A B Anti-SRP-Myopathy Non-severe Severe* ± IV steroids 0.5-1 g/d, 3-5 d oral steroids 1 mg/kg/d IV steroids 0.5-1 g/d, 3-5 d oral steroids 1 mg/kg/d At the same time or within 1 month, start 2nd or 3rd agent Oral/sc Methotrexate° (0.3 mg/kg/w, max 15 & 25 mg/w in child & adult, respectively) Rituximab£,§ 750 mg/m2 (max 1g) D1 + D7-D15 eventually IVIG 2 g/kg/m, 3-6 times Initial treatment If no adequate response within 6m: reconsider Rituximab (if not started) and keep giving other treatments. Anti-HMGCR-Myopathy B Non-severe Severe* ± IV steroids 0.5-1 g/d, 3-5 d oral steroids 1 mg/kg/d IV steroids 0.5-1 g/d, 3-5 d oral steroids 1 mg/kg/d At the same time or within 1 month, start 2nd or 3rd agent Oral/sc Methotrexate° (0.3 mg/kg/w, max 15 & 25 mg/w in child & adult, respectively) IVIG 2 g/kg/m, 3-6 times (possibly alone in case of steroids contraindication or intolerance) eventually Rituximab£ 750 mg/m2 (max 1g) D1 + D7-D15 Initial treatment If no adequate response within 6m: reconsider Rituximab (if not started) and keep giving other treatments.

Anti-SRP & anti-HMGCR-Myopathy Taper oral steroids to the lowest dose as tolareted or as soon as possible (regarding the maximum benefit) Continue Methotrexate at least 2 years of well-controlled disease (slowly tapered later: 2.5 mg/w each month) Continue Rituximab every 6 months, at least 2 years of well-controlled disease If started IVIG are stopped or tapered, as tolerated.Nota bene: many HMGCR+patients may required IVIG Maintenance treatment Figure X: Consensus for the initial treatment of IMNM A. Anti-SRP-myopathy. B. Anti-HMGCR myopathy IV: intavenous, IVIG: intravenous immunoglobulins, d: day, m: month, sc: sub-cutaneous,*: walking difficulties and/or dysphagia, °: Azathioprine or Mycophenolate mofetil in case of Methotrexate intolerance, £: rigourous data from the literature are however missing, §: along with Methotrexate, especially in severe cases