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Psoriasis is common, affecting 1.5-2.0% of the population. It affects all ages, with bimodal peaks of incidence at ages 20-30 and 50-60 years. Plaque-type psoriasis is easily identified by its discrete round orange-red plaques with silver scale on the scalp, elbows, knees, or trunk. When psoriasis appears as intertriginous, guttate, or nail-limited psoriasis, the diagnosis can be challenging.

Etiology. Psoriasis exhibits abnormalities of epidermal proliferation and migration of inflammatory cells, resulting in accelerated epidermal turnover time. Psoriasis, particularly extensive psoriasis, has a strong genetic component. Exacerbants of psoriasis include stress, probably alcohol, many drugs, infection.

Diagnosis. Plaque-type psoriasis is usually diagnosed by clinical exam alone; no other condition closely approximates its discrete plaques and silver scale. Inverse psoriasis, however, lacks surface scale because it is hydrated in intertriginous areas, and appears as shiny red patches. Guttate (meaning "drop-like") psoriasis has an abrupt onset of small red macules over the trunk.

Therapy. Psoriasis can be significantly improved with hydration followed by application of heavy emollients. Topical steroids are the most prescribed therapy for psoriasis in the United States. Creams, ointments, or gels are applied thinly once or twice daily.

The lowest strength steroid is used to avoid tachyphylaxis and skin atrophy; however, any strength fluorinated steroid can cause skin atrophy if used over a long period.

Tar is an effective treatment for psoriasis, but its odor and staining limits its acceptance. It may be used at night under occlusion and washed off in the morning. Tars can be irritating in intertriginous areas.

Anthralin preparations are widely used in Europe, and are equal in efficacy to topical steroids. Anthralin is available in gels and creams from 0.1% to 5% and, like tar, stains clothing and can be irritating. Short-contact anthralin therapy, in which stronger anthralin is applied to plaques and washed off after minutes to two hours, is as effective as overnight application. New formulations of liposomal anthralin are being developed to reduce irritancy and staining. Anthralin does not carry the risk of atrophy with long-term use.

Calcipotriene, a vitamin D3 analog, is available in the United States in ointment form. It is about as effective as medium-potency steroids in most studies. The total dose must be limited to 100 g of 50 mcg/g preparation per week to avoid hypercalcemia.

Light therapy is useful for widespread psoriasis. Ultraviolet B alone, or ultraviolet A combined with oral psoralen (PUVA) can be used for psoriasis flares and for maintenance therapy. UVB units such as the Jordan light system may be purchased and used as home maintenance therapy. It may be better tolerated and less expensive than drug therapy, but it increases the risk.

Retinoids are vitamin A derivatives, and the oral retinoids isotretinoin (Accutane) and, particularly, etretinate (Tegison) are useful for erythrodermic and pustular psoriasis. Either may be used with PUVA for additive effect. Side effects of retinoids are several, including dry skin, headache, joint inflammation, osteoporosis and osteophyte formation, temporary hair loss, emotional depression, and a peculiar "sticky skin" sensation. Triglycerides and rarely liver enzymes may be elevated. Both isotretinoin and etretinate are teratogens; etretinate.

Methotrexate, long used for rheumatoid arthritis, has gained wide use for the treatment of extensive psoriasis. Weekly doses of 10-20 mg can give rapid clearance of psoriasis, which can be held in remission with low doses in many cases.

Cyclosporine is used in low doses for refractory psoriasis. Although 5 mg/kg/d is the generally recommended dose, it is best to check serum trough levels for the first few weeks to maintain a concentration of 200-300 ng/mL. Remissions may be rapid.

Systemic steroids are relatively contraindicated for the treatment of psoriasis because of the risk of precipitating a flare of erythrodermic psoriasis.