From the Guidelines
Respiratory muscle testing is crucial for patients with Myasthenia Gravis (MG) to monitor disease progression and prevent respiratory crises, with regular assessment including spirometry, maximal inspiratory pressure (MIP), and maximal expiratory pressure (MEP) measurements, as recommended by the most recent guidelines 1.
Importance of Respiratory Muscle Testing
Respiratory muscle testing is essential to evaluate the endurance and strength of respiratory muscles in patients with MG, as it can help identify those at risk of respiratory compromise, which is a life-threatening condition. The American Thoracic Society (ATS) and European Respiratory Society (ERS) statement on respiratory muscle testing highlights the importance of measuring muscle endurance, which is the ability to sustain a specific muscular task over time 1.
Recommendations for Testing
Regular assessment should include:
- Spirometry with forced vital capacity (FVC)
- Maximal inspiratory pressure (MIP)
- Maximal expiratory pressure (MEP) These tests should be performed at baseline and during regular follow-up visits, with increased frequency during exacerbations or when patients report increased fatigue or breathing difficulties. Values below 30 mL/kg for FVC or less than 40 cm H2O for MIP indicate significant respiratory compromise and may necessitate intensive monitoring or intervention 1.
Clinical Implications
Respiratory muscle weakness in MG results from autoantibodies targeting acetylcholine receptors at the neuromuscular junction, impairing diaphragm and intercostal muscle function. This weakness often worsens with fatigue and can rapidly deteriorate, making consistent monitoring crucial. Patients showing signs of respiratory compromise should be evaluated for possible myasthenic crisis, which may require hospitalization, ventilatory support, and treatments such as plasma exchange or intravenous immunoglobulin alongside optimization of their regular immunosuppressive regimen 1.
Management of Respiratory Compromise
According to the ASCO guideline update, patients with MG should be managed based on the grading of their symptoms, with all grades warranting workup and intervention given the potential for progressive MG to lead to respiratory compromise 1. For example, patients with G2 symptoms (some symptoms interfering with activities of daily living) may require holding immune checkpoint inhibitor therapy, neurology consultation, and treatment with pyridostigmine and corticosteroids. In contrast, patients with G3-4 symptoms (limiting self-care, weakness limiting walking, or respiratory muscle weakness) require permanent discontinuation of immune checkpoint inhibitor therapy, admission to the hospital, and initiation of intravenous immunoglobulin or plasmapheresis, in addition to corticosteroids and frequent pulmonary function assessment 1.
From the Research
Respiratory Muscle Testing in Myasthenia Gravis
- Myasthenia gravis (MG) is an autoimmune disease characterized by fluctuating, fatigable muscle weakness, frequently involving bulbar and respiratory muscles 2.
- Routine evaluation of respiratory function is essential in patients with MG due to the severity of respiratory involvement 2, 3.
- Respiratory muscle testing, including peak expiratory flow (PEF) and the single-breath counting test, can be useful markers of respiratory function in patients with MG 2.
Correlations with Respiratory Function
- Positive correlations have been found between single-breath counting test scores and forced vital capacity (FVC) values, as well as between PEF and FVC values 2.
- Severity of dyspnea according to the modified Medical Research Council dyspnoea scale showed a negative correlation with FVC values 2.
- PEF also showed a significant correlation with maximal expiratory pressure (MEP) 2.
Effects of Respiratory Muscle Training
- Respiratory muscle training (RMT) has been shown to improve respiratory muscle strength and endurance in patients with MG 4, 5, 6.
- RMT can also improve lung function, reduce fatigue, and decrease dyspnea symptoms in patients with MG 4, 5, 6.
- Long-term RMT has been found to be feasible and beneficial for patients with mild to moderate MG, and can be a valuable supplement to conventional drug treatment 6.
Clinical Applications
- PEF, the single-breath counting test, and the modified Medical Research Council dyspnoea scale are useful measures for evaluating respiratory function in patients with MG 2.
- RMT can be used as a complementary therapy to reduce dyspnea symptoms, delay the breathing crisis, and reduce the need for mechanical ventilation in patients with MG 4, 5, 6.