Inclusion criteria

• Adult patients (≥ 18 years) with IIM, according to diagnostic criteria:
  • Dermatomyositis 
  • Immune mediated necrotizing myopathy 
  • Anti-synthetase syndrome 
  • Overlap/non-specific myositis, including polymyositis 
• Disease duration < 12 months (defined as duration of symptoms attributed to myositis before screening).
• Disease activity with a minimum of at least 10% loss on Manual Muscle Testing (MMT) in 13 muscle groups 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
  • 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) and is not known for its use in idiopathic inflammatory myopathy
• The biological or other immunosuppressive or immunomodulatory treatment is not known to induce inflammatory myopathy
• Signed informed consent.

Exclusion criteria

A potentially eligible patient who meets any of the following criteria will be excluded from participation in this study:

• Severe muscle weakness (i.e. bedridden, severe dysphagia requiring a nasogastric tube, or symptomatic respiratory muscle weakness (respiratory symptoms in combination with a forced vital capacity below 50% of predicted in upright position)) necessitating more intensive treatment than standard glucocorticoids from the start.
• Signs of severe or rapidly progressive ILD, necessitating respiratory support or progressive ILD on HRCT, with ≥ 10% decrease in vital capacity and/or ≥ 15% decrease of diffusion.
• A known malignancy, which is likely to interfere with outcome assessment.
• Related to IVIg:
  • History of thrombotic episodes within 10 years prior to enrolment
  • Known allergic reactions or other severe reactions to any blood-derived product
  • Known IgA deficiency and IgA serum antibodies
  • Pregnancy or trying to conceive
  • Use of loop diuretics
  • Use of nephrotoxic medication
• Conditions that are likely to interfere with:
  • Compliance (legally incompetent and/or incapacitated patients are excluded), or,
  • Evaluation of efficacy (e.g. due to severe pre-existing disability as a result of any disease other than myositis or due to a language barrier)
• Immunosuppressive medication or immunomodulatory treatment within the last 3 months (e.g. azathioprine, methotrexate, mycophenolate mofetil, tacrolimus, cyclophosphamide, cyclosporine, IVIg, biologicals, Janus kinase inhibitors, plasmapheresis), with evident clinical response (as judged by the treating physician at a screening visit)