Assessment of Breathing in Patients with Myasthenia Gravis
Breathing assessment in myasthenia gravis patients requires measurement of negative inspiratory force (NIF) and vital capacity (VC) as critical components of pulmonary function evaluation, especially when monitoring for respiratory compromise. 1
Initial Assessment Parameters
- Measure negative inspiratory force (NIF) and vital capacity (VC) as baseline parameters in all myasthenia gravis patients, regardless of symptom severity 1
- Apply the "20/30/40 rule" to identify patients at risk of respiratory failure: vital capacity <20 ml/kg, maximum inspiratory pressure <30 cmH₂O, or maximum expiratory pressure <40 cmH₂O 1
- Perform complete sensorimotor examination with attention to variable strabismus, ptosis, and slow saccades that may indicate generalized weakness affecting respiratory muscles 2
- Assess for clinical signs of respiratory fatigue including difficulty holding up the head, slurred speech, and trouble with chewing or swallowing 2
Monitoring Techniques
- Conduct frequent pulmonary function assessments with NIF and VC measurements in patients with moderate to severe generalized weakness (MGFA class III-V) 1
- Monitor respiratory function regularly even when patients don't show clinical signs of dyspnea, as respiratory insufficiency may develop without obvious symptoms 1
- Evaluate forced vital capacity (FVC), which is typically less than 60% of predicted values in myasthenia gravis patients 3
- Perform respiratory assessments every two hours in hospitalized patients, particularly after medication administration when muscles are strongest 3
Special Considerations
- Be vigilant for respiratory failure as a potential first presentation of myasthenia gravis, especially in patients with previous episodes of unexplained respiratory failure 4
- Recognize that respiratory muscle weakness may be present even in patients without symptomatic dyspnea 5
- Consider the increased sensitivity to non-depolarizing neuromuscular blocking agents when assessing breathing in myasthenic patients who require anesthesia 6
- Implement aggressive respiratory treatment including suction, intermittent positive-pressure breathing, bronchodilator treatments, and chest physiotherapy to reduce the risk of prolonged respiratory complications 7
Warning Signs Requiring Immediate Intervention
- Watch for signs of impending myasthenic crisis requiring ICU-level monitoring and immediate intervention 1
- Monitor for development of pneumonia or atelectasis, which are common complications in myasthenic patients requiring mechanical ventilation 7
- Immediately discontinue medications that can worsen myasthenia gravis and precipitate respiratory failure, including beta-blockers, IV magnesium, fluoroquinolones, aminoglycosides, and macrolide antibiotics 1
- Hospitalize patients showing signs of respiratory compromise for close monitoring and potential mechanical ventilation 8
Diagnostic Confirmation
- Consider the ice test (application of ice pack over closed eyes) which may temporarily improve muscle strength, including respiratory muscles 2
- Check for acetylcholine receptor antibodies, anti-MuSK antibodies, and LRP4 antibodies to confirm diagnosis in patients presenting with respiratory symptoms 1
- Perform single-fiber electromyography, which is considered the gold standard for diagnosis in many centers and is positive in over 90% of patients with ocular myasthenia 2