## 5 Discussion  
The main finding of the study is that the number of cases of pustular psoriasis observed over 17 years was relatively small, particularly when compared to those of plaque psoriasis, being 1:34 the ratio of pustular over plaque psoriasis. The University Hospital of Verona is the main public hospital of the city, serving a population of about one million inhabitants and the division of Dermatology represents a regional and extra-regional referral center treatment of psoriasis, thus it is unlikely that there have been a relevant number of pustular psoriasis cases that have escaped our analysis. We found that pustular psoriasis was more common in females (65%) than in males. This is consistent with several Asian studies, whereas this finding is controversial in other Western countries [4; 7; 8; 9; 10; 11; 12; 13]. A large Japanese claim-based study involving 718 patients with GPP and 27,773 patients  
Figure 3: The number of incident cases of plaque versus pustular psoriasis diagnosed per year from 2005 to 2021. Plaque psoriasis (white histograms), pustular psoriasis (black histograms). Figure 2: Clinical presentation of three forms of pustular psoriasis. Generalized pustular psoriasis showing diffuse erythroedema covered by confluent pustules coalescing into pustular lakes (**A**). Palmoplantar pustulosis showing pustular lesions involving the palmar area (**B**). Typical lesions of acrodermatitis continua of Hallopeau involving the toe and leading to the destruction of the nail apparatus (**C**). with plaque psoriasis found a slight female preponderance in patients with GPP (51.5%) compared with those with plaque psoriasis, in which a minority of patients were female (38.7%) [13]. In the study, we found that more than 80% of pustular psoriasis was PPP, while GPP and ACH occurred more rarely. This finding is consistent with a large multi-cohort study by [PERSON] S et al. in which PPP accounted for 560 out of 863 (65%) patients, whereas GPP and ACH were much rarer [14]. In contrast, some Asian studies including those from [PERSON] et al. reported that GPP was more common than localized pustular psoriasis including PPP [10; 11; 12]. Ethnicity and genetics, but also different categorizations of pustular psoriasis, may contribute to explain the different prevalence between the GPP and PPP subtypes. We found that 27% of patients with pustular psoriasis also had concomitant PsA, which is a similar prevalence to PsA in patients with plaque psoriasis [15]. This finding is interesting considering that there are few data in the literature on the matter and is consistent with the finding of [PERSON] et al. in Japanese patients with GPP (24 out of 102, i.e., 23%) [16]. Of note, the anatomical proximity between the nail apparatus and distal interphalangeal joints supports such an association in ACH [6]. We found a higher prevalence of tobacco smoking in patients with PPP, reaching a prevalence of 70%, as expected [6]. Local and systemic exposure to smoke may trigger pustular psoriasis in patients with a predisposed background through the excretion of stress-induced cytokines in keratinocytes as a Koebner phenomenon [14]. In our study, the proportion of concomitant plaque psoriasis was 31% in GPP, 20% in PPP, and 18% in ACH, which is consistent with [PERSON] et al. [17]. Pustular psoriasis is a rare disease whose incidence and prevalence vary across geographical region and ethnicities. Interpreting the available data is challenging because of several issues including misdiagnosis, lack of awareness, different data-sources, and the low number of patients [18; 19]. As an example, different definitions of pustular psoriasis can be used across studies as well as the inclusion (or lack of differentiation from) palmoplantar pustulosis. In other cases, data from hospital audits or claims database analyses have not been extrapolated to reflect the national population-level prevalence estimates [20]. A French study with a questionnaire-based assessment, estimated a disease occurrence of 1.76 cases per million [21].