What are the diagnostic tests for dermatomyositis?

Diagnostic Tests for Dermatomyositis

The diagnosis of dermatomyositis requires a comprehensive panel of tests including muscle enzymes, myositis-specific antibodies, MRI with T2-weighted/STIR sequences, nailfold capillaroscopy, and in some cases, muscle biopsy. 1, 2

Core Diagnostic Tests

• Muscle enzymes - Measure creatine kinase (CPK), lactate dehydrogenase (LDH), aspartate aminotransferase (AST), alanine aminotransferase (ALT), and aldolase, though these may be normal despite active disease 1, 2

• Myositis-specific and myositis-associated antibodies - These should be measured when available, particularly anti-TIF 1-γ (p155), anti-NXP2/(p140/MJ), anti-MDA5, and anti-SRP 1

• Muscle MRI - Using T2-weighted/STIR sequences to enhance detection of muscle inflammation, interpreted by an expert radiologist 1, 3

• Nailfold capillaroscopy - Standardized assessment should be performed at diagnosis and regularly during follow-up 1

• Cutaneous Assessment Tool (CAT) - Formal assessment tools like DAS (skin) or MITAX (skin) should be used to aid diagnosis and monitor skin disease activity 1

Additional Diagnostic Tests

• Complete blood count and inflammatory markers - Including ESR (erythrocyte sedimentation rate) and CRP (C-reactive protein) 1

• Electromyography (EMG) - Should be considered to differentiate myopathy from neuropathy when diagnosis is uncertain 1, 2

• Muscle biopsy - Essential in all cases where presentation is atypical, particularly in the absence of characteristic skin signs; considered the gold standard for diagnosis 1, 4

• Pulmonary function tests - Including carbon monoxide diffusion; if abnormal, further investigations (chest X-ray/HRCT) are needed 1

• Cardiac assessment - Echocardiogram and ECG should be performed at diagnosis 1

Specialized Testing

• In overlap syndromes - Consider testing for myositis-associated antibodies such as anti-PmScl, anti-U1-RNP, anti-La ('SSB'), anti-Ro ('SSA'), and anti-Sm 1

• Swallow function assessment - May include speech and language therapy assessment, video fluoroscopy/barium studies 1

• Imaging for calcinosis - Plain radiographs can be used for evaluation 1

Diagnostic Algorithm

1. Initial evaluation: Muscle enzymes, inflammatory markers, complete blood count 2

2. Autoantibody testing: Myositis-specific and myositis-associated antibodies 1, 2

3. Imaging: Muscle MRI with T2-weighted/STIR sequences 1, 3

4. Nailfold capillaroscopy: To detect characteristic microvascular changes 1

5. Electrophysiological studies: EMG if diagnosis remains uncertain 1

6. Muscle biopsy: Particularly important in atypical presentations or when other tests are inconclusive 1, 4

Common Pitfalls and Caveats

• Normal muscle enzymes do not rule out disease - Muscle enzymes may be normal despite active disease 1, 5

• Perifascicular atrophy on muscle biopsy - While characteristic, it is not specific to dermatomyositis and can be present in overlap myositis 6

• Adermatopathic dermatomyositis - Some patients may have muscle inflammation without skin manifestations, requiring careful evaluation 6

• Malignancy screening - All patients with dermatomyositis should be evaluated for associated malignancy at diagnosis 5

• Alternative diagnoses - Consider metabolic or mitochondrial myopathies and dystrophies if cutaneous signs are absent or response to therapy is poor 1