Psoriasis – Epidemiology, pathophysiology and presentation

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Psoriasis – Epidemiology, pathophysiology and presentation

Definition

Psoriasis is a chronic, non-infectious, inflammatory dermatosis characterized by well-demarcated erythematous plaques topped by silvery scales (Fig. 1).

image

Fig. 1 Histopathology of psoriasis.

Epidemiology

Psoriasis affects 1.5–3% of the population in Europe and North America, but is less common in Africa, China and Japan. The sex incidence is equal. The condition may start at any age, even in the elderly. The two peaks of onset are the second to third and the sixth decades. It is unusual in children under 8 years old.

Aetiopathogenesis

Genetics

Inherited factors predispose to the development of psoriasis: genetic factors appear to be polygenic (p. 12). About 35% of patients show a family history, and identical twin studies show a concordance of 64%. There is a 14% probability that a child with one parent who has psoriasis will be affected, but this increases to 41% if both parents have psoriasis. There are strong correlations with the human leucocyte antigens (HLAs), e.g. HLA-Cw6. Environmental factors are thought to trigger the disease in susceptible individuals.

Epidermal kinetics and metabolism

Psoriatic lesions result from abnormal reactivity of skin T cells with alteration of the epidermal barrier and of inflammatory signalling. The number of cycling epidermal cells is increased seven-fold because of an increase in the basal and suprabasal proliferating cell compartment. The cell cycle time is not reduced. Growth factors, especially transforming growth factor-α, mediate these events. The upper dermal capillary plexus is expanded. At least nine possible chromosomal loci for genes linked to psoriasis have been found, e.g. PSOR1 on chromosome 6p.

Precipitating factors

A number of precipitating factors are associated with the disorder:

image Koebner phenomenon. Trauma to the epidermis and dermis, such as a scratch or surgical scar (p. 19), can precipitate psoriasis in the damaged skin (Fig. 2).

image Infection. Typically, a streptococcal sore throat may precipitate guttate psoriasis.

image Drugs. Beta-blockers, lithium and antimalarials can make psoriasis worse or precipitate it.

image Sunlight. Exposure to sunlight can aggravate psoriasis (in about 6%), although it has a beneficial effect in the majority.

image Psychological stress. Stress can cause and exacerbate psoriasis.

image Cigarettes and alcohol. These seem to make psoriasis worse.

image

Fig. 2 The Koebner phenomenon.

Psoriasis has developed in a surgical scar.

From Buxton PK, Morris-Jones R 2009 ABC of Dermatology, 5th Edn, Wiley-Blackwell, with permission.

Pathology

The epidermis is thickened, with keratinocytes retaining their nuclei (see Fig. 1

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