Overall and by subtype epidemiology of idiopathic inflammatory myopathies among African descent patients with free access to high standard healthcare

• This study reports a high incidence of inflammatory myopathies, making it the third most common systemic inflammatory rheumatic disease after systemic lupus and rheumatoid arthritis in patients of African descent. • Dermatomyositis is the second most common subtype, and inclusion body myopathy is almost non-existent in this population. Anti-MDA-5 myositis accounts for 20% of dermatomyositis cases, which is similar to the rates observed in Asian patients. • At the time of diagnosis of myositis, more than half of African descent patients present with ILD, which is fibrotic in half of the cases. • Patients of African descent require rapid assessment, diagnosis and treatment, especially with regard to the high frequency of pulmonary and cardiac involvement. To describe the epidemiology, characteristics and evolution of African descent patients with IIM in the Caribbean Island of Martinique with free access to high standard socialized healthcare. Retrospective longitudinal study from January 1, 2000 to June 2023 at the University Hospital of Martinique, the island’s only tertiary care center. Patients with clinically myopathic or amyopathic adult-onset dermatomyositis (DM), polymyositis (PM), anti-synthetase syndrome (ASyS), immune-mediated necrotizing myositis (IMNM), overlap myositis (OM) were included. Baseline clinical, immunological, radiological characteristics, and treatments were collected along with occurrence of relapses and deaths Mean IIM incidence was 2.34/100,000. The 2022 prevalence of IIM reached 32.2/100,000 inhabitants, with variations between subtypes. 174 patients were included, of whom 55 (32%) had ASyS, 44 (25%) OM, 44 (25%) DM, 16 (9%) PM, 14 (8%) IMNM and 1 (0.6%) IBM. Median time from first symptoms to diagnosis was 4.2 months [2.0 to 12.3]. Anti-Jo1, anti-PL12 and anti-SRP were the most frequent myositis specific autoantibody detected. Most frequent manifestations were arthralgia (65%), muscle weakness (61%) and skin DM lesions (53%). At diagnosis, 77 (52%) and 28 patients (16%) had interstitial lung disease (ILD) and cardiac involvement, respectively. ASyS was more frequently clinically amyopathic (p<0.05) and had more frequently ILD (p<0.05). After treatments, sixty-nine patients (40%) relapsed in a median time of 2.8 years [1.1 to 7.2]. There is a high incidence and prevalence of IIM in the Caribbean Island of Martinique, and a pulmonary and cardiac threat at IIM diagnosis in African descent population.