The incidence of idiopathic inflammatory myopathies in the adult Slovenian population.

Idiopathic Inflammatory myopathies (IIM) are rare disorders. The aim of our study was to determine the incidence of IIM in a well-defined Slovenian region. This retrospective study was conducted at the Department of Rheumatology, University Medical Centre Ljubljana, the only secondary/tertiary rheumatology center in a region with a population of 704,342 adults. We identified potential IIM cases by searching the electronic patient records for ICD-10 codes M33, M35.1, M35.8, M60, G72, G73, and J84. We included incipient IIM cases between January 2010 and December 2017, who were at the time of the diagnosis, residents of the inspected region. To avoid under-reporting due to miscoded cases, we obtained a list of the patients who had histological patterns consistent with IIM on muscle biopsy from the Institute of Pathology. The annual incidence rate for IIM was calculated. During the eight-year observation period, we identified 65 IIM cases (72.3% female, median (IQR) patient age 64.8 (54.8-73.2) years). The estimated annual incidence of IIM in the studied population was 11.5 (95% CI 9.0-14.6) per 10 adults, in females 16.2 (95% CI 12.1-21.4), and in males 6.6 (95% CI 4.0-10.2) per 10 adults. The incidence rate of IIM in Slovenia is consistent with data from the literature.