Psoriasis

Psoriasis, chronic recurring disease of the skin, characterized by the appearance of plaques, patches, or papules on the skin surface. The lesions are usually slightly elevated above the normal skin surface, sharply distinguishable from normal skin, and red to reddish-brown in color. They are usually covered with small whitish-silver scales that adhere to the underlying eruption and, if scraped off, leave bleeding points. The extent of the disease may vary from a few tiny lesions to generalized involvement of most of the skin. Characteristically the elbows, knees, scalp, and chest are involved.

The cause of the disease is unknown. It almost never occurs in persons of naturally dark skin pigment, and the affliction is often seen in families. A variety of preparations applied locally may be helpful, and the rash often subsides on exposure to sunlight. In 1979 a successful treatment for severe, chronic psoriasis was introduced. It consists of first coating the skin with a dye that absorbs ultraviolet light and then shining ultraviolet light on the painted area. Because this technique causes an increased incidence of mild skin cancers, it is reserved for severe cases of psoriasis.