clinical characteristics associated with INMN
• Formerly lumped with PM. As its own separate category, it may account for 10% to 20% of all IIM. Also called necrotizing autoimmune myositis.
• Muscle disease predominates with acute or subacute onset of progressive proximal muscle weakness, myalgias, very high CPK >10 to 50 times upper limit of normal. No ILD or rash. Rarely associated with cancer, HIV, ASA syndrome, or another connective tissue disease (e.g., systemic sclerosis).
• MSAs found in 80% and identify clinical subsets.
• Muscle histology: scattered necrotic fibers with macrophages. Minimal inflammatory infiltrate. No CD8+ T cells. MHC class I antigens and C5b-9 on sarcolemma of non-necrotic fibers.
• Prognosis guarded. May be refractory to therapy.