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



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