Eczema, or dermatitis, generally refers to any inflammation of the skin. Discoid eczema is a type of dermatitis characterized by round or oval, extremely itchy rashes over the extremities. Since discoid eczema has the same size and shape as small coins, the condition also goes by the name nummular (meaning "coin-shaped") dermatitis.

Cause of Discoid Eczema

The etiology and pathophysiology of discoid eczema remain shrouded in mystery. However, it is believed that discoid eczema is related to xerosis, or dryness of the skin, whereby there is a marked reduction in the layer of oils and lipids overlying the epidermis. In the absence of this protective barrier of epidermal lipids, environmental allergens gain access to the epidermis, induce an inflammatory and allergic reaction, and cause discoid eczema.

The intense itchiness associated with discoid eczema is thought to be due to the increased numbers of mast cells found in lesions of the disease. Mast cells are specialized cells of the immune system that store and release histamine and other mediators of inflammation. Histamine is a substance responsible for itching, as well as dilation of blood vessels.

Discoid eczema affects all races, but is more commonly seen in males than in females. If you are in the sixth to seventh decades of your life, you are more prone to having discoid eczema. The predilection of discoid eczema among older age groups is probably due to the greater incidence of xerosis among these age groups.

If you are a young female in your 20s or 30s, you can also be afflicted with discoid eczema. However, this tendency is related to atopy and family history rather than to dryness of the skin. Discoid eczema is virtually unheard of among infants and children.

Signs and Symptoms

Discoid eczema is characterized by a chronic and itchy rash on the legs or arms, although any part of the body, with the exception of the face and scalp, may be affected. Rashes are characteristically round to oval, red and raised, dry and scaly. Some rashes may also be studded by small, bubble-like lesions called vesicles, which have the propensity to be secondarily colonized and infected by bacteria such as Staphylococcus aureus.

Since the rashes are extremely itchy, it is not unusual to find thickened rashes with superficial excoriations due to persistent scratching. As the rashes heal, they eventually flatten and resemble normal skin. However, in almost all cases, there is residual hyperpigmentation in the affected areas, which does not completely fade with time.

Environmental conditions that promote skin drying, especially during the winter and cold weather, often exacerbate discoid eczema. On the other hand, improvements in symptoms are observed with sun exposure, with moisturizer use and in humid environments.

Treatment

The mainstay of treatment involves rehydration of the skin and repair of the epidermal lipid layer. Most commonly, this can be accomplished by frequent lukewarm showers followed by generous applications of moisturizers over the skin. Medications that can be used for severe inflammation and itching include topical steroids and immunomodulators, or oral antihistamines. Antibiotics may also be used in cases of secondary bacterial infection of lesions.