Add-on Intravenous Immunoglobulins in Early Myositis

Inclusion Criteria: * Adult patients (≥ 18 years) with IIM, according to diagnostic criteria: * Dermatomyositis * Polymyositis * Anti-synthetase syndrome * Immune mediated necrotizing myopathy * Overlap myositis * Disease duration \< 12 months * Minimal disability defined as at least 10% loss on Manual Muscle Testing (MMT) and abnormal scores on two other Core Set Measures (CSMs) of the international Myositis Assessment and Clinical Studies (IMACS) group (see 'Primary and secondary outcomes'). * Patients are eligible for inclusion if they are treatment-naive, or if there is no clinical evident response (as carefully judged by the treating physician at a screening visit) to prior treatment with: * High dosed glucocorticoids, such as dexamethasone (e.g. 40 mg per day up to 4 days) or intravenous methylprednisolone (e.g. 1000 mg daily for three days), within 1 week prior to screening visit. * Daily dosed prednisone 1 mg/kg, or equivalent, used for up to 2 weeks prior to screening visit. * Treatment with low-dosed prednisone (max 20 mg daily) up to three months prior to screening visit. * Treatment with biologicals or other immunosuppressive or immunomodulatory treatment when meeting all of the following criteria: * Stable dose for the last 6 months * The biological or other immunosuppressive or immunomodulatory treatment has been approved for a non-muscular condition (e.g. hematological condition, eczema) * The biological or other immunosuppressive or immunomo…