Diagnostic Accuracy of Whole Body Magnetic Resonance Imaging in Inflammatory Myopathies

Idiopathic inflammatory myopathies (IIM) are a great concern in acquired muscle illnesses. An appropriate and rapid diagnosis is necessary, because morbidity and mortality are high and as a specific treatment is needed (corticosteroids, immunosuppressant, and "biotherapies").

Currently the use of muscle MRI in departments managing IIM is common. In absence of recommendations fixing their place in the diagnostic phase, the practices observed are extremely heterogeneous. Some systematically order a muscle MRI for patients suspected to suffer from IIM, others hold this examination for patients selected according to non defined criteria, and others sometimes use MRI to follow-up patients. This practices diversity well reflects the lack of data in the literature, making it impossible to appreciate the real contribution of this test. The results published in the great majority of studies are obtained with patients whose IIM diagnosis is already established. The diagnostic accuracy of muscle MRI in real conditions of clinical practice is thus unknown for patients having a simple suspicion of IIM.

Primary scientific aim: Evaluation of diagnostic accuracy of muscle MRI (in terms of sensitivity, specificity, predictive positive value and predictive negative value) for patients who are suspected to suffer from IIM.

Secondary scientific aim: Comparison of two diagnostic strategies: MRI-oriented muscle biopsy versus non MRI-oriented muscle biopsy.

Gold standard: muscle biopsy Index test: whole body MRI, STIR and T1 sequences Patients and setting of diagnosis tests: Screening will be realized on any patient suspected to suffer from inflammatory and addressed for muscle biopsy in one of the participating reference centres for neuro-muscular disorders. -Evaluate if RMI is efficiency to reduce false negative results during a second biopsy.