Section 3

Rare
Not so common
Common
Low morbidity
Considerable morbidity
Serious

• Psoriasis affects 1.5–2% of the population in western countries. World wide 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 arthritis occurs in 10–25% of the patients.
• The pathogenesis is determined by a polygenic trait with an ongoing T cell–driven autoreactive immune response.

### Classification

Psoriasis vulgaris

Acute guttate

Chronic stable plaque

Palmoplantar

Inverse

Psoriatic erythroderma

Pustular psoriasis

Pustular psoriasis of von Zumbusch

Palmoplantar pustulosis

Acrodermatitis continua

ICD-9 : 696.1 • ICD-10 : L 40.0

### Epidemiology

#### Age of Onset

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

#### Incidence

Psoriasis affects 1.5–2% of the population in western countries. In the United States, there are 3 to 5 million persons with psoriasis. Most have localized psoriasis, but approximately 300,000 persons have generalized psoriasis.

#### 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 offspring develop psoriasis; when both parents have psoriasis, 41% of children develop psoriasis. HLA types most frequently associated with psoriasis are HLA- B13, -B17,-Bw57, and, most importantly, HLA-Cw6, which presents antigens to CD8+ T cells and is thus a candidate for functional involvement.

#### Trigger Factors

Physical trauma (Koebner phenomenon) is a major factor in eliciting lesions; rubbing and scratching stimulate the psoriatic proliferative process. Infections: acute streptococcal infection precipitates guttate psoriasis. Stress: a factor in flares of psoriasis is said to be as high as 40% in adults and higher in children. Drugs: systemic glucocorticoids, oral lithium, antimalarial drugs, interferon, and β-adrenergic blockers can cause flares in existing psoriasis 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 from 311 to 36 h, resulting in 28 times the normal production of epidermal cells, and (2) ...

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