The most common group of acquired and potentially treatable skeletal muscle disorders. Five major forms: dermatomyositis (DM), inclusion body myositis (IBM), polymyositis (PM), immune-mediated necrotizing myopathy, and antisynthetase syndrome. Usually present as progressive and symmetric muscle weakness; extraocular muscles spared but pharyngeal weakness (dysphagia) and head drop from neck muscle weakness are common. Respiratory muscles may be affected in advanced cases. IBM is characterized by early involvement of quadriceps (leading to falls) and distal muscles; IBM may have an asymmetric pattern. Progression is over weeks or months in PM and DM, but typically over years in IBM. Skin involvement in DM may consist of a heliotrope rash (blue-purple discoloration) on the upper eyelids with edema, a flat red rash on the face and upper trunk, or erythema over knuckles (Gottron’s sign). A variety of cancers are associated with DM. Features of each disorder are summarized in Table 198-1.
|DISORDER||SEX||AGE OF ONSET||RASH||PATTERN OF WEAKNESS||LABORATORY FEATURES||MUSCLE BIOPSY||CELLULAR INFILTRATE||RESPONSE TO IS THERAPY||COMMON ASSOCIATED CONDITIONS|
|DM||F > M||Childhood and adult||Yes||Proximal > distal||Normal or increased CK (up to 50× normal or higher); various MSAs (anti-MDA5, anti-TIF1, anti-Mi-2, anti-NXP2)||Perimysial and perivascular inflammation; IFN-1 regulated proteins (MHC-1, MxA), MAC deposition on capillaries||CD4+ Dendritic cells; B cells; macrophages||Yes||Myocarditis, ILD, malignancy, vasculitis, other CTDs|
|PM||F > M||Adult||No||Proximal > distal||Increased CK (up to 50× normal or higher)||Endomysial and perivascular inflammation; ubiquitous expression of MHC-1||CD8+ T-cells; macrophages; plasma cells||Yes||Myocarditis, ILD, other CTDs|
|NM||M = F||Children and adults||No||Proximal > distal||Elevated CK (> 10× normal or higher); anti-HMGCR or anti-SRP antibodies||Necrotic muscle fibers; minimal inflammatory infiltrate||Macrophages in necrotic fibers undergoing phagocytosis||Yes||Malignancy, CTD, HMGCR antibody cases can be triggered by statin use|
|ASS||F > M||Children and adults||Sometimes||Proximal > distal||Elevated CK (>10× normal or higher); antisynthetase antibodies||Perimysial and perivascular inflammation; perimysial fragmentation with alkaline phosphatase staining; perimysial muscle damage with necrosis||CD4+ Dendritic cells; B cells; macrophages||Yes||Non-erosive arthritis, ILD, Raynaud phenomenon, mechanic hands, and fever|
|IBM||M > F|
|No||Proximal and distal; predilection for: finger/wrist flexors, knee extensors||Normal or mildly increased CK (usually <10× normal); anti-cN-1A antibodies; large granular lymphocytes on flow cytometry and reduced CD4/CD8 ratio with increased CD8 count||Endomysial and perivascular inflammation; ubiquitous expression of MHC-1; rimmed vacuoles; p62, LC3, TDP-43 aggregates; EM: 15–18 nm tubulofilaments; ragged red and COX negative fibers||CD8+ T-cells; macrophages; plasma cells; myeloid dendritic cells; large granular lymphocytes||None or minimal||Granular lymphocytic leukemia/lymphocytosis, sarcoidosis, SICCA or Sjogren’s syndrome|
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