Myositis is a rare autoimmune disease in which the immune system mistakenly attacks the patient's own peripheral muscles. This aggression manifests by muscle inflammation and necrosis responsible for a motor deficit of varying severity. The treatments available today are insufficient and are non-specific. Biological criteria, issued from simple blood or muscle tests are missing, and they will help to define the activity of the disease and the efficacy of treatments. The MASC protocol will include patients with myositis, and investigators will collect clinical, radiological, electrophysiological, histological and biological data to be used for researches aiming at better understanding this entity. A biobank (muscle biopsy, DNA, serum, plasma, PBMCs) will be acquired on this prospective cohort. The study itself will be composed of a baseline visit and monthly to yearly follow-up visits which will assess:~Clinical examination with an evaluation of the muscle strength and function impairment/handicap, including but not limited to:~Manual testing of proximal axial and distal muscles on the five points Medical Research Council (MRC) scale~Barré tests and Mingazzini tests, number of stand-up / sitting, leg crossing~Biometry, lab and radiological measurements: muscle enzymes (creatine phosphokinase CPK, troponin, C-reactive protein, quantification of autoantibodies, muscle MRI, muscle biopsy, thorax tomodensitometry, pulmonary test function~Extra-muscular evaluation: cardiac examination and work-up (echocardiography, cardiac MRI and Positron Emission Tomography (PET) scanner, cardiac biopsies), pulmonary evaluation, rheumatological and dermatological assessment, history of thromboembolic disease and cancer Patient activity assessment: evaluation of daily life activity by both patient and physician using a Visual Analogue Scale~Quality of life questionnaires~Evaluation of the efficacy and toxicity of specific treatments For each patient, the date of last visit or contact will be collected as well as outcomes, particularly for the cause of death if relevant. Data from the biobank MASC  Muscles DNA/RNA Serum and Cells  will be added to other data. The biobank has been fully registered with local authorities and ethical committees (Committee for Personal Protection (CPP) CPP agreement). It contains peripheral blood mononuclear cells (PBMC), serum, DNA and RNA from blood and muscular biopsies collected at the diagnosis stage. The database contains immunological and genetical data.~This prospective study will also aim at:~Identify the differential pathophysiological processes between the different subgroups of myositis~Identify prognostic factors, including the different treatment modalities used~Improve physiopathological knowledge (clinico-anatomobiological characteristics and identification of other biomarkers through the biobank)~Improve the evaluation of the clinical outcomes/endpoints for future trials~Develop clinical trials for homogeneous subgroups of patients, based on their pathophysiology and evaluated on the appropriate endpoints.
Myositis are rare diseases for which the development of a cohort associated with a bank of biological samples (biobank) will allow for the conduct of researches to better delineate the underlying pathophysiology and find cures. This prospective cohort of patients with myositis will allow for identification of factors favouring the occurrence of myositis, whether they are constitutional (genetic) or acquired (environmental or drug). Different subgroups of myositis used for prognostication will be identified based on clinico-demographical variables, the nature of the organs involved beyond peripheral muscles (cardiac, diaphragm) and biomarkers abnormalities