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- A chronic relapsing skin disease most often characterized by thick white scales over erythematous plaques often involving elbows, knees, and scalp (i.e., psoriasis vulgaris). In children, psoriasis may present with less scaling and more erythema than in adults; may also present as diaper rash.
- Other variants include:
- Guttate psoriasis—drop like plaques on the chest, back, and limbs; often in children following strep throat
- Inverse psoriasis—lesions in flexures, also called intertriginous
- Erythrodermic psoriasis—a severe variant with widespread erythematous skin involvement often accompanied by fever; can be life-threatening
- Pustular psoriasis—generalized or localized, often affecting palms and soles; potentially lethal
- Psoriatic arthritis—joint pain can precede skin involvement.
- Involves both genders equally
- Onset of psoriasis is bimodal, commonly presenting in the 3rd decade with a smaller second peak of onset in the 6th decade; however, it can present at any age. Mean age of onset in children is 8.1 years.
- Earlier onset is associated with more severe disease.
Psoriasis is universal in occurrence, but prevalence varies in different populations. The average prevalence in the United States is ~1–3%.
- Although psoriasis has a strong genetic influence, mode of transmission is not defined. It is likely multifactorial with more than one gene involved and is modified by environmental influence.
- 1/3 of patients with psoriasis report a relative with the disease.
- In family studies, 8.1% of children develop psoriasis when one parent is affected.
- When both parents have psoriasis, the affected percentage increases to 41%.
- In twin studies, 65% of monozygotic twins are concordant for the disease, whereas only 30% of dizygotic twins are concordant.
- Plaque-type psoriasis is characterized by a thickened parakeratotic epidermis with an absent granular layer above dermal papillae containing dilated tortuous capillaries.
- Collections of polymorphonuclear leukocytes extend from the dermal papillae into the epidermidis stratum corneum (i.e., Munro microabscesses).
- A mixed perivascular infiltrate is confined to the papillary dermis.
The pathogenesis is unknown, possibly an immune-mediated inflammatory disease. Genetic factors are important. Well-defined trigger factors include the following:
- Trauma—psoriasis can develop in areas of trauma (i.e., isomorphic response, sometimes called the Koebner phenomenon).
- Infections (e.g., upper respiratory infections, Streptococcus pyogenes, human immunodeficiency virus)
- Winter in colder climates in Northern Hemisphere
- Some drugs (i.e., systemic corticosteroids, lithium, NSAIDs, and antimalarials)
Commonly Associated Conditions
- Metabolic syndrome
- Rheumatoid arthritis
- Crohn disease
- Diabetes mellitus
- Lymphoma (rare)