Psoriasis, seborrheic dermatitis, pityriasis rosea, and lichen planus are diseases that present with papulosquamous lesions (scaly papules and plaques). Although these diseases may have a similar morphology, their underlying etiologies vary. Secondary syphilis, cutaneous T-cell lymphomas, and connective tissue disease may also present with papulosquamous lesions and should be included in the differential diagnosis.
Psoriasis is a common, chronic, inflammatory disease that can result in decreased quality of life. Clinicians have long been vexed by this ancient affliction. Although most medical literature prior to Willan (1757-1812) lumped psoriasis, leprosy, eczema, and other inflammatory dermatoses into a confusing menagerie, Celsus gave a convincing account of psoriasis vulgaris almost 2000 years ago. His description included many of the morphologic features that physicians today utilize to diagnose psoriasis, including the "ruddy" or salmon-colored plaques with silvery scales that often are associated with punctate hemorrhage or "erosions" when removed.1
More than 7.5 million adults (2.1% of the population) in the United States are affected and 30% of these individuals will develop psoriatic arthritis.2 About 1.5 million of them are considered to have moderate to severe disease. Psoriasis may have a significant negative impact on a patient's quality of life. Patient often are self-conscious, depressed, or frustrated over the appearance of their skin.
Psoriasis spans all socioeconomic groups, and its prevalence varies by geographic location. Historically, the disease is more common in the northern latitudes. The rate of psoriatic disease is lower in African Americans compared with that in European Americans.2
The primary cause of psoriasis is a dysregulation of the cell-mediated, adaptive immune response. This dysregulation is likely triggered by hyperactivity of the innate immunological surveillance system to environmental antigens. In genetically predisposed individuals, the Th1 pathway response is overstimulated. This overproduction of Th1-related cytokines along with IL-12, 17, and 23 causes hyperproliferation of epidermal keratinocytes. These events lead to the formation of the psoriatic plaques.3
Environmental factors and disease states that interact with polygenic inheritance patterns most likely account for the variable expression of psoriatic disease. These include streptococcal pharyngitis (guttate psoriasis), stressful life events, low humidity, human immunodeficiency virus (HIV), trauma, medications, cold, and obesity. Diets high in fish oils seem to be protective against the development of psoriasis.2,4
Most patients with mild to moderate psoriasis are asymptomatic, but pruritus is common in severe or widespread disease. Patients may also give a history of joint pain and swelling, especially in the fingers and toes.
Psoriasis can vary in appearance and distribution. However, there are clues in the physical examination that allow the clinician to properly diagnose psoriasis and identify its subtypes.