What is the role of pulmonary function tests (PFTs) in a patient with multiple sclerosis (MS) experiencing a flare with respiratory concerns?

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 3, 4
• Orthopnea suggesting diaphragmatic weakness 4
• 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 3, 4
• Maximal inspiratory pressure (MIP) to evaluate inspiratory muscle strength 3, 4
• Maximal expiratory pressure (MEP) to assess expiratory muscle function 3, 4
• Peak cough flow (PCF) to determine secretion clearance ability 3, 4
• Testing in both upright and supine positions when possible, as the change in FVC between positions (Delta FVC) can reveal early diaphragmatic weakness 5

Critical Thresholds Requiring Intervention

Initiate noninvasive ventilation (NIV) when:

• MIP falls below -60 cm H₂O indicating significant inspiratory muscle weakness 3, 4
• MEP drops below 40 cm H₂O reflecting severe expiratory muscle weakness 3, 4
• PCF < 270 L/min in patients ≥12 years, indicating impaired secretion clearance 3, 4
• FVC < 50% predicted even without symptoms 4

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:

1. Patient-reported difficulty clearing pulmonary secretions 2
2. Patient-reported weakened cough 2
3. Examiner's observation of cough strength 2
4. 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 3, 4
• More frequent testing (every 3 months) may be warranted in rapidly progressive disease or when approaching critical thresholds 3
• Serial PFTs are valuable to monitor disease progression, especially in patients with EDSS ≥4 where respiratory impairment becomes more likely 3, 6

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, 5
• Do not wait for severe symptoms - respiratory impairment can be insidious and often not reported by patients 3
• Do not overlook expiratory muscle function - MEP is more severely affected than MIP in MS and correlates with functional capacity and disease severity 1, 7
• Do not delay NIV initiation when MIP/MEP thresholds are met, even in asymptomatic patients with FVC <50% predicted 4

Contraindications to PFT Testing

Absolute contraindications include:

• Recent myocardial infarction 6
• Unstable angina 6
• Hemodynamically significant arrhythmias 6

Relative contraindications include:

• Uncontrolled asthma 6
• Severe hypertension 6
• Mental impairment affecting cooperation 6