Clinical Aspects of Itch: Psoriasis

Psoriasis is a chronic, inflammatory skin disease affecting about 1% to 3% of the Caucasian population and slightly less frequently occurring also in other races. The most common variant of psoriasis, namely, plaque-type psoriasis, is clinically characterized by the presence of well-demarcated papules and plaques covered by silvery scales, which classically demonstrate symmetric distribution involving most commonly scalp, sacral area, and extensor surfaces of elbows and knees (Figure 4.1). Less often skin lesions may occur within the flexures and on the face. Other clinical subtypes include guttate, erythrodermic, and generalized or localized pustular psoriasis. Many patients (up to 80%) have nail abnormalities, and some of them (about 5%–30%) develop psoriatic arthritis. The disease may occur at any age, but two peaks of morbidity can be observed: the first one between 20 and 30 years of age, and the second one between 50 and 60 years of age (van de Kerkhof 2003). The pathogenesis of psoriasis is still not completely understood. The genetic background seems to be the most important factor, and many genes have been identified to predispose to this skin disease so far (van de Kerkhof 2003; Reich and Szepietowski 2007). However, environmental factors like infections, stress, some drugs, smoking, or alcohol also play a role. Altogether, genetic and extrinsic factors lead to abnormal keratinocyte proliferation, cutaneous inflammation, and skin vessel disturbances finally resulting in clinical features of psoriasis (Reich and Szepietowski 2007). To date, a number of treatment options of psoriasis have been developed, but none is a curative one. Patients with psoriasis frequently experience relapses of skin lesions, causing a need of a lifelong therapy. This may also lead to discouragement and abandonment of treatment. Because of its chronicity and visibility, psoriasis is responsible for significant distress, suffering, decrease of quality of life level, and stigmatization (Böhm et al. 2013; Hrehorów et al. 2012; Raho et al. 2012). Several studies documented that patients with psoriasis often suffer from chronic pruritus, which further contributes to lowering of psoriatic patients’ well-being (Yosipovitch et al. 2000; Reich et al. 2010a).