Section 3: Psoriasis, Psoriasiform, and Pityriasiform Dermatoses

PSORIASIS

• Psoriasis affects 1.5% to 2% of the population in Western countries but has worldwide occurrence.

• A chronic disorder with polygenic predisposition and triggering environmental factors such as bacterial infection, trauma, or drugs.

• Several clinical expressions. Typical lesions are chronic, recurring, scaly papules, and plaques. Pustular eruptions and erythroderma occur.

• Clinical presentation varies among individuals, from those with only a few localized plaques to those with generalized skin involvement.

• Psoriatic erythroderma is psoriasis involving the entire skin.

• Psoriatic arthritis occurs in 7% to 25% of psoriasis patients.

Classification

• Psoriasis vulgaris

  • Acute guttate

  • Chronic stable plaque

  • Palmoplantar

  • Inverse

• Psoriatic erythroderma

• Pustular psoriasis

  • Pustular psoriasis of von Zumbusch

  • Palmoplantar pustulosis

  • Acrodermatitis continua

PSORIASIS VULGARIS ICD-10: L40.0

EPIDEMIOLOGY

AGE OF ONSET All ages. Early: Peak incidence occurs at 22.5 years of age (in children, the mean age of onset is 8 years). Late: Presents around age 55 years. Early onset predicts a more severe and long-lasting disease, and there is usually a positive family history of psoriasis.

INCIDENCE Occurs in about 1.5% to 2% of the population in Western countries. In the United States, there are 3 to 5 million persons with psoriasis. Most have localized psoriasis, but in approximately 300,000 persons psoriasis is generalized.

SEX Equal incidence in males and females.

RACE Low incidence in West Africans, Japanese, and Inuits; very low incidence or absence in North and South American Indians.

HEREDITY Polygenic trait. When one parent has psoriasis, 8% of his or her offspring develop psoriasis; when both parents have psoriasis, 41% of their children develop psoriasis. Human leukocyte antigen (HLA) types most frequently associated with psoriasis are HLA-B13, -B37, -B57, and, most importantly, HLA-Cw6, which is a candidate for functional involvement.

TRIGGER FACTORS Physical trauma (rubbing and scratching) is a major factor in eliciting lesions. Acute streptococcal infection precipitates guttate psoriasis. Stress is a factor in flares of psoriasis and is said to be as high as 40% in adults and even higher in children. Drugs: Systemic glucocorticoids, oral lithium, antimalarial drugs, interferon, and β-adrenergic blockers can cause flares and cause a psoriasiform drug eruption. Alcohol ingestion is a putative trigger factor.

PATHOGENESIS

The most obvious abnormalities in psoriasis are (1) an alteration of the cell kinetics of keratinocytes with a shortening of the cell cycle resulting in 28 times the normal production of epidermal cells and (2) CD8+ T cells, which are the overwhelming T-cell population in lesions. Psoriasis is a T-cell–driven disease and the cytokine spectrum is that of a T_H1 response. Maintenance of psoriatic lesions is considered an ongoing autoreactive immune response driven by TNF-α, IL-17, and IL-23.

CLINICAL MANIFESTATION

There are two major types:

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