Seborrheic dermatitis is a relatively common skin condition that usually is easily recognized and differentiated from other eczematous conditions. Its characteristics are:

• Reddened, somewhat swollen patches of skin; and,
• Yellowish-brown scales and crusts on affected areas of the skin, frequently on the scalp (a common cause of dandruff), and around the nose and mouth.

Seborrheic dermatitis is common in infancy and can occur again after puberty and in adulthood. Its cause is unknown. Unlike acne, which also has increased sebum production, seborrheic dermatitis is not a disease of the hair follicles and attendant sebaceous glands.

The following photos are characteristic examples of seborrheic dermatitis:

(Photos are used with permission of the American Academy of Dermatology National Library of Dermatologic Teaching Slides and the Sulzberger Institute for Dermatologic Education)

In adults, seborrheic dermatitis can resemble psoriasis, especially on the scalp. In some patients with a genetic predisposition for psoriasis, seborrheic dermatitis may be a forerunner or trigger for psoriasis (Look at PsoriasisNet for information about causes and treatment of psoriasis).

Seborrheic Dermatitis in Infants

The typical form of seborrheic dermatitis in infancy is "cradle cap"—oily scales and crusts on hairy portions of the scalp that may extend to the face and neck. The groin and armpits may also be affected, resembling "diaper rash." The infant form of seborrheic dermatitis is usually self-limiting and disappears after a period of weeks to months. Seborrheic dermatitis should be treated by a dermatologist to relieve symptoms and to be sure that the condition is not atopic dermatitis—a condition requiring different management.

Seborrheic Dermatitis in Adults

The manifestations of the mildest form of seborrheic dermatitis in adults are oily-appearing skin, scaling skin, mild skin redness, and itching of eyelids, scalp, face and body. Scaling skin on the scalp often presents as dandruff. In young women, especially, seborrheic dermatitis often occurs in the facial areas around the mouth and nose.

In its more severe form of patchy seborrheic dermatitis, more areas of the body are covered and lesions are more severe.

Adult seborrheic dermatitis often persists for years to decades and may be associated with Parkinson’s disease and other neurological disorders. The disease may worsen in cold weather and improve in warm weather. A dermatologist’s examination must differentiate seborrheic dermatitis from other skin conditions such as atopic dermatitis, contact dermatitis, rosacea, and psoriasis.

Treatment of seborrheic dermatitis is directed toward loosening and removing scales and crusts, preventing skin infections, and reducing inflammation and itching. Standbys of treatment include daily shampoos with a product recommended by a dermatologist, topical steroids, anti-bacterial and anti-fungal agents, and anti-itch medications. Infant and adult forms of seborrheic dermatitis may require different formulations and applications of standard treatment.

An eczema that gets its name from its unique lesions is nummular eczematous dermatitis, also called discoid eczema. The lesions are coin-shaped areas of red, inflamed skin containing papules and small blisters. The lesions may clear in the center, leaving an appearance similar to the fungal infection known as ringworm. The photo shows a patient with nummular eczematous dermatitis:

(Photo used with permission of the American Academy of Dermatology National Library of Dermatologic Teaching Slides and the Sulzberger Institute for Dermatologic Education)

The cause of nummular eczematous dermatitis is unknown. Treatment includes moisturization of the skin, topical coal tar preparations, anti-inflammatory agents, corticosteroids, and anti-histamines to control itching.

Skin photosensitivity can produce skin conditions ranging from rashes to severe blistering. These conditions are mostly found on sun-exposed areas of the body, such as the head, neck, and the tops of the hands and arms. Two agents are required to induce a photosensitivity reaction: (1) a photosensitizing agent—usually a complex chemical, and (2) ultraviolet light of a wavelength that will react with the photosensitizer. The two types of photosensitivity are:

• Phototoxicity
• Photoallergy

Photosensitivity is usually acute but occasionally may progress to a chronic condition. It is sometimes an occupational skin condition. Photoallergy tends to produce eczematous, intensely itchy skin lesions.

Phototoxicity

An acute phototoxic reaction usually occurs within a few hours after exposure to the photosensitizing agent and ultraviolet light (most frequently sunlight). Unlike photoallergy, phototoxicity does not require that a person have a previous exposure and sensitivity. The initial symptoms are usually itching and burning on exposed areas of skin, followed by skin redness and swelling. Large blisters may form in severe cases. The skin reaction eventually resolves, but may repeat upon re-exposure to the same photosensitizer and ultraviolet radiation. Typical phototoxic sensitizers include a group of natural chemical constituents (fucocoumarins) found in fruits and vegetables, and tar. Lime juice, celery, parsley, and some perfumes are frequently encountered agents in this group. Systemic phototoxic sensitizers include many prescription drugs. These photos show examples of phototoxic reactions:

A phototoxic reaction may resemble a chronic hand eczema—for example, a farm worker who handles parsley, celery, etc., every day while working in the sun may have a phototoxic skin condition for the entire growing season.

Identification and avoidance of the phototoxic sensitizing agent is the most important step in treatment. Topical steroids may be used to suppress inflammation and itching.

Photoallergy

Photoallergy is a form of allergic contact dermatitis in which the allergen must be activated by light to sensitize the allergic response, and to cause a rash on subsequent exposure. The second and subsequent exposures produce photoallergic skin conditions which are often eczematous. Common photoallergens include sunscreen products and fragrances applied to the skin’s surface, as well as some drugs taken internally. The photos show some examples of photoallergic reactions:

Treatment of photoallergy consists of identifying the photoallergy agent by photopatch testing and subsequently avoiding it, sun protection, and treatment of inflammation and itching.

Why Does Eczema Itch?

Itching is a common symptom of eczema and other skin conditions. Many persons with eczema may regard itching as the major and most troublesome symptom. Itching is difficult to define. It is described as the sensation that leads to a desire to scratch. It is a complex sensation that research is helping to understand.

Not only is itching not well understood, there is not even a good definition for it. A major dermatology text states that itching may best be defined as a sensation that leads to a desire to scratch.

Itch seems to have a two-phase mechanism:

• A well-defined area of itch that can often be identified with a lesion or localized patch of skin; followed by,

• A more diffuse area of "itchy skin" that can be stimulated to intense itch by light touch.

In this two-phase mechanism, itch is similar to pain—a sharply defined pain at the site of an injury, followed by a wider area that can be rendered more painful by pressure against the skin. Itch and pain do share some neurophysiologic mechanisms, although they are separate phenomena. As anyone with severe itching can attest, the sensation most like severe itching is severe pain.

It is interesting to know—but not very helpful to the person who itches—that skin, the body’s mucous membranes, and the cornea of the eye are the only tissues that produce the itch sensation. This fact has lead to many—so far fruitless—searches for specific "itch nerves" in the skin. Although itch and pain share some neurophysiologic mechanisms, receptors for pain are not receptors for itch. As with pain, the neurophysiologic pathways of itching also have a central nervous system component that is under some degree of control from the brainstem. Research into the origins of itch still continue, guided by several hypotheses.

Itching in atopic dermatitis and itching in aging skin seems to be influenced by overly dry skin. Thus, adequate moisturization and avoidance of skin-drying soaps and detergents is a basic treatment to reduce itching in these conditions.

Environmental triggers that stimulate itching should be avoided (Click on What is Eczema for more information on itch triggers). The desire to scratch should be resisted as much as possible. Scratching irritates the skin, contributing to dryness and inflammation and thus to more itching. Scratching may also open the skin to infection. Topical steroids may relieve itching, but their use should be prescribed by a dermatologist to assure safe and effective use. Systemic antihistamines are effective in some patients with atopic dermatitis.