Inflammatory Muscle Disease

Abstract

The main types of inflammatory muscle disease (IMD) are dermatomyositis (DM), polymyositis (PM), necrotizing autoimmune myositis (NAM), and sporadic inclusion body myositis (sIBM). Major clinical and laboratory manifestations are proximal muscle weakness, skin manifestations (Gottron’s sign, heliotropic rash, mechanic’s hands, and calcinosis cutis), lung involvement, elevated muscle enzymes, and myositis-specific autoantibodies. The latter can be biomarkers for certain disease features: (a) anti-PMScl for interstitial lung disease (ILD), inflammatory arthritis, Raynaud’s phenomenon (RP), and mechanic’s hands; (b) anti-U1snRNP for myositis in the context of mixed connective tissue disease (MCTD); (c) anti-Ku for overlap myositis with other autoimmune systemic diseases; (d) anti-Ro52 for mechanic’s hands and malignancy when it co-occurs with anti-Jo1 antibodies; (e) antibodies to aminoacyl-tRNA synthetases (anti-Jo1) for mechanic’s hands, arthritis, ILD, and RP; (f) anti-SRP (signal recognition particle), for severe muscle and heart involvement; (g) anti-HMGCR (3-hydroxy-3-methylglutaryl-coenzyme A reductase) for statin use-related necrotizing myopathy; (h) anti-MDA5 (melanoma differentiation-associated gene 5) for amyopathic myositis with ILD; and (i) anti-cN1A (cytosolic 5’nucleotidase 1A) for sIBM. Paraneoplastic myositis is characterized by autoantibodies to TIF-1 (transcriptional intermediary factor 1) antigen. In terms of histopathology, DM is characterized by prominent perivascular and perifascicular inflammation, PM by endomysial inflammation, and sIBM by a combination of inflammation and degeneration of muscle fibers with internal rimmed vacuoles. NAM is considered a macrophage-mediated inflammatory process. Scattered necrotic muscle fibers surrounded by sparse inflammatory cells (predominantly lymphocytes) is characteristic. Endocrine conditions (Cushing’s disease, thyroid dysfunction, and diabetes mellitus) as well as genetic abnormalities (mitochondrial myopathies) induce muscle diseases which have to be distinguished from IMDs.