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Autoantibodies in PM and DM. Autoantibodies: >90% Positive ANA: 60-80% More in overlap Low in IBM Defined antibodies: 50% Myositis-specific antibodies: 35-40% Most common Ab (Jo-1): 20%. Myositis-Specific Antibodies. High disease specificity Appear prior to disease onset
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Autoantibodies in PM and DM • Autoantibodies: >90% • Positive ANA: 60-80% • More in overlap • Low in IBM • Defined antibodies: 50% • Myositis-specific antibodies: 35-40% • Most common Ab (Jo-1): 20%
Myositis-Specific Antibodies • High disease specificity • Appear prior to disease onset • Absence does not exclude • Assist in diagnosis and classification
Established Myositis-Specific Autoantibodies • Anti-synthetases 25% • Anti-Jo-1 20% • Non-Jo-1 4 - 8% • Anti-Mi-2 5 - 10% • Anti-SRP 5% • “signal recognition particle” • (Anti-PM-Scl) 5 - 10%
Anti-Synthetases: Indirect Immunofluorescence Anti-Jo-1 Anti-OJ Anti-PL-12
Anti-Synthetase Syndrome Myositis 95% Interstitial Lung Disease 70-90% Arthritis 50-90% Raynaud’s Phenomenon 60% Fevers 85% Recurrences 60% Mechanic’s Hands 70%
Anti-Mi-2: Clinical Picture • High specificity for myositis • low sensitivity • Relative specificity for dermatomyositis • Adults and children • Rash often prominent
Signal Recognition Particle Autoantibodies • Acute onset • Severe weakness • No skin involvement • Biopsy may lack inflammation • Immune-mediated necrotizing myopathy
DIAGNOSIS and the Myositis Autoantibodies • High specificity, Low sensitivity • Define patient subgroups • May help when extra-muscular features predominate
Inclusion Body Myositis • Older age of onset • Slow Progression • Longer duration to diagnosis • Distal involvement: IBM>PM/DM • Forearm flexors • Quadriceps • Lesser degree of CreatineKinase elevation
HISTOLOGIC FEATURES INCLUSION BODY MYOSITIS • Light Microscopy • Rimmed vacuoles with basophilic granules • Eosinophilic inclusions • Electron Microscopy • Inclusion bodies with microtubular filaments
Histopathology of IBMDalakas: Rheum Dis Clin NA 28:779,2002 Endomysial Inflammation Sourrounding Non-necrotic fibers
IBM PathologyDalakas Rheum Dis Clin NA 28:779,2002 Rimmed vacuoles; Basophilic Granules EM: Tubulofilaments
Extra-Muscular Manifestations • Systemic • Pulmonary • Interstitial Lung Disease • Respiratory Muscle Weakness • Aspiration • Infection • Toxicity • Cardiac • GI
Malignancy and Myositis • 32% DM, 15% PM • Risk greatest in 1st year • Increased risk in those age >45 • May have paraneoplastic course • Ovarian cancer overrepresented • Colon, lung, pancreatic, breast, lymphoma • Risk is higher with anti-p155/140 • lower with other myositis antibodies
Differential Diagnosis of Idiopathic Inflammatory Myopathy • Other myopathies (dystrophy, metabolic) • Neuropathies (ALS, Myasthenia gravis) • Drug-induced myopathies • Infectious myopathies • Endocrine myopathies (hypothyroidism, Cushing’s syndrome)
DRUG-INDUCED MYOPATHY • D-Penicillamine • Zidovudine • Lipid Lowering Agents • Colchicine • L-Tryptophan • Drugs of Abuse • Endocrine • Chloroquine
Suggested Reading • Primer on the rheumatic diseases • For greater depth: • Miller FW: Inflammatory Myopathies: Polymyositis, Dermatomyositis, and Related Conditions (Chapter 75). In: Koopman et al, Arthritis and Allied Conditions