Autoimmune Myositis - Differential Diagnosis Framework Inflammatory Myopathies: Commonly symmetric proximal muscle weakness, no substantial muscle pain or tenderness; serum creatine kinase ↑ Muscle Tenderness → Consider infectious, thyroid, drug-induced myopathies Diagnostic Tests: • Measure CK • Aldolase levels • EMG DERMATOMYOSITIS: • Subacute • Proximal muscle (symmetric) • CK > 50 ULN Antibodies: • Anti-MDA5 (RP-ILD), dermato-rheumatologic symptoms • Anti-CADM-140 (amyopathic dermatomyositis) • Anti-Mi2 (skin lesions) • Anti-TIF1 (cancer-associated adult dermatomyositis) • Anti-NXP2 (ANA present 80% of the time) Dermatomyositis Symptoms: • Gottron rash (Gottron papules and Gottron sign) • Shawl sign • Heliotrope • Interstitial lung disease Malignancy Risk: Ovarian, lung, pancreas, stomach, colon, and lymphoma POLYMYOSITIS: • Progressive, symmetric, proximal muscle weakness • CK > 50 ULN Antibodies: • Anti-Synthetase Ab • Anti-Jo-1 antibodies (ANA present 80% of the time) Polymyositis Symptoms: • Muscle pain and tenderness • Muscle atrophy • Weakness of respiratory muscles • Interstitial lung disease • Dysphagia, dysmotility, and increased risk of aspiration pneumonia • Antisynthetase syndrome (also seen in association with dermatomyositis) Malignancy Risk: Ovarian, lung, pancreas, stomach, colon, and lymphoma Overlap Syndrome: Fever, joint pain and Raynaud phenomenon INCLUSION BODY MYOSITIS: • M > F • Age > 50 • Misdiagnosed as: Polymyositis or ALS • Very slow onset and pattern of muscle involvement/Early weakness • History of preexisting weakness averaging 5 years • Proximal + distal muscle involvement • Symmetric, but asymmetry may occur • Frequent falls due to quadriceps muscle weakness PE: Muscle weakness + atrophy- Hip flexors, quadriceps, finger flexors, and forearm flexors Labs: • Serum creatine kinase levels are elevated < PM and DM • CPK 10X ULN • + Camptocormia (bending forward of the spine) or head drop • ANA < 20 % IBM • Anti-cytoplasmic 5'-nucleotidase 1A (Anti cN-1A) 50% Cricopharyngeal muscle involvement -> dysphagia and increased risk of aspiration > 50% of pts ANTI-SYNTHETASE SYNDROME: • Fever • Interstitial lung disease • Myositis • Raynaud phenomenon • Nonerosive arthritis • Mechanic's hands Autoantibodies: • Anti-aminoacyl-tRNA synthetase enzymes • Anti-Jo-1 antibodies Symptoms: • Pericarditis/Pericardial tamponade • Conduction system abnormalities/Arrhythmias • Dysphagia, dysmotility, and increased risk of aspiration pneumonia (esophagus upper 2/3 striated muscle) NECROTIZING AUTOIMMUNE MYOPATHY: • Proximal muscle (symmetric) • Prominent myonecrosis on biopsy • Severe, rapidly progressive weakness • Very high serum CK levels 50X ULN • Myocyte necrosis and regeneration without significant inflammation • Extramuscular manifestations rare • Possible paraneoplastic syndrome • If myopathy improves within 4 to 6 weeks after discontinuation of statins - probably caused by toxic effects of the drug Antibodies: • Signal recognition particles (SRP's) • 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase ANTI-MITOCHONDRIAL MYOPATHY: • Chronic skeletal muscle disease/muscular atrophy • Chronic progressive respiratory muscle weakness • Severe cardiac involvement: (20-30%) arrhythmias, cardiomyopathy, and myocarditis Other Diseases: • PBC, autoimmune hepatitis, Psoriasis, Hashimoto's Antibodies: • Anti-mitochondrial antibodies #Autoimmune #Myositis #Myopathy #rheumatology #differential #diagnosis
