Causes of Dyspnea in Ankylosing Spondylitis
Dyspnea in ankylosing spondylitis is primarily caused by restrictive lung disease due to reduced chest wall mobility and thoracic cage rigidity, which progressively worsens with disease duration.
Pathophysiological Mechanisms
1. Chest Wall Restriction
- Costovertebral joint fusion: Progressive ankylosis of the thoracic skeleton and costovertebral joints leads to decreased chest wall mobility 1, 2
- Thoracic kyphosis: Increasing kyphotic spinal posture contributes to restrictive breathing patterns 3
- Reduced chest expansion: Significantly lower in patients with long-standing AS (>10 years) compared to early disease 1
2. Respiratory Muscle Changes
- Respiratory muscle weakness: Decreased respiratory muscle strength (MIP, MEP) correlates with reduced chest expansion 4
- Reduced respiratory endurance: Maximum voluntary ventilation (MVV) is significantly lower in late-stage AS 1
- Diaphragmatic compensation: While diaphragmatic function remains intact and compensates for chest wall restriction, this compensation is often incomplete 3
3. Pulmonary Parenchymal Involvement
- Apical fibrobullous disease: A characteristic upper lobe fibrotic process that can progress to cyst formation and cavitation 2
- May be unilateral initially but often becomes bilateral with disease progression
- Can lead to secondary complications including:
- Mycobacterial or fungal superinfection (especially Aspergillus fumigatus)
- Spontaneous pneumothorax
4. Pulmonary Function Changes
- Restrictive ventilatory pattern: Characterized by reduced forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV1) 1
- Normal pulmonary compliance and diffusion capacity: Despite restrictive changes, these parameters often remain normal 1
- Normal arterial blood gases: Generally preserved in uncomplicated AS 1
Clinical Correlation with Disease Progression
- Disease duration impact: Patients with AS >10 years show significantly worse pulmonary function compared to those with early disease (<10 years) 1
- Chest expansion correlation: Chest expansion negatively correlates with disease duration (r=-0.502; p=0.03) 4
- Functional status: Interestingly, BASFI scores (functional status) do not always correlate with chest expansion limitations 4, suggesting patients may adapt to gradual respiratory changes
Management Implications
- Early intervention: Respiratory exercises should be implemented early to prevent limitation of chest expansion 1
- Regular physical therapy: The ACR/SAA/SPARTAN guidelines strongly recommend physical therapy for AS patients 5
- NSAIDs: While effective for managing AS symptoms, NSAIDs have not been shown to improve vital capacity even with prolonged treatment 6
- TNF inhibitors: Recommended for patients with persistent disease activity despite NSAID treatment 5, though their specific effect on pulmonary manifestations remains unclear 2
Diagnostic Considerations
- Chest radiography: First-line imaging for evaluation of dyspnea, can reveal structural abnormalities of the thoracic spine and ribs 5
- Pulmonary function tests: Essential for documenting restrictive pattern and monitoring progression 1
- Consider alternative causes: If significant changes in dyspnea occur, other causes such as spinal fracture should be considered with appropriate imaging 5
Clinical Pitfalls to Avoid
- Missing cardiac involvement: Cardiac manifestations are actually the most common cause of breathlessness in AS patients 3 and should always be considered
- Overlooking superinfection: In patients with apical fibrobullous disease, respiratory deterioration may indicate superinfection requiring prompt evaluation 2
- Neglecting physical therapy: Focusing solely on pharmacological management without addressing chest mobility can lead to worse long-term outcomes 1
Understanding these mechanisms is crucial for proper management of AS patients with respiratory symptoms, with early intervention focusing on maintaining chest wall mobility being particularly important for preventing progressive respiratory limitation.