Seborrheic dermatitis (SD) is a very common, chronic inflammatory dermatitis. Its characteristic distribution involves areas that have the greatest concentration of sebaceous glands: the scalp, face, presternal region, interscapular area, umbilicus, and body folds (intertriginous areas).

Many people experience some degree of dandruff—a whitish scaling of the scalp that is sometimes itchy and is fairly easily controlled with antidandruff shampoos. When dandruff is accompanied by erythema, a sign of inflammation, it is referred to as seborrheic dermatitis.

SD is seen more commonly in males and often begins after puberty. There appears to be a hereditary predisposition to its development. When it appears in patients who are infected with the human immunodeficiency virus, SD may serve as an early marker of the acquired immunodeficiency syndrome. SD is also seen commonly in patients with Parkinson disease and in patients taking phenothiazines.

SD has many features in common with chronic eczema and psoriasis. Typical lesions of SD often appear in patients with psoriasis, and its histologic features resemble those of both eczema and psoriasis. In fact, some dermatologists do not consider SD as a distinct nosologic entity but instead assign it to various forms of eczema or psoriasis. In the latter case, the term “seborrhiasis” has been used. In the United Kingdom, seborrheic dermatitis is referred to as “seborrhoeic eczema.”

Described as idiopathic, some evidence indicates that an abnormal response to Malassezia, a small yeast, may play a part in its pathogenesis because SD occasionally responds to antifungal medications. Malassezia organisms are probably not the cause but are a cofactor in SD. Because SD occurs only where the sebaceous glands are found, sebum has also been thought to play a role, although no link has yet been shown.

The eruption of SD tends to be bilaterally symmetric in its distribution and the appearance of the lesions varies depending on their location.

On the face, lesions are red, with or without an overlying whitish scale, or they may appear as orange-yellow greasy patches and are typically found on the forehead, eyebrows, eyelashes, cheeks, beard, and nasolabial folds (Figs. 13.40 to 13.42). Lesions can also occur behind the ears and in the external ear canal.

On the scalp, SD may range from a mild erythema and scaling to thick, armor-like plaques that are indistinguishable from psoriasis (“sebopsoriasis”). There may be itching and scale with resultant dandruff that, embarrassingly, often falls on clothing.

When lesions occur in body folds, they often consist of sharply defined, bright red plaques that may develop fissures. Typically involved areas include inframammary, axillae, inguinal creases, intragluteal crease, perianal area, and umbilicus. In these areas, SD is often clinically indistinguishable from tinea, intertrigo, and inverse psoriasis.

Presternal lesions are scaly or papular (Fig. 13.43).

Facial SD usually flares in the winter and improves in the summer. However, some patients report provocation of the condition after sun exposure.

When fissures develop in the body folds and umbilicus, symptoms may consist of burning, itching, oozing, and pain.