The Incidence of IgG4-Related Disease in Slovenia-Single-Centre Experience.

Background: Data on the incidence of IgG4-related disease (IgG4-RD) are scarce. Our aim was to determine the incidence of IgG4-RD in a well-defined region. Methods: This retrospective study covered the Ljubljana region over the period from January 2012 to December 2024. A review of cases diagnosed with IgG4-RD was performed at several departments of the University Medical Centre Ljubljana-an integrated secondary/tertiary university teaching hospital (rheumatology, nephrology, angiology, gastroenterology, abdominal surgery, ENT surgery, ophthalmology). While IgG4-RD cases at the Department of Rheumatology were collected prospectively, potential cases at other departments were retrieved by searching electronic medical database for the keyword "IgG4". In addition, the Institute of Pathology, Faculty of Medicine, University of Ljubljana, provided a list of patients with histological features consistent with IgG4-RD. Year-specific incidence rates and an average incidence rate over the 13-year period were determined. Clinical features of patients were analysed. Results: During the observation period, 58 cases of IgG4-RD were diagnosed. Of these, 35 patients were residents of the Ljubljana region, which had an average adult population of 541,600. The estimated average annual incidence rate of IgG4-RD was 5.0 per million (95% confidence interval: 3.5; 6.9), with year-specific incidence rates fluctuating between 1.8 and 9.3 per million adults. The cases were stratified into four phenotypic categories: pancreato-hepato-biliary (17%), retroperitoneal fibrosis-aortitis (43%), head and neck-limited (14%), and Mikulicz syndrome with systemic involvement (26%). Conclusions: The average annual incidence rate of IgG4-RD was 5 per million adults, with the retroperitoneal fibrosis-aortitis phenotype predominating in our cohort.