What is the initial workup for a patient suspected of having Myasthenia Gravis?

Initial Workup for Suspected Myasthenia Gravis

The initial workup for a patient suspected of having Myasthenia Gravis should include acetylcholine receptor (AChR) antibody testing, antistriated muscle antibody testing, pulmonary function assessment, and electrodiagnostic studies to establish the diagnosis and evaluate disease severity. 1, 2

Clinical Presentation

• Fatigable or fluctuating muscle weakness, generally more proximal than distal, is the hallmark presentation of Myasthenia Gravis 1, 3
• Ocular weakness with asymmetric ptosis and binocular diplopia is the most typical initial presentation 3, 4
• Bulbar involvement (dysphagia, dysarthria, facial muscle weakness) may be present 1, 4
• Neck and/or respiratory muscle weakness can occur in more severe cases 1
• Symptoms typically worsen with fatigue and improve with rest 4

Diagnostic Workup Algorithm

Step 1: Serologic Testing

• AChR antibody testing in blood (positive in 80-85% of patients) 1, 4
• Antistriated muscle antibody testing in blood 1
• If AChR antibodies are negative, test for:
  • Muscle-specific kinase (MuSK) antibodies (positive in 5-8% of patients) 1, 4
  • Lipoprotein-related protein 4 (LRP4) antibodies (positive in <1% of patients) 1, 4

Step 2: Pulmonary Function Assessment

• Negative inspiratory force (NIF) measurement 1
• Vital capacity (VC) measurement 1
• These tests are crucial to evaluate for respiratory compromise 1

Step 3: Laboratory Testing for Concurrent Conditions

• Creatine phosphokinase (CPK) and aldolase to evaluate for concurrent myositis 1
• Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) 1
• Troponin T if respiratory insufficiency or elevated CPK is present (to rule out myocarditis) 1

Step 4: Electrodiagnostic Studies

• Neuromuscular junction testing with repetitive nerve stimulation 1
• Jitter studies to evaluate neuromuscular junction function 1
• Nerve conduction studies to exclude neuropathy 1
• Needle electromyography (EMG) to evaluate for myositis 1

Step 5: Imaging Studies (if indicated)

• MRI of brain and/or spine depending on symptoms to rule out CNS involvement or alternative diagnoses 1
• Chest imaging to evaluate for thymoma, which is present in 10-20% of patients with AChR antibody-positive MG 4

Step 6: Cardiac Evaluation (if indicated)

• ECG if respiratory insufficiency or elevated CPK/troponin is present 1
• Transthoracic echocardiogram (TTE) to evaluate for possible concomitant myocarditis 1

Specialist Consultation

• Neurologic consultation is essential for diagnosis confirmation and treatment planning 1
• Ophthalmology consultation may be beneficial for patients with ocular symptoms 1

Common Pitfalls and Considerations

• Avoid medications that can worsen myasthenia: β-blockers, IV magnesium, fluoroquinolones, aminoglycosides, and macrolides 1, 2
• Consider overlapping conditions such as Miller Fisher variant of Guillain-Barré syndrome and oculobulbar myositis, which may have similar presentations 1
• Be aware that 50% of patients initially present with ocular symptoms only, but 50-80% of these patients develop generalized systemic myasthenia within a few years 1
• The ice test (applying an ice pack over closed eyes for 2 minutes) can be a highly specific bedside test for ocular myasthenia 2
• Approximately 10% of patients remain seronegative despite comprehensive antibody testing 4

By following this systematic approach to the initial workup of suspected Myasthenia Gravis, clinicians can establish the diagnosis, assess disease severity, and prepare for appropriate treatment planning.