Chapter 2. Psoriasis
Psoriasis may be confused with atopic dermatitis, although it is more commonly mistaken for seborrheic dermatitis. Psoriasis is a papulosquamous disorder characterized by red, well-demarcated plaques with thick, silvery scale. (PICTURE A). Psoriasis affects from 1-3% of Americans and in about 20% of cases the disease begins before the age of 20 years. Infantile psoriasis is much less common than atopic dermatitis and only 2% of psoriasis cases present during the first two years of life. There exists a genetic component to this disorder, and multiple family members may be affected. The lesions of psoriasis appear in a usual morphology and distribution pattern. The predominant morphology consists of red, sharply demarcated papules and plaques. It is markedly different then the typical diffuse eczematous lesions of atopic dermatitis. The plaques of psoriasis tend to locate to the scalp, lower back, and extremities.
In contrast to atopic dermatitis, the extensor surfaces are the most commonly affected. Isolated scalp involvement may also be seen in psoriasis, and patients commonly have thick plaques along the frontal hairline. Rarely, psoriasis may present in infants as persistent diaper dermatitis whereas atopic dermatitis does not usually involve the diaper area. Children with psoriasis tend to have prominent involvement of the eyelids, genital area and periumbilical area. Some children have a type of disease termed "guttate" psoriasis with many, small, drop-like lesions scattered all over the body. Psoriatic lesions are often induced in areas of trauma, the so-called Koebner phenomenon. Additionally, removal of scale may result in multiple, small bleeding points, termed the "Auspitz sign."
There are many additional clinical features that, if present, aid in distinguishing psoriasis from atopic dermatitis. Psoriasis patients may have nail changes such as onycholysis (separation of the nail plate from the nail bed), yellowing, pitting (PICTURE B) or increased friability. Approximately 8% of psoriasis patients will suffer from psoriatic arthritis. The arthritis may develop prior to the rash, making accurate diagnosis challenging. Psoriatic arthritis is characterized by involvement of multiple hand and foot joints, most commonly the distal interphalangeal joints. Arthritis may be severe and progress to deformity. Patients are usually HLA-B27 positive. Diagnosis of psoriasis is usually made by history and recognition of the typical morphology and distribution pattern. Skin biopsy may be performed in attempt to clarify the diagnosis in an atypical case.
There are a number of treatments for psoriasis including topical lubricants, corticosteroids, tar, keratolytics, calcipitriol, and tazarotene. The disease is often recurrent, and it can be difficult to achieve complete clearing of the rash. Ultra-high potency steroid preparations are often required to manage the thickest plaques. Recalcitrant cases may benefit from phototherapy or systemic treatment with retinoids, anti-metabolites, new systemic biologic agents and selective immunomodulatory agents. These treatments require close clinical and laboratory monitoring under the care of a specialist.