Psoriasis: Epidemiology, Clinical and Histological Features, Triggering Factors, Assessment of Severity and Psychosocial Aspects
What is Psoriasis?
Psoriasis, a chronic erythematosquamous dermatitis that affects about 2-3% of the population, is characterized by abnormal keratinocyte hyperproliferation, resulting in thickening of the epidermis and of the stratum corneum. Psoriasis Vulgaris accounts for 90% of the psoriasis cases and is characterized by well-delineated reddish and scaly papules and plaques, typically on the elbows, knees, and scalp or in other cutaneous surfaces.
Psoriatic skin changes have been described since biblical times. The first documented description is found in the Old Testament in the third book of Moses. It was confused with leprosy for hundreds of years, and, therefore, many people with psoriasis was ostracised in the Middle Age. At the beginning of the 19th century, Robert Willan, an English physician, was the first to clinically describe psoriasis (Crissey and Parish 1998). Humankind suffered and studied this disease for at least 3000 years, and, naturally, several possible causes for the disease have been hypothesized.
Nowadays it is accepted that psoriasis is a chronic, recurrent, immune-mediated inflammatory disease, with a recognized genetic predisposition. The primary immune defect appears to be an increase in cell signaling via chemokines and cytokines that act upregulating gene expression, causing keratinocyte hyperproliferation. T lymphocytes and their cytokines and chemokines appear to be the driver of lesion development and persistence, although other cells, such as endothelial cells, dendritic cells, neutrophils and keratinocytes play also an important role, along with other cytokines and growth factors (Chen, de Groot et al.; Wollenberg, Wagner et al. 2002; Sano, Chan et al. 2005). Currently, it is proposed that psoriasis development depends on skin infiltration of T helper (Th)1/Th17 cells that stimulate macrophages and dermal dendritic cells to release mediators that sustain inflammation and cause abnormal keratinocyte proliferation. Interleukin (IL)-23 has the potential to activate Th17 cells, stimulating their survival and proliferation and serving as a key master cytokine regulator in psoriasis (Blauvelt 2008). Th17 cells secrete IL-17, IL-21 and IL-22, with the latter mediating IL-23 induced acanthosis and dermal inflammation (Zheng, Danilenko et al. 2007; Kunz 2009).
Therefore, the IL-23/Th17 axis seems to play an important role in psoriasis and explains the hyperplasia of psoriatic keratinocytes (by IL-22), and why neutrophils appear in a chronic inflammatory disease, such as psoriasis (IL-8 production induced by IL-17) (Di Cesare, Di Meglio et al. 2009). More recently, functional interactions between IL-33 and mast cells were also found to contribute to inflammatory conditions, such as psoriasis (Xu, Jiang et al. 2008; Castellani, Kempuraj et al. 2009; Theoharides, Zhang et al. 2010). Nonetheless, the immunologic target molecule that would allow classifying psoriasis as an autoimmune disease, as well as, the events that trigger the inflammatory process, remain to be determined. Patients with psoriasis require an individual management and long-term planning of therapeutic strategies. The ratio risk versus benefit and the cost-effectiveness of the different treatments should be carefully evaluated. The therapy is chosen in accordance with skin type, clinical history, patient’s age, the severity of psoriasis and the response to previous treatments.
Topical agents are, usually, chosen for milder forms and limited psoriasis; phototherapy, photochemotherapy and systemic agents for moderate and severe psoriasis. Biological therapies, the more recent therapies for psoriasis, are particularly used for severe psoriasis.
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Susana Coimbra, Hugo Oliveira, Américo Figueiredo, Petronila Rocha-Pereira and Alice Santos-Silva (2012). Psoriasis: Epidemiology, Clinical and Histological Features, Triggering Factors, Assessment of Severity and Psychosocial Aspects, Psoriasis – A Systemic Disease, Dr. Jose O’ Daly (Ed.), InTech, DOI: 10.5772/26474. Available from: http://www.intechopen.com/books/psoriasis-a-systemic-disease/psoriasis-epidemiology-clinical-and-histological-features-triggering-factors-assessment-of-severity-