## 2 Material and Methods  
This retrospective observational study from 16 dermatology centres in Italy included a consecutive series of 66 patients experiencing a flare of GPP between 1 January 2018 and 31 December 2022. Data were collected on a shared case report form that has been completed by a selected group of dermatologists who have recognized experience in the diagnosis and treatment of GPP. ERASPEN network criteria were applied to classify GPP (von Zumbusch type) [2]. In particular, GPP was defined in case of macroscopically visible sterile pastules occurring on non-acral skin and not within psoriasis plaques, with at least one relapse or persisting for over 3 months. The diagnosis of GPP was clinically confirmed by the dermatologist and was supported by histological examination when deemed necessary. Flares of GPP were defined as acute appearance of pastules affecting large body areas on an erythemat skin that could be accompanied by systemic symptoms, such as fever, leukocytosis, and fatigue [2]. Inclusion criteria was the occurrence of the GPP flare during the observational period (i.e., between 2018 and 2022) and given consensus to the anonymous collection of data. Exclusion criteria were localized forms of pustular psoriasis (i.e., acrodermatitis continua of Hallopeau [ACH] and palmoplantar pustular psoriasis [PPP]). PPP was defined only on a clinical basis, i.e., in the presence of chronic pustular dermatitis involving the palms and soles characterized by vesicles, pastules, erythema, lichenification, and abnormal desquamation according to ERASPEN criteria [2]. Demographic and clinical characteristics of the patients including age, gender, body mass index (BMI), personal and family history of chronic plaque psoriasis, smoking habit, age of GPP onset, history of PPP and ACH, disease course of GPP (relapsing, i.e., had had more than a single episode and/or persistent; i.e., flare persisting for over 3 months), and the number of previous flares were collected. Disease severity at flare was assessed by Generalized Pustular Soriasis Area and Severity Index (GPPASI) ranging from 0 to 72, and Generalized Pustular Soriasis Physician Global Assessment (GPPGA) ranging from 0 to 5 [16]. The presence of fever (\(\geq\)38 \({}^{\circ}\)C) and leukocytosis (white blood cells > 12 \(\times\) 10\({}^{9}\)/L) were also investigated [2]. Disease localization was estimated by proportion of the body surface area involvement (BSA %) and detailed for body regions, including mucosal and nail involvement. Oral and genital mucosal involvement was confirmed in the case of geographic tongue, and erythema/pustules in the genital area, respectively. Nail involvement was recognized in case of any of the following manifestations: leukonychia, onycholysis, salmon patch, splinter haemorrhage, hyperkeratotic lamina, subungual hyperkeratosis, onycholysis, and/or pitting. Subjective symptoms of itch and pain were assessed by Visual Analogue Scale (range 0-10). Dermatology life quality index (DLQI) questionnaire was also administered [17]. Concomitant psoriatic arthritis was assessed according to the CASPAR criteria [18]. History of previous medical diagnosis of and/or treatment of type 2 diabetes, obesity, arterial hypertension, dyslipidemia, hyperuricemia, and malignancy were investigated. The Charlson Comorbidity Index (CCI), which identifies different comorbidities that are associated with mortality, was calculated [16]. CCI final score ranges from 0 (no disease burden) to 24 (maximal disease burden), and the severity of comorbidity can be categorized into three grades: mild, with CCI scores of 1-2; moderate, with CCI scores of 3-4; and severe, with CCI scores \(\geq\)5. The risk for mortality increases with CCI scores [19; 20]. Finally, possible triggers, complications, and number and type of treatments prescribed for managing the flare were collected.