What is the workup for myasthenia gravis (MG)?

Diagnostic Workup for Myasthenia Gravis

The comprehensive workup for myasthenia gravis must include neurology consultation, antibody testing, electrodiagnostic studies, and chest imaging to evaluate for thymoma, with additional tests based on clinical presentation. 1

Initial Clinical Assessment

• Antibody testing:

  • AChR antibodies (present in 80-85% of patients)
  • MuSK antibodies (present in 5-8% of patients)
  • Lrp4 antibodies (present in <1% of patients)
  • Note: ~10% of patients may be seronegative 2

• Electrodiagnostic studies:

  • Repetitive nerve stimulation
  • Single-fiber EMG (gold standard with >90% sensitivity for ocular MG) 1
  • Neuromuscular junction testing with jitter studies
  • Nerve conduction studies to exclude neuropathy 3

• Pharmacological testing:

  • Edrophonium (Tensilon) test:
    • Adults: 2 mg IV initially, followed by 8 mg if no reaction after 45 seconds
    • Children: Weight-based dosing (1 mg IV for children up to 75 lbs) 4
  • Ice pack test (particularly useful for ocular symptoms)

Imaging Studies

• Chest imaging:

  • CT or MRI to evaluate for thymoma (present in 10-20% of AChR-positive patients) 1

• Brain/spine MRI:

  • To rule out other neurological conditions
  • Consider with or without contrast depending on symptoms 3, 1

Additional Testing

• Pulmonary function assessment:

  • Negative inspiratory force (NIF)
  • Vital capacity (VC)
  • Essential for evaluating respiratory muscle strength 1

• Cardiac evaluation:

  • ECG
  • Troponin levels
  • Consider transthoracic echocardiogram (TTE) if respiratory insufficiency or elevated cardiac markers to rule out myocarditis 3, 1

• Laboratory screening:

  • Screen for concurrent autoimmune conditions (thyroid function tests, ANA)
  • Basic metabolic panel
  • Complete blood count

Subgroup Classification

Classify patients based on:

1. Antibody status (AChR, MuSK, Lrp4, or seronegative)
2. Age of onset (early vs. late)
3. Thymus pathology (thymoma vs. non-thymoma)
4. Clinical manifestations (ocular vs. generalized) 5

Special Considerations

• Comorbidity screening:

  • Thyroid disease, systemic lupus erythematosus, and rheumatoid arthritis are common comorbidities, especially in early-onset MG 6

• Medication review:

  • Identify and discontinue medications that can worsen myasthenia, including β-blockers, IV magnesium, fluoroquinolones, aminoglycosides, and macrolides 3, 1

• Crisis evaluation:

  • For patients presenting with respiratory distress, immediate assessment of ventilatory function is critical
  • Edrophonium testing in crisis should be performed cautiously with 1 mg IV initially, followed by another 1 mg if no adverse effect 4

Interpretation of Results

• Understanding antibody status helps guide prognosis and treatment selection:
  • AChR-positive MG generally has better prognosis than MuSK-positive MG
  • Seronegative status does not rule out MG
  • Cell-based assays may be more sensitive in detecting antibodies 1

The diagnostic approach should be tailored based on the presenting symptoms, with more extensive testing for patients with generalized weakness compared to those with purely ocular symptoms. Early neurology consultation is essential for coordinating the diagnostic workup and interpreting test results.