across the body. Without proper treatment, nummular dermatitis can persist for a long time, sometimes lasting for years. However, with the right treatment, people with nummular dermatitis can feel better within 10 days.
Nummular dermatitis is often a chronic condition, so it may never fully resolve. It may clear up completely in some people. For others, it keeps coming back, often in the same areas that were previously affected.
There is no known single cause of nummular dermatitis. This condition can affect anyone regardless of age. However, it is slightly more common in women 15 to 25 years of age and older men in their 50s and 60s. Although it’s not clear why it is more prevalent among young adult women, nummular eczema has been associated with chronic alcoholism in men.
Nummular dermatitis, like other forms of eczema, is often attributed to triggers like dry skin. Having particularly dry skin weakens the barrier that protects against irritating substances and chemicals. As a result, coming into contact with otherwise harmless substances may cause skin irritation and itchiness. This itching leads to scratching, which can expose the skin to infection.
Nummular dermatitis may be caused by allergic or irritant contact dermatitis and its triggers, such as nickel, mercury, and gold. Contact dermatitis is a type of eczema that develops as a result of direct contact with skin irritants or chemicals to which one has an allergy.
Some drugs can cause drug-induced nummular dermatitis. This condition occurs when certain medications compromise the functioning of the skin’s outermost layer or the immune response.
Medications that can induce nummular dermatitis include diuretics, statins used in cholesterol-lowering treatments, and immune-modulating medications used to treat hepatitis C (such as ribavirin and interferon), tumor necrosis factor (TNF) inhibitors used in the treatment of arthritis, and intravenous immunoglobulin (IVIG).
Some people with nummular dermatitis also have a history of atopic dermatitis (eczema).
In some cases, a minor skin injury like a burn or insect bite may trigger nummular dermatitis.
When it develops on the legs, nummular dermatitis can be linked to poor blood circulation in the lower body and swelling of the legs. This form of eczema is known as stasis dermatitis.
Generally, nummular dermatitis is characterized by itchy, circular or oval-shaped patches of affected skin. It is not unusual to have just one patch of nummular dermatitis, but most people with the condition have several patches with dry skin between them. These patches can appear anywhere, although they rarely affect the face or scalp.
Nummular dermatitis on the arm (DermNet NZ)
Initially, clusters of small spots or bumps appear on the skin. These rapidly merge into larger patches (called plaques) that can vary in size, up to several centimeters in diameter.
At first, these patches are often oozy, swollen, and covered with vesicles (bumps). They can also be itchy, especially at night. Over time, they may dry up, forming crusts, cracks, and flakes. These patches may also have clear spots in their centers, forming a ring of clear skin that can be mistaken for ringworm.
Nummular dermatitis is clinically divided into two forms:
Exudative nummular dermatitis begins acutely and may become chronic, persisting for weeks, months, or, in rare cases, years. This form of nummular dermatitis can occur at any age, but it most commonly affects people over the age of 50. It’s also more common in men than in women.
Dry nummular dermatitis may be localized or generalized (affect multiple areas of skin). This form of nummular dermatitis often arises as the result of dry skin, either from atopy (the tendency to develop allergic diseases), excessive washing with soap and water, or as a complication from using oral retinoids.
In some cases, dry nummular dermatitis causes elevated skin lesions, such as Meyerson’s nevus (moles) and seborrheic keratoses (common, noncancerous skin growths that arise after the age of 30).
Autosensitization dermatitis, a variant of nummular dermatitis, develops from severe dermatitis in another site. This is known as an id reaction — an itchy form of dermatitis usually distantly associated with another inflammatory or infectious skin lesion. Autosensitization dermatitis affects the trunk and limbs, with the lower legs as the typical primary site due to stasis dermatitis. It can be extensive.
In some cases, patches of nummular eczema can become infected, usually as a result of scratching. Signs that infection has occurred may include:
If you suspect you have nummular dermatitis, seek medical advice to receive the proper treatment — especially if you show signs of an infection.
In many cases, nummular dermatitis can be diagnosed by its appearance. Skin swabs may be taken to test for infection with bacteria, such as Staphylococcus aureus. Scrapings may also be taken for evaluation. Fungal cultures may be used to rule out ringworm infection (tinea corporis), which closely resembles nummular dermatitis with its ring-shaped appearance.
Your doctor will likely perform a differential diagnosis to rule out the possibility of other skin conditions that may resemble nummular dermatitis. These conditions, which include the following, may differ slightly from nummular dermatitis:
Patch testing may also be performed to rule out contact allergy to certain elements, such as chromate and nickel.
Treatment options for nummular dermatitis include medication, phototherapy, and home remedies. The symptoms of nummular dermatitis can be stubborn and long-lasting, so it is important to see a doctor to receive the right treatment. You will also need an accurate diagnosis before beginning treatment for nummular dermatitis.
There are several types of medications used to treat nummular dermatitis.
Steroids can be administered topically, orally, or via injections. Topical steroids include anti-inflammatory creams and ointments that are applied to the affected areas once or twice a day for two to four weeks. Stronger steroids may be necessary for this stubborn form of eczema, but stronger steroid creams will thin out the skin over time.
Oral steroids are systemic and are only used in severe and extensive cases of nummular dermatitis. They tend to give immediate, but short-lived relief. Steroids can also be injected into particularly stubborn areas of nummular dermatitis.
Your doctor may prescribe antihistamine pills or recommend over-the-counter antihistamines to relieve itching associated with nummular dermatitis. These are particularly helpful at nighttime to aid in sleeping.
Antibiotics, such as cephalexin or amoxicillin, may be prescribed for infection or blistered, crusted, or sticky nummular dermatitis.
Occasionally, persistent and more severe nummular dermatitis may be treated with the immunosuppressant medications Azasan (azathioprine), Sandimmune (cyclosporine), or Trexall (methotrexate). These may be more effective and safer than long-term systemic steroids. Careful monitoring of blood work and side effects by a dermatologist is required for these treatments.
Eucrisa (crisaborole) is a newer topical medication used for multiple forms of eczema. It is safe for adults and children over the age of 2.
Dupixent (dipilumab) is an injectable medicine that suppresses the inflammatory pathway that is overactive in many forms of eczema.
Generalized or widespread nummular dermatitis may be treated with phototherapy (also called light therapy). This technique helps relieve itching and rashes. It works by exposing the affected skin to either natural sunlight, ultraviolet A light, or ultraviolet B light.
Home care tips for nummular dermatitis include the following suggestions.
Injuries to the skin — such as cuts, scrapes, insect bites, or burns — can lead to or exacerbate nummular dermatitis. Avoid scratching itchy areas, as this could open up your skin and expose you to bacterial infections.
Bathing for too long or too frequently strips the skin of its natural oils, causing dryness. To avoid this, take short baths of 20 minutes or less in lukewarm water. Avoid harsh soaps and detergents that may irritate your skin, especially if you have atopic dermatitis. Using gentle cleansers on your skin will be less drying.
Moisturizers and emollients help keep your skin hydrated. Moisturizing will prevent dryness that may contribute to nummular dermatitis. Apply moisturizer at least once a day, especially while your skin is still damp after bathing. This step can also help relieve itching and scaling. Thicker creams and ointments will provide more skin protection.
In case of contact allergy, avoid touching or coming into contact with skin irritants such as gold, nickel, chromate, and mercury.
Once you are out of the doctor’s office, treatment is largely up to you. Take all medication until it’s finished, even when you feel like your health has improved. In case of any negative and unexpected side effects from medication, do not hesitate to talk to your dermatologist.
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