AM is a clinically and etiologically heterogenous group of myopathies. It is rarely associated with myositis, although there have been occasional reports of AM masquerading idiopathic inflammatory myopathies (IIMs) such as polymyositis and inclusion body myositis. Pathologically, AM is characterized by amyloid deposits in the intramuscular blood vessels or connective tissue elements, and reportedly not in myofibers.

Case 1: 62-year-old male with a 1-year history of dilated cardiomyopathy presented with significant proximal muscle weakness. His serum creatine kinase (CK) levels were moderately elevated. A right deltoid muscle biopsy demonstrated myopathic changes with necrotic fibers, many CD68+ macrophages, occasional CD8+ T-cells, MHC-1 sarcolemmal/sarcoplasmic positivity, and amyloid deposits in the intramuscular blood vessels. A subsequent bone marrow biopsy revealed the IGH-CCND1 rearrangement with malignant 25% lambda light-chain restricted plasma cells.

Case 2: 58-year-old female presented with generalized weakness, lower extremity edema, decreased exercise tolerance and lethargy over 4 weeks. She was recently diagnosed with kappa light-chain multiple myeloma by a bone marrow biopsy showing 80% kappa light-chain restricted plasma cells. Despite normal CK while receiving dexamethasone, the thigh muscle magnetic resonance imaging and clinical features were consistent with IM. A left thigh muscle biopsy demonstrated myopathic changes including necrotic fibers and amyloid deposits in the necrotic fibers, endomysial/perimysial tissue, and blood vessels. It also revealed abundant inflammatory cell infiltrates including CD68+ macrophages, CD8+ T-cells, and CD20+ B-cells with MHC-1 sarcolemmal/sarcoplasmic positivity.

Our cases and literature review suggest that AM may present with IM features including unusual features such as B-cell inflammatory foci. IM associated with AM (amyloid IM) may be secondary or a subtype of IIMs.