Bedside Screening for Myasthenia Gravis
The ice pack test is the most effective and safest bedside screening method for myasthenia gravis, demonstrating high specificity for the disorder by showing improvement in ptosis and ocular misalignment after 2-5 minutes of application. 1
Clinical Presentation to Consider for Screening
When considering myasthenia gravis at the bedside, look for these key features:
- Variable muscle weakness that worsens with fatigue and improves with rest
- Ocular symptoms: ptosis, diplopia, ophthalmoplegia
- Strabismus that changes pattern during prolonged examination
- Slow ocular saccades
- Possible bulbar symptoms (dysarthria, dysphagia)
- Possible limb weakness (typically proximal)
Bedside Screening Algorithm
1. Ice Pack Test (First-Line)
- For ptosis: Apply ice pack over closed eyes for 2 minutes
- Positive result: ≥2mm improvement in ptosis
- For strabismus: Apply ice pack for 5 minutes
- Positive result: Reduction in ocular misalignment
- Highly specific for myasthenia gravis 1
- Mechanism: Cold reduces anticholinesterase activity at neuromuscular junction
2. Rest Test
- Have patient close eyes and rest for several minutes
- Observe for improvement in ptosis or ocular alignment after rest
- Simple alternative when ice is not available 1
3. Fatigue Testing
- Sustained upgaze for 1-2 minutes to induce ptosis
- Observe for Cogan's lid twitch sign (brief overshoot of lid on returning gaze from downward to primary position)
- Look for worsening of contralateral ptosis when manually elevating the more ptotic lid 1
4. Edrophonium (Tensilon) Test
- CAUTION: Only perform if experienced with proper administration and in a monitored setting with atropine available
- Sensitivity: 95% for generalized MG, 86% for ocular MG 1
- Risks: Muscarinic side effects including bradycardia, bronchospasm, hypotension 1
- Due to safety concerns, this test is now less commonly used than the ice pack test
Important Considerations
- Pupillary involvement is rare in MG; its presence should prompt consideration of other diagnoses like third nerve palsy 1
- Bedside tests are screening tools; definitive diagnosis requires additional testing
- Early recognition is critical as 50-80% of patients with ocular MG develop generalized disease within a few years 1
Next Steps After Positive Bedside Screening
If bedside screening suggests MG, proceed with:
Laboratory testing:
- Acetylcholine receptor antibodies (AChR-Ab) - positive in 80% of generalized MG but only 50% of ocular MG
- Anti-MuSK antibodies - check in seronegative patients
- Anti-LRP4 antibodies - newer test for seronegative patients 1
Electrophysiologic testing:
- Single-fiber EMG (gold standard) - positive in >90% of ocular MG
- Repetitive nerve stimulation - positive in only one-third of ocular MG 1
Imaging:
- Chest CT to evaluate for thymoma, especially in adult-onset MG
Pitfalls to Avoid
- Relying solely on antibody testing (seronegative MG is common, especially in ocular forms)
- Performing Tensilon test without proper monitoring and emergency equipment
- Failing to recognize that variable strabismus and ptosis that worsen with fatigue are hallmark signs
- Missing concurrent conditions (10-20% of AChR-positive patients have thymoma) 2
- Using medications that can worsen MG: beta-blockers, IV magnesium, fluoroquinolones, aminoglycosides, and macrolides 1
The bedside screening approach should prioritize patient safety while maximizing diagnostic yield, with the ice pack test representing the optimal balance of safety, specificity, and ease of administration for initial evaluation of suspected myasthenia gravis.