Psoriasis is a chronic, recurrent disease, marked by epidermal proliferation. Its lesions, which appear as erythematous papules and plaques covered with silver scales, vary widely in severity and distribution. Psoriasis affects about 21% of the population in the United States.
Although this disorder commonly affects young adults, it may strike at any age, including during infancy. Psoriasis is characterized by recurring partial remissions and exacerbations. Flare-ups are commonly related to specific systemic and environmental factors but may be unpredictable; they can usually be controlled with therapy.
The tendency to develop psoriasis is genetically determined. Researchers have discovered a significantly higher-than-normal incidence of certain human leukocyte antigens (HLA) in families with psoriasis, suggesting a possible immune disorder. Onset of the disease is also influenced by environmental factors.
Trauma can trigger the isomorphic effect or Koebner’s phenomenon, in which lesions develop at sites of injury. Infections, especially those resulting from beta-hemolytic streptococci, may cause a flare-up of guttate (drop-shaped) lesions. Other contributing factors include pregnancy, endocrine changes, climate (cold weather tends to exacerbate psoriasis), and emotional stress.
Generally, a skin cell takes 14 days to move from the basal layer to the stratum corneum, where after 14 days of normal wear and tear, it’s sloughed off. The life cycle of a normal skin cell is 28 days, compared to only 4 days for a psoriatic skin cell. This markedly shortened cycle doesn’t allow time for the cell to mature. Consequently, the stratum corneum becomes thick and flaky, producing the cardinal manifestations of psoriasis.
Signs and symptoms
The most common complaint of the patient with psoriasis is itching and occasional pain from dry, cracked, encrusted lesions.
Psoriatic lesions are erythematous and usually form well-defined plaques, sometimes covering large areas of the body. (See Viewing psoriasis.) Such lesions usually appear on the scalp, chest, elbows, knees, back, and buttocks.
The plaques consist of characteristic silver scales that either flake off easily or can thicken, covering the lesion. Removal of psoriatic scales typically produces fine bleeding points (Auspitz sign). Occasionally, small guttate lesions appear, either alone or with plaques; these lesions are typically thin and erythematous, with few scales.
Widespread involvement of scales and erythema is called exfoliative or erythrodermic psoriasis. In about 60% of patients, psoriasis spreads to the fingernails, producing small indentations or pits and yellow or brown discoloration. In some cases, the accumulation of thick, crumbly debris under the nail causes it to separate from the nail-bed (onycholysis).