Types of Eczema
Nummular Dermatitis

Often appearing after a skin injury, such as a burn, abrasion, or insect bite, the hallmark of this common eczema is unique, coin-shaped (nummular) or oval lesions. One or many patches can develop that may last for weeks or months.

Other names

• Discoid eczema

• Nummular eczema

• Nummular eczematous dermatitis

Signs and Symptoms

• One or several patches may appear and tend to:

  • Begin as a group of tiny reddish spots and blister-like lesions that enlarge and grow together to form a reddened, coin-shaped patch that ranges in size from less than 1 inch to more than 4 inches.

  • Weep fluid in the beginning and then become crusty. Long-term patches are scaly.

  • Be pink, red, or brown and well-defined.

  • Develop on the legs, but also occur on the torso, arms, hands, and feet.

  • Clear in the center, forming a “ring” around the clear skin that causes the patch to resemble a ringworm infection.
     

• Patches often itch and burn. Itching and burning range from severe to barely noticeable. The itch may intensify at night, disturbing sleep.
   

• A yellowish crust may develop on the patches if a Staphylococcus aureus (staph) infection develops.
   

• While the skin between the patches usually remains clear, the skin may be very dry and easily irritated.
   

• When this type of eczema clears, it often leaves the skin a bit darker (hyperpigmentation) or lighter (hypopigmentation) than the surrounding skin. The discoloration may never fade completely when a lesion occurs below the knee.

Who Gets

• In the United States, approximately 2 out of every 1,000 people develop nummular eczema.
   

• Men develop it more frequently and tend to have their first outbreak between 55 and 65 years of age. Another peak period for developing nummular dermatitis is between the ages of 15 and 25 years. Women are more likely to develop the condition then.
   

• Rare in children.

Causes
While the cause remains unknown, researchers believe that heightened sensitivity plays a role in most cases. Some patients show sensitivity to:

• Mercury. It is believed that in rare cases, exposure to mercury, which is a common component of dental fillings, can cause nummular dermatitis. Research shows that a few patients with nummular dermatitis only cleared, despite numerous treatments, when all of their mercury-containing fillings were removed. Even handling the materials in a dental office or lab has been shown to cause an outbreak, which leads researchers to believe that inhaling the mercury vapor may be the cause in these patients.
   

• Rubber, nickel, formaldehyde, or neomycin. If the patient has an allergy to any of these, the skin will only clear when the substance is avoided.

Risk Factors
The following seem to increase the risk of developing nummular dermatitis:

• Medical condition. A history of very dry skin (xerosis) or eczema, especially atopic dermatitis or stasis dermatitis, increases the risk as does having poor blood flow and/or swelling in the legs.
   

• Environment. A low-humidity environment, especially one that also is cold seems to increase the risk as well as worsen existing nummular dermatitis.
   

• Injury to the skin. An insect bite, contact with chemicals, or an abrasion may trigger an outbreak.
   

• Bacterial skin infection.
   

• Certain medications. Isotretinoin, a prescription medication used to treat severe acne that proves resistant to other treatments, seems to increase the risk. Some patients who are treated for hepatitis C with interferon develop severe, generalized (widespread lesions on the body) nummular dermatitis.

Duration

• Some patients clear within a year. Others have persistent or recurring lesions for many years.
   

• Lesions that recur after clearing tend to do so at the sites of the original outbreak.

How Diagnosed

• Visual examination. Diagnosis usually begins with a visual exam of the patient’s skin. During the exam, the dermatologist may take samples if a skin infection is suspected.
   

• Patch testing. If an allergy is suspected, patch testing (medical testing to find allergies) is advised. Patch testing also is advised when treatment does not effectively clear the condition.

Treatment
With proper treatment, nummular dermatitis can clear completely. However, lesions on the lower extremities often take longer to heal and tend to leave spots. The treatment plan for nummular eczema consists of:

• Protecting the skin from further injury.
   

• Hydrating the skin. Taking a short, lukewarm bath or shower once a day and immediately applying a cream or ointment to still-damp skin can help hydrate dry skin as well as relieve the itch and scaling. Adding a bath oil to the water also may help.
   

• Using medication as directed. Topical corticosteroids and tar preparations help reduce inflammation and itch. If a bacterial skin infection develops, it is treated with an antibiotic. An antihistamine can help a patient sleep.
  
  When a patient develops severe or generalized (widespread lesions) nummular dermatitis, treatment may require special dressings, phototherapy (treatment with ultraviolet light), oral antibiotics, systemic (taken by mouth or injected) corticosteroids, and bed rest in a cool, moist environment.

To prevent nummular dermatitis from recurring once the skin clears, dermatologists recommend:

• Moisturizing. Applying a moisturizer at least daily, and after bathing helps, especially in a dry climate. Moisturizer helps trap water in the skin. A dermatologist can recommend suitable products that will not irritate the skin.
   

• Avoiding certain activities. Anything that dries, heats, or irritates the skin, such as hot baths, frequent bathing, or sitting next to a fire, can cause a flare-up.
   

• Modifying skin care. Using only a mild, non-drying cleanser (not soap) when bathing helps.
   

• Using a humidifier. When heating or air-conditioning is necessary, use a humidifier to add moisture to the air.
   

• Dressing for success. Wear loose clothing, and avoid wearing rough fabrics, such as wool, which can irritate the skin.

References:
Adachi A et al. “Mercury-induced nummular dermatitis.” Journal of the American Academy of Dermatology, 2000 August;43(2, part 2) 383-385.

Jarvikallio A et al. “Quantitative analysis of tryptase- and chymase-containing mast cells in atopic dermatitis and nummular eczema.” British Journal of Dermatology, 1997 June;136(6):871-877.

Sotor NA. “Nummular eczematous eczema.” In: Freedberg IM et al (eds.) Fitzpatrick’s Dermatology in General Medicine, 5th ed. New York: McGraw Hill; 1999:1480-1482.

All content solely developed by the American Academy of Dermatology