Role of Pulmonary Function Tests in MS Flare with Respiratory Concerns
Pulmonary function tests should be performed in MS patients experiencing a flare with respiratory symptoms, as respiratory muscle weakness is common even in moderate disease and can be clinically significant despite normal spirometry. 1, 2
When to Perform PFTs in MS Patients
PFTs are indicated when MS patients present with any of the following respiratory concerns during a flare:
- Dyspnea at rest or with exertion that is increased compared to baseline or unexplained 3
- Difficulty clearing pulmonary secretions or weakened cough 2
- Chest pain at rest or with exertion that is out of proportion to known condition 3
- Morning headaches, concentration difficulties, or memory changes suggesting nocturnal hypoventilation 4, 5
- Orthopnea suggesting diaphragmatic weakness 5
- Unexplained desaturation or hypoxemia during sleep, while awake, or with exertion 3
Essential PFT Components for MS Patients
The comprehensive respiratory assessment must include:
- Forced vital capacity (FVC) or slow vital capacity (SVC) to assess restrictive patterns 4, 5
- Maximal inspiratory pressure (MIP) to evaluate inspiratory muscle strength 4, 5
- Maximal expiratory pressure (MEP) to assess expiratory muscle function 4, 5
- Peak cough flow (PCF) to determine secretion clearance ability 4, 5
- Testing in both upright and supine positions when possible, as the change in FVC between positions (Delta FVC) can reveal early diaphragmatic weakness 6
Critical Thresholds Requiring Intervention
Initiate noninvasive ventilation (NIV) when:
- MIP falls below -60 cm H₂O indicating significant inspiratory muscle weakness 4, 5
- MEP drops below 40 cm H₂O reflecting severe expiratory muscle weakness 4, 5
- PCF < 270 L/min in patients ≥12 years, indicating impaired secretion clearance 4, 5
- FVC < 50% predicted even without symptoms 5
Respiratory Patterns Specific to MS
MS patients demonstrate distinct respiratory dysfunction patterns:
- Predominant expiratory muscle weakness rather than inspiratory weakness, with MEP more severely affected than MIP 1
- Restrictive respiratory failure is frequent in severely impaired patients (EDSS ≥7), occurring in 72.6% of wheelchair-bound patients 1
- Respiratory muscle weakness can occur even with normal spirometry - mean MVV (68% predicted), MIP (74% predicted), and MEP (51% predicted) may be reduced despite normal vital capacity 2
- Clinical assessment is superior to spirometry alone for detecting respiratory muscle weakness in MS 2
Clinical Assessment Index
A four-point clinical index is the best predictor of expiratory muscle weakness in MS patients 2:
- Patient-reported difficulty clearing pulmonary secretions 2
- Patient-reported weakened cough 2
- Examiner's observation of cough strength 2
- Ability to count on a single exhalation 2
This clinical index combined with upper extremity weakness assessment accounts for 60% of the variance in MEP 2
Monitoring Frequency
- Perform PFTs every 6 months in MS patients with respiratory symptoms or significant disability 4, 5
- More frequent testing (every 3 months) may be warranted in rapidly progressive disease or when approaching critical thresholds 4
- Serial PFTs are valuable to monitor disease progression, especially in patients with EDSS ≥4 where respiratory impairment becomes more likely 7, 8
Common Pitfalls to Avoid
- Do not rely solely on spirometry - respiratory muscle strength testing (MIP/MEP) is essential as spirometry may be normal despite significant respiratory muscle weakness 2, 6
- Do not wait for severe symptoms - respiratory impairment can be insidious and often not reported by patients 7
- Do not overlook expiratory muscle function - MEP is more severely affected than MIP in MS and correlates with functional capacity and disease severity 1, 9
- Do not delay NIV initiation when MIP/MEP thresholds are met, even in asymptomatic patients with FVC <50% predicted 5
Contraindications to PFT Testing
Absolute contraindications include:
Relative contraindications include: