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  • Worldwide occurrence; affects 2% to 3% of Americans; prevalence ranges from 0.1% to 3% in various populations.

  • A chronic disorder with polygenic predisposition combined with triggering environmental factors such as trauma, infection, or medication.

  • Erythematous scaly papules and plaques; pustular and erythrodermic eruptions occur.

  • Most common sites of involvement are the scalp, elbows, knees, hands, feet, trunk, and nails.

  • Psoriatic arthritis occurs in 10% to 25% of patients; pustular and erythrodermic forms may be associated with fever.

  • Pathology of fully developed lesions is characterized by uniform elongation of the rete ridges, with dilated blood vessels, thinning of the suprapapillary plate, and intermittent parakeratosis. Epidermal and perivascular dermal infiltrates of lymphocytes, with neutrophils occasionally in aggregates in the epidermis.



Psoriasis is a common, immunologically mediated, inflammatory disease characterized by skin inflammation, epidermal hyperplasia, and increased risk of a painful and destructive arthritis as well as cardiovascular morbidity and psychosocial challenges. The economic and health burden of this constellation of pathologies is very substantial, yet its cause remains unknown.


More than 2000 years ago, Hippocrates used the terms psora and lepra for conditions that can be recognized as psoriasis. Later, Celsus (ca. 25 BC) described a form of impetigo that was interpreted by Robert Willan (1757–1812) as being psoriasis. Willan separated two diseases as psoriasiform entities, a discoid lepra Graecorum and a polycyclic confluent psora leprosa, which later was called psoriasis. In 1841, the Viennese dermatologist Ferdinand von Hebra (1816–1880) unequivocally showed that Willan’s lepra Graecorum and psora leprosa were one disease that had caused much confusion because of differences in the size, distribution, growth, and involution of lesions.



Psoriasis is universal in occurrence. However, its reported prevalence in different populations varies considerably, from 0.91% in the United States to 8.5% in Norway.1 The prevalence of psoriasis is lower in Asians, and in an examination of more than 25,000 Andean Indians, not a single case was seen.2 Psoriasis appears to be equally common in males and females.


Psoriasis may begin at any age, but it is uncommon before the age of 10 years. It is most likely to appear between the ages of 15 and 30 years. Possession of certain human leukocyte antigen (HLA) class I antigens, particularly HLA-Cw6, is associated with an earlier age of onset and with a positive family history. This finding led Henseler and Christophers3 to propose that two different forms of psoriasis exist: type I, with age of onset before 40 years and HLA associated, and type II, with age of onset after 40 years, although many patients do not fit into this classification.

Genetic Epidemiology


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