## 4 Discussion
The GPP index showed consistent results (\(\rho\) ranged from 0.31 to 0.63), albeit probably due to the small studied group, these results were not always statistically significant (\(p=0.04\)-0.16). It is difficult to uniformly characterize itch; even patients in the same subtypes of psoriasis used various terms to describe their feelings. The most common associated symptoms with large plaque psoriasis were burning and pure itch. Likewise, tingling, pinching, and painful pruritus were commonly used by the patients. Published data regarding individual patients' sensations of itch seems to be limited. In one study, tickling, crawling, and burning were the terms most often used by patients to name pruritus. Inthis research, plaque, guttate, and erythrodermic subtypes of psoriasis were taken into consideration; unfortunately authors did not provide a separate analysis for certain types. It is worth mentioning that most of these patients suffered from plaque psoriasis (\(n=92\)), while guttate and erythrodermic subtypes were in the great minority (\(n=5\) and \(n=4\), respectively); thus, one can assume that this result is related mostly to plaque psoriasis [18]. In our study, painful itch accompanied mainly palmoplantar types of psoriasis, and it was probably due to a higher density of free nerve endings in this area and the pressure causing additional pain [32]. On the other hand, 27% of responders with GPP also declared painful itch, which could suggest that different factors related to the formation of pustules in psoriasis may be relevant. In most subtypes, itching occurred during the appearance of skin lesions (10-49%) or when they were fully developed (13-60%). The only exception was GPP, in which the highest intensity of pruritus was during the extension of the skin lesion. It is difficult to compare this data with previous studies as in many cases, no distinction of psoriasis subtypes was provided [6]. It has been well demonstrated that pruritus has a major impact on patients' quality of life [33; 34; 35]. In concordance with previous studies, our data indicated that psoriatic patients suffering from intensive pruritus had significantly impaired quality of life [36; 37]. Interestingly, in erythrodermic psoriasis, a lower quality of life did not depend on pruritus, which means that pruritus was not the major factor that impacted patients' quality of life. This can be explained by the severe course of this disease subtype. The pathogenesis of pruritus in psoriasis still remains unclear. Several potential mediators have been proposed, such as substance P, calcitonin gene-related peptide, neuropeptide Y, nerve growth factor and its receptors, lipocalin 2, and interleukin 31; however, data remain controversial and inconclusive [5; 16; 38; 39; 40]. Other authors suggested disturbances in the endogenous opioid system homeostasis, but there is still no final proof that indeed the observed abnormalities are a primary phenomenon in psoriasis leading to pruritus [41; 42]. Recent data also indicated the role of transient receptor potential (TRP) channels, such as TRPV1, TRPV4, or TRPM8, but further studies are needed to confirm their relevance [43; 44; 45].