What Is Psoriasis?

Psoriasis is a chronic, lifelong skin disease characterized by skin with white scale, redness, swelling, and itching or pain. It appears without a trigger or warning in the teens to 30s (for most people) and waxes and wanes in severity for life. Psoriasis is almost never fatal but can cause severe discomfort, disfigurement, and disability for sufferers. It can also be associated with a destructive form of arthritis. Although the name psoriasis was not introduced for many years, the description of psoriasis and the beneficial effects of sunlight (which can ease psoriasis) were noted in the ancient Greek world. References to this skin disease are found in writings by the Greek physician Hippocrates, who lived from 460 to 377 BC. The English dermatologist Robert Willan, who lived from 1757 to 1812, was the first to recognize psoriasis as an independent disease.

He described the scaly  plaques of psoriasis as  leprosy graecorum, an active, severe dis-ease like psora leprosa, although there is no connection between psoriasis and leprosy. In 1841, Ferdinand Hebra, a Viennese dermatologist working from Dr. Willan’s notes, was the first to use the word “psoriasis” to describe the disease. Dr. Hebra first described the clinical picture of psoriasis that is used today. Hereditary associations in psoriasis had been established by this time. The word psoriasis comes from the Greek psora, meaning itch or rash, and -iasis, a suffix that indicates a condition characterized or produced by an itch or rash. Over the last century physicians have considered it a disease of skin differentiation (the way skin forms complete layers) and have developed therapies against scale and thickness. Recent research into the disease and therapy points toward an immune cause for the thick-ness, redness, and scaling of psoriasis. The immune sys-tem, when it is overactive in the skin, appears to cause changes in skin differentiation that lead to scaling and thickening. This new understanding of immune disruption in psoriasis has led to targeted therapies for the disease.

A formal description of psoriasis, developed by psoriasis experts at the American Academy of Dermatology, is “[a] chronic skin disease that is classically characterized by thickened, red areas of skin covered with silvery scale.” In psoriatic skin (skin changed by psoriasis), plaques become red, thickened, or scaly and may be itchy or painful. The characteristics of a typical skin plaque depend on the type of psoriasis and body location affected. Psoriasis frequently affects the elbows, knees, scalp, and trunk. It may rarely appear on the hands and feet, mouth, skin folds, or genitals. When compared to other common skin diseases, such as eczema, psoriasis shows less swelling or oozing and a dry, white, shiny scale. Other skin diseases that can look similar to psoriasis include seborrheic dermatitis, pityriasis rosea, lichen planus, and skin forms of lupus.

A few characteristics of psoriasis have historically been used to diagnose the disease. These signs may be visible, especially in untreated psoriasis. Auspitz’s sign is positive when the skin bleeds in pinpoint locations after peeling off a piece of scale from the skin. A positive Auspitz’s sign is considered characteristic for psoriasis because it does not occur after removing scale from other skin diseases. The Woronoff ring is seen in a ring of skin around the edges of a psoriasis plaque, when the surrounding unaffected skin becomes a paler, whiter color than the rest of the unaffected skin (see color plate A). Some types of psoriasis appear different than the classic description.

Other varieties include  inverse psoriasis (affecting the skin folds), guttate psoriasis (in small spots all over the body; see color plate C), palmar-plantar psoriasis (on the hands and feet), erythrodermic psoriasis (where the entire body may turn red; see color plate D), and  pustular psoriasis (with sterile, noninfected pustules). These varieties of psoriasis are caused by the same processes but appear as distinct forms of the disease. Because their locations and appearance are different, they may require different treatments for effective control.