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Psoriasis is a common chronic disease characterized by recurrent exacerbations of thickened, erythematous, and scaling plaques. It is universal in occurrence and affects approximately 2 percent of the US population.(1) It is equally common in males and females.(2) The mean age of onset is 27 years, with approximately 50 percent of cases occurring in the most productive years between the ages of 20 and 60; however, the age of onset is widely variable from infancy to old age.(3)
The exact cause of psoriasis is unknown but there are a number of theories regarding its pathophysiology. These include: defects in the epidermal cell cycle, disruption in arachidonic acid metabolism, genetics, immunologic mechanisms, and endogenous trigger factors such as climate, stress, infection, trauma, and drugs. It has been found that the psoriatic epidermal cells proliferate at a rate seven-fold faster than normal epidermal cells.(4) The germinative cell population increases in psoriatic skin, and duration of the cell cycle is 37.5 hours (versus 300 hours in normal skin).(5) Other factors under investigation include genetics and exogenous trigger factors.
Factors such as climate, stress, infection, trauma, and drugs may aggravate psoriasis. Warm seasons and sunlight reportedly improve psoriasis in 80 percent of patients, while 90 percent of patients report worsening in cold weather. Stress seems to play a role with 30-40 percent of patients reporting exacerbations during times of stress; however, the exact role is uncertain. Genetics seems to most definitely be a factor, predisposing individuals to psoriasis, but once again, the mode is not known. Another abnormality found is that arachidonic acid is present in psoriatic lesions at 30 times the normal levels.
Studies have reported that exacerbation of psoriasis often occurs following infections, with one study involving children citing a 54 percent occurrence rate in a two to three week interval after having an upper respiratory tract infection.(6) Lesions that occur at the site of trauma or injury to skin are called the Koebner response. The incidence is variable and not specific to psoriasis. Drugs most commonly known to exacerbate psoriasis are lithium carbonate and beta-adrenergic blocking agents.
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