What tests can be done to confirm dermatomyositis and/or polymyositis and/or thymoma associated myasthenia gravis?

Diagnostic Tests for Dermatomyositis, Polymyositis, and Thymoma-Associated Myasthenia Gravis

A comprehensive diagnostic workup for dermatomyositis, polymyositis, and thymoma-associated myasthenia gravis should include muscle enzyme testing, autoantibody panels, imaging studies, electrophysiological testing, and tissue biopsy to confirm diagnosis and guide treatment decisions. 1

Diagnostic Tests for Dermatomyositis and Polymyositis

Laboratory Tests

• Muscle enzymes including creatine kinase (CK), lactate dehydrogenase (LDH), aspartate aminotransferase (AST), alanine aminotransferase (ALT), and aldolase - these may be elevated but can sometimes be normal despite active disease 1
• Complete blood count and blood film 1
• Inflammatory markers - erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) 1
• Myositis-specific and myositis-associated antibodies - crucial for classification and prognosis 1
• Renal and liver function tests 1
• Infection screen to rule out infectious causes of myopathy 1
• Thyroid function tests and electrolyte panel to exclude alternative causes of muscle weakness 1

Imaging Studies

• Muscle MRI with T2-weighted/STIR sequences - highly sensitive for detecting muscle inflammation 1
• Chest X-ray and high-resolution CT if pulmonary involvement is suspected 1
• Abdominal ultrasound scan 1

Functional Tests

• Manual Muscle Testing (MMT8) and Childhood Myositis Assessment Scale (CMAS) for formal muscle strength and function evaluation 1
• Pulmonary function tests including carbon monoxide diffusion capacity 1
• Nailfold capillaroscopy - particularly useful for dermatomyositis diagnosis 1

Electrophysiological Testing

• Electromyography (EMG) - to differentiate myopathy from neuropathy and disorders of neuromuscular junction 1
• Nerve conduction studies - particularly when diagnosis is uncertain 1

Tissue Sampling

• Muscle biopsy - essential when presentation is atypical or when skin signs are absent 1
• Standardized biopsy score tool should be used to quantify histological abnormalities 1
• Expert histopathological interpretation is required 1

Diagnostic Tests for Thymoma-Associated Myasthenia Gravis

Laboratory Tests

• Anti-acetylcholine receptor (AChR) antibodies - highly specific for myasthenia gravis 1, 2
• Anti-striated muscle antibodies - often present in thymoma-associated MG 1
• Paraneoplastic autoantibody panel 1

Imaging Studies

• Chest CT or MRI - to detect thymoma 3, 4
• PET scan - may be useful in detecting occult thymoma 1

Electrophysiological Testing

• Repetitive nerve stimulation - demonstrates decremental response characteristic of neuromuscular junction disorders 1, 2
• Single-fiber EMG - most sensitive test for neuromuscular junction disorders 1

Functional Tests

• Edrophonium (Tensilon) test - rapid, short-acting anticholinesterase that temporarily improves myasthenic weakness 2
• Ice pack test - non-pharmacological test for ocular myasthenia 2

Special Considerations for Overlap Syndromes

Clinical Features Suggesting Myositis-Myasthenia Gravis Overlap

• Presence of bulbar weakness (83% of overlap cases) 2
• Ptosis (33% of overlap cases) 2
• Diplopia (17% of overlap cases) 2
• Fatigable weakness (83% of overlap cases) 2
• Improvement with pyridostigmine (83% of overlap cases) 2
• Worsening with high-dose steroids (33% of overlap cases) 2

Additional Tests for Suspected Overlap Syndromes

• Cardiac evaluation with echocardiogram and ECG - essential for all patients with suspected myositis 1
• Swallow function assessment - particularly important when bulbar symptoms are present 1, 2
• Testing for co-existing autoimmune conditions 5

Diagnostic Pitfalls and Caveats

• Muscle enzymes may be normal despite active myositis 1
• High-dose steroids may worsen symptoms in patients with myasthenia gravis-myositis overlap 2
• EMG does not reliably detect metabolic myopathies, requiring additional workup if these are suspected 1
• Overlap of myasthenia gravis and myositis occurs in approximately 2.3% of myasthenia gravis cases 5
• Thymoma can present with paraneoplastic autoimmune disorders including polymyositis, dermatomyositis, and subclinical myasthenia gravis 3, 4
• Diagnostic criteria may overlap in patients with both conditions, complicating the diagnostic approach 3

Diagnostic Algorithm

1. Initial evaluation: Muscle enzyme panel, inflammatory markers, complete blood count 1
2. Autoantibody testing: Myositis-specific antibodies, AChR antibodies, anti-striated muscle antibodies 1
3. Imaging: Muscle MRI, chest CT/MRI to evaluate for thymoma 1
4. Electrophysiological studies: EMG, nerve conduction studies, repetitive nerve stimulation 1
5. Tissue sampling: Muscle biopsy when diagnosis remains uncertain 1
6. Functional testing: Manual muscle testing, pulmonary function tests, swallow assessment 1