What is the recommended workup for suspected myositis?

Workup for Myositis

The workup for suspected myositis should include muscle enzyme testing (CK, aldolase), myositis-specific autoantibodies, MRI of affected muscles, electromyography, and muscle biopsy when diagnosis remains uncertain, with the goal of distinguishing idiopathic inflammatory myopathy from mimicking conditions and identifying specific subgroups that guide prognosis and treatment. 1, 2

Clinical Assessment

Begin by documenting the pattern and tempo of muscle weakness:

• Proximal symmetric weakness developing over weeks to months is characteristic of polymyositis and dermatomyositis 2
• Distal and proximal weakness with slow progression over months to years suggests inclusion body myositis 2
• Document extramuscular manifestations including characteristic rashes (Gottron papules, heliotrope rash, periungual telangiectasias), fever, weight loss, dysphagia, and respiratory symptoms 2

Laboratory Testing

Muscle Enzymes

• Creatine kinase (CK) is the primary screening test and may be elevated 10-fold or more, particularly in immune-mediated necrotizing myopathy 2
• Aldolase, AST, ALT, and LDH provide additional supportive evidence 2

Autoantibody Panel

Myositis-specific autoantibodies are critical for diagnosis, subgroup classification, predicting extramuscular organ involvement (especially pulmonary and cardiac), and determining long-term prognosis 2. Test for:

• Anti-aminoacyl-tRNA synthetase antibodies (anti-Jo-1, anti-PL-7, anti-PL-12) - associated with antisynthetase syndrome and interstitial lung disease 2, 3
• Anti-Mi-2 - associated with classic dermatomyositis 2, 3
• Anti-SRP (signal recognition particle) - associated with necrotizing myopathy 4
• Anti-MDA5 - associated with rapidly progressive interstitial lung disease 5
• Anti-PM/Scl and anti-Ku - associated with overlap syndromes 3

Line blot immunoassays provide high specificity (96-100% for most antibodies) and are suitable for routine diagnostic workup 3. However, be aware that anti-Ro52 has lower specificity (76%) and coexisting autoantibodies occur in up to 32% of cases 3, 4.

Imaging Studies

MRI of Skeletal Muscle

MRI with T2-weighted sequences using fat suppression techniques or short tau inversion recovery (STIR) is the most sensitive and specific routine imaging method for detecting muscle inflammation 2. MRI serves three critical functions:

• Confirms the presence of muscle inflammation 6
• Identifies optimal biopsy sites by visualizing areas of active inflammation 2, 6
• Monitors treatment response 2

Whole-body MRI can sensitively visualize the distribution of muscle involvement throughout the body 6.

Electromyography (EMG)

EMG demonstrates characteristic abnormalities including 2:

• Polyphasic motor unit action potentials of short duration and low amplitude
• Increased insertional and spontaneous activity with fibrillation potentials
• Sharp waves and repetitive discharges

Muscle Biopsy

Muscle biopsy is often necessary to confirm the diagnosis and distinguish myositis from mimicking conditions 7, 8. The biopsy should be obtained from:

• A muscle that is clinically weak but not end-stage (avoid severely atrophied muscles) 8
• A site identified by MRI as showing active inflammation 2, 6
• The contralateral side if EMG was performed unilaterally to avoid needle artifact 8

Histopathologic evaluation distinguishes key subgroups 2, 8:

• Dermatomyositis: Perifascicular atrophy, perivascular inflammation
• Polymyositis: CD8+ cytotoxic T cells invading non-necrotic muscle fibers
• Necrotizing myopathy: Minimal inflammatory infiltrate with prominent necrosis
• Inclusion body myositis: Rimmed vacuoles, amyloid-β and phosphorylated tau accumulation

Screening for Complications and Comorbidities

Pulmonary Assessment

• High-resolution chest CT to screen for interstitial lung disease, particularly in patients with antisynthetase antibodies or anti-MDA5 2, 7
• Pulmonary function tests including DLCO 2

Cardiac Evaluation

• ECG and echocardiogram to assess for cardiac involvement 2
• Consider troponin levels 2

Malignancy Screening

In adult dermatomyositis patients, screen for underlying malignancy as cancer-associated myositis occurs in a significant subset 2, 7. Age-appropriate cancer screening should be performed, with consideration of:

• CT chest/abdomen/pelvis 7
• PET scan can simultaneously detect muscle inflammation and screen for malignancy 6
• In patients with cancer diagnosed within 3 years before myositis onset, approximately 25% develop recurrent or new primary cancer, typically within the first 3 years after myositis symptom onset 9

Critical Pitfalls to Avoid

• Do not diagnose polymyositis without muscle biopsy, as inclusion body myositis is frequently misclassified as polymyositis and requires different management 1, 8
• Recognize myositis mimics including muscular dystrophies with inflammation, statin-induced myopathy, denervation, diabetic myonecrosis, and deep venous thrombosis 10
• Evaluation at a myositis referral center may be necessary when differential diagnoses are challenging 7
• The 2017 EULAR/ACR classification criteria require a probability score ≥55% (corresponding to score ≥5.5 without biopsy or ≥6.7 with biopsy) for "probable IIM" classification 1