Psoriasis is a chronic skin disease that affects about 3% of the U.S. population and about 125 million people worldwide. While not an autoimmune disease per se, the human immune system plays a large part in the disease process. The most common variant of psoriasis is plaque psoriasis, which accounts for more than 80 to 90% of cases. Plaque psoriasis is characterized by red, scaly patches of thickened skin that most commonly occur on extensor surfaces (typically the elbows and knees), but it can also affect the areas between fingers and toes (intertriginous areas), palms, soles, and finger and toe nails, as well as large amounts of skin on the scalp, trunk, back, and buttocks. Psoriasis affects men and women equally, and adults more than children. Genetic factors play a critical role in the development of the disease, and certain environmental factors can worsen it.
How the disease develops is not yet fully understood, however excessive activation of the immune system is thought to be central to disease development. In basic terms, immune system factors activate dendritic cells, which then activate keratinocytes. Once activated, keratinocytes, cells that are responsible for skin formation, proliferate at a rate much higher than normal, causing the formation of excess, thickened skin. Affected areas of skin are usually well-demarcated and symmetric. In the keobner phenomenon, new psoriasis lesions can develop in areas of skin trauma, such as from scratching, cuts, or pressure. Pruritis (itchiness) can be moderate to severe, especially during exacerbations. Plaques can be so thick on the hands and feet that they limit functioning of these areas. When nails are affected, onycholysis (separation of the nail plate from the nail bed) may occur. One third of psoriasis patients develop psoriatic arthritis, which can lead to debilitating joint destruction.
Risk factors include genetic, environmental, and behavioral factors. One study showed the risk of developing psoriasis is 40% if both parents are affected, 14% if one parent is affected, and 6% if a sibling is affected. In genetically susceptible individuals, psoriasis can be exacerbated by skin trauma, infections (streptococcal i.e. strep throat), smoking, certain medications like lithium and interferon, and stress. Treatment for mild disease (less than 3 to 5% of skin area affected) is typically steroid creams, vitamin D compounds, keratinolytic agents (skin break down agents), and phototherapy.
For many years it has been recognized that sun exposure improves psoriasis, however it also increases skin cancer risk. Targeted phototherapy, which uses specific wavelengths of light and minimizes the wavelengths responsible for skin cancer, has had significant success. Phototherapy is also used for moderate to severe disease. One of the most significant advances in dermatology has been the development and use of biologic agents and oral systemic treatments for moderate to severe psoriasis, which have made the “heartbreak” of psoriasis much less common. While I use the term heartbreak in reference to an old TV commercial, the incidence of debilitating side effects of psoriasis such as depression and inflammatory bowel disease have thankfully decreased significantly.
By Peter Galvin, MD
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