Psoriasis (psoriasis vulgaris) is a chronic, immune-mediated skin condition characterized by red, scaly plaques that are most often found on the scalp, elbows, and knees. Psoriasis is common and is estimated to affect 1% to 2% of the world's population. The condition is more common in whites and occurs less often in West Africans, African-Americans, Native Americans, and Asiatic people. Psoriasis is found equally in men and women and most frequently begins in the second or third decade of life, but can first present in infants or in the elderly.

Psoriasis is considered a systemic disease. Psoriatic arthritis will develop in 5% to 30% of patients and it may precede or follow the onset of skin lesions. Patients with moderate to severe psoriasis are at increased risk for developing diabetes, obesity, hyperlipidemia, and hypertension (components of the metabolic syndrome). There also appears to be an increased risk of myocardial infarction, particularly in those who have severe psoriasis at an early age.

The primary concern to most patients is the unsightly appearance of lesions whose visibility and persistence often lead to feelings of self-consciousness and uncleanliness. The emotional toll and the personal struggle to come to terms with psoriasis are expressed in an autobiographic short story, “At War with my Skin,” by John Updike, who had severe psoriasis. After undergoing an operation for a broken leg, Updike reflected, “I chiefly remember amid my pain and helplessness being pleased that my shins, at that time, were clear and I would not offend the surgeon.”

The clinical features, differential diagnoses, and management of the most common clinical forms of psoriasis will be discussed in this chapter: