The idiopathic inflammatory myopathies are a diverse group of connective tissue diseases of unknown etiology characterized by chronic inflammation of skeletal muscle, or myositis. The most common types of those disorders are consisted of dermatomyositis (DM), polymyositis (PM), necrotizing myopathy (NM) or immune mediated necrotizing myopathy, and sporadic inclusion body myositis (sIBM). Patients ordinarily present with sub-acute to chronic onset of proximal weakness revealed by difficulty with getting up from a chair, climbing stairs, pick up objects, and brushing hair.
Panel test:
• Creatine kinase (CK). elevated in most idiopathic inflammatory myopathies (IIM)
• Aldolase, aspartate aminotransferase (AST), alanine aminotransferase (ALT), lactate dehydrogenase (LD), serum myoglobin- Not usually recommended, variably elevated, Highly nonspecific
• Thyroid-stimulating hormone – rule out thyroid disease as etiology for muscle weekness
• Antinuclear antibodies Antibody– This test help to rule out connective tissue disease (CTD) or overlap diseases. Staining pattern may be useful in determining the type of confirmatory test(s) to perform. This test could be positive in 50-80% of patients with inflammatory myopathies
• Antisynthetase antibodies
o anti-Jo-1. Moderate to severe disease, Arthritis common, Low rate of mechanic’s hand
o anti-PL-7. Higher incidence of ILD (interstitial lung disease), Severe arthritis, Raynaud phenomenon, Infrequent myositis
o anti-PL-12. Raynaud phenomenon, more common to have myositis-associated antibodies present concurrently
o anti-EJ. Mechanic’s hands, DM.
o anti-OJ. Arthritis, High incidence of ILD (interstitial lung disease), myositis
• Other myositis-specific antibodies
o Anti SRP (signal recognition particle). Acute, severe necrotizing myopathy
o Severe cardiac involvement
o Anti-Mi-2 (nuclear helicase protein). Classic DM
o Anti P155/140. Aggressive skin lesions in DM
o Anti NXP-2 (nuclear matrix protein-2), DM, Increased malignancy risk, ILD
o Anti TIF1-gamma (TIF1-y), Aggressive skin lesions in DM, Cancer in adults >50 years
o Anti MDA5 (CADM-140), Rapidly progressive ILD, Poor prognosis, CADM
o Anti SAE1 (SUMO activating enzyme), Prognosis is favorable, Initial amyopathic DM, DM, Severe skin disease, dysphagia, and systemic features
o Anti HMGCR. Necrotizing autoimmune myopathy, could be associated with statin therapy
o Anti Mup44. sIBM, also seen in different autoimmune systemic diseases
• Myositis- Associated antibodies– Target autoantigen and overlap syndromes-
o anti-PM/Scl-100. Polymyositis and scleroderma, overlap myositis, MCTD
o anti-SSA(RO). ILD in IM patients
o anti-U1 RNP. MCTD, overlap myositis
o Ku. PM-SSC Overlap
Algorithm
References:
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