Respiratory Status Testing in Myasthenia Gravis
The assessment of respiratory function in myasthenia gravis requires regular monitoring of pulmonary function tests, with specific focus on negative inspiratory force (NIF), vital capacity (VC), and maximum expiratory pressure (MEP) measurements to identify patients at risk for respiratory failure. 1
Core Assessment Parameters
- 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 2, 1
- Measure negative inspiratory force (NIF) and vital capacity (VC) as baseline parameters in all myasthenia gravis patients, regardless of symptom severity 1, 3
- Monitor forced vital capacity (FVC), which is often reduced to less than 60% of predicted values in myasthenia gravis patients 4
- Perform sniff nasal inspiratory pressure testing to evaluate diaphragm strength and inspiratory muscle function (values greater than −70 cm H2O in males or −60 cm H2O in females reflect absence of clinically significant inspiratory muscle weakness) 2, 3
- Use the single breath count test, which involves taking a deep breath and counting at a rate of two numbers per second while exhaling (counting to ≥25 correlates with normal respiratory muscle function) 2
Monitoring Frequency and Approach
- Conduct respiratory function assessments every 6 months in patients with stable disease 2
- Increase monitoring frequency to every 2 hours in hospitalized patients, especially after medication administration when muscles are strongest 4
- Perform more frequent assessments in patients with moderate to severe generalized weakness (MGFA class III-V) 1, 3
- Monitor respiratory function even when patients don't show clinical signs of dyspnea, as respiratory insufficiency may develop without obvious symptoms 3
Warning Signs Requiring Immediate Intervention
- Watch for clinical signs of respiratory fatigue including difficulty holding up the head, slurred speech, and trouble with chewing or swallowing 3
- Monitor for symptoms of sleep-disordered breathing such as morning headaches, fatigue, concentration difficulties, and witnessed apneas 2
- Consider electrical activity of the diaphragm (E-di) monitoring in ventilated patients as a potentially more reliable tool than MIP alone for assessing weaning readiness 5
- Be vigilant for respiratory failure as a possible initial presentation of myasthenia gravis, particularly in patients with anti-MuSK antibodies 6
Special Considerations
- End-tidal carbon dioxide (EtCO2) monitoring is an optional modality for monitoring early respiratory failure 2
- Pulse oximetry and arterial blood gases might not be reliable early indicators of emerging respiratory failure as hypoxia and hypercapnia might develop only in later stages 2
- Immediately discontinue medications that can worsen myasthenia gravis and precipitate respiratory failure, including beta-blockers, IV magnesium, fluoroquinolones, aminoglycosides, and macrolide antibiotics 1, 7, 3
- Consider polysomnography when there is concern that pulmonary function tests and clinical evaluation are not capturing complications such as hypoventilation 2
Practical Implementation
- Ensure testing is performed by adequately trained practitioners familiar with assessing individuals with neuromuscular disorders 2
- Implement a program of coughing, deep breathing, and sighing after medication administration when muscles are strongest 4
- Compare test results with predicted values, with a reduction greater than one standard deviation considered abnormal 8
- Recognize that respiratory muscle weakness may be present even in patients with mild disease (grade I) where MIP can be reduced in 40% and MEP in 60% of patients 8