Diagnostic Approach for Myasthenia Gravis
The diagnosis of myasthenia gravis requires serological antibody testing (AChR, MuSK, or LRP4 antibodies) combined with electrodiagnostic confirmation via single-fiber electromyography, which is the gold standard with >90% sensitivity in ocular myasthenia. 1, 2
Clinical Presentation to Recognize
Look specifically for:
- Fluctuating, fatigable muscle weakness that worsens with repetitive use and improves with rest
- Variable ptosis that worsens with prolonged upgaze and may show Cogan lid-twitch sign
- Variable strabismus and diplopia that changes throughout examination
- Bulbar symptoms: difficulty chewing, swallowing, slurred speech
- Respiratory muscle weakness in severe cases
- Symptoms worsen throughout the day and with fatigue 1, 2, 3
Diagnostic Algorithm
Step 1: Bedside Clinical Tests
Ice Pack Test (highly specific):
- Apply ice pack over closed eyes for 2 minutes for ptosis or 5 minutes for strabismus
- Positive if ptosis reduces by ≥2mm or strabismus improves
- Specificity is excellent for MG 2, 4
Rest Test: Alternative without ice pack, observe improvement after period of rest 2
Edrophonium (Tensilon) Test:
- Only perform if experienced with IV administration in monitored setting with atropine available
- 95% sensitive for generalized MG, 86% sensitive for ocular MG
- Risk of muscarinic side effects (bradycardia, bronchospasm, hypotension) 2, 4
Step 2: Serological Testing (Essential)
Primary antibody panel:
Acetylcholine receptor (AChR) antibodies - binding, blocking, or modulating types
If AChR negative, test for:
Anti-striated muscle antibodies - check in all patients 5, 1
Critical caveat: Absence of antibodies does NOT rule out MG - proceed with electrodiagnostic testing 1
Step 3: Electrodiagnostic Studies (Confirmatory)
Single-fiber electromyography (SF-EMG):
Repetitive nerve stimulation (RNS):
- Positive in only 1/3 of ocular myasthenia patients
- Less sensitive than SF-EMG but more widely available 2, 4
Additional studies:
- Nerve conduction studies to exclude neuropathy
- Needle EMG to evaluate for concurrent myositis 5, 1, 5
Step 4: Assess for Concurrent Conditions and Complications
Pulmonary function testing:
- Negative inspiratory force (NIF)
- Vital capacity (VC)
- Essential for all grades given risk of respiratory compromise 5, 1, 5
Cardiac evaluation (if respiratory insufficiency or elevated CPK):
- Troponin T
- ECG
- Transthoracic echocardiogram (TTE) or cardiac MRI
- Rule out concurrent myocarditis 5, 1, 5
Laboratory markers:
Neuroimaging:
- MRI brain and/or spine depending on symptoms
- Rule out CNS involvement or alternative diagnosis 5, 1, 5
Paraneoplastic workup: Consider in appropriate clinical context 1
Step 5: Mandatory Neurology Consultation
All suspected MG cases warrant neurology consultation for confirmation and management planning 5, 1, 5
Key Diagnostic Pitfalls to Avoid
- Do NOT rely solely on antibody testing - 50% of ocular MG patients are seronegative 2
- Do NOT skip electrodiagnostic testing in seronegative patients - SF-EMG remains highly sensitive 2, 4
- Do NOT dismiss pupillary involvement - this should prompt consideration of alternative diagnoses like third nerve palsy 2
- Do NOT delay pulmonary function assessment - respiratory compromise can develop rapidly even in mild cases 1, 5
- Review and STOP medications that worsen myasthenia: β-blockers, IV magnesium, fluoroquinolones, aminoglycosides, macrolide antibiotics 5, 1, 5
Disease Subtyping for Treatment Planning
Once diagnosed, classify by:
- Antibody status: AChR-positive, MuSK-positive, LRP4-positive, or seronegative
- Clinical distribution: Ocular vs. generalized (using MGFA classification)
- Severity grading: Determines treatment intensity 1, 3, 6
This classification directly impacts treatment selection, as MuSK-positive patients respond better to rituximab, while AChR-positive patients are candidates for complement inhibitors and neonatal Fc receptor antagonists 1, 3, 6.