What are the causes of variable intrathoracic and extrathoracic airway obstructions?

Causes of Variable Intrathoracic and Extrathoracic Airway Obstructions

Variable Extrathoracic Obstructions

Variable extrathoracic obstructions primarily affect inspiratory flow because atmospheric pressure cannot oppose the negative intraluminal pressure generated during inspiration, causing the airway to collapse inward. 1

Common Causes:

• Vocal cord dysfunction (VCD) - the most common cause, characterized by inappropriate adduction of vocal cords during inspiration 2
• Exercise-induced laryngeal dysfunction including laryngeal prolapse, laryngomalacia, and arytenoid collapse 2
• Vocal cord paralysis - may present with normal maximum flows despite obstruction 1
• Laryngeal edema from chronic coughing - violent coughing generates intrathoracic pressures up to 300 mm Hg, traumatizing laryngeal structures and causing reversible extrathoracic obstruction 3
• Redundant supraglottic mucosa - histologically normal but abnormally mobile tissue that collapses during inspiration 4
• Chronic laryngeal irritation from postnasal drip or gastroesophageal reflux, which can trigger functional upper airway obstruction 5

Physiologic Pattern:

• Maximum inspiratory flow is markedly decreased while expiratory flow remains relatively preserved 1
• Flow-volume loop shows flattening or plateau on the inspiratory limb only 2, 6

Variable Intrathoracic Obstructions

Variable intrathoracic obstructions primarily affect expiratory flow because positive pleural pressure during expiration compresses the airway from outside, worsening the obstruction. 1

Common Causes:

• Tracheomalacia and excessive dynamic airway collapse - weakness of airway walls causing collapse during expiration 1, 7
• Endobronchial tumors - both malignant (lung cancer, metastases) and benign lesions that move with respiration 1, 8
• Post-surgical complications including valve-like mechanisms from surgical scars occluding bronchi during specific respiratory phases 1
• Compression by vascular structures such as tortuous innominate artery 7
• Inflammatory conditions including granulomatosis with polyangiitis, relapsing polychondritis, and sarcoidosis 1
• Post-transplant stenosis and bronchomalacia in lung transplant recipients 1

Physiologic Pattern:

• Expiratory flow shows plateau or flattening while inspiratory flow is relatively preserved 1
• Maximum inspiratory flow is less affected because pleural pressure opposes the negative intraluminal pressure, limiting obstruction effects 1
• Peak expiratory flow is generally decreased 1

Critical Diagnostic Considerations

Endoscopic and radiological confirmation is mandatory because spirometric patterns alone do not accurately predict the presence or absence of pathology. 1

Important Caveats:

• Classic spirometric patterns may be absent in patients with coexisting lower airway disease (COPD, asthma), masking the upper airway obstruction 6
• Flow-volume loop quality depends entirely on patient effort and cooperation 6
• Direct laryngoscopy during symptoms is the gold standard for confirming vocal cord dysfunction 2
• Computed tomography with multiplanar reconstructions and virtual bronchoscopy provides excellent evaluation of airway walls and adjacent tissues that direct bronchoscopy cannot assess 8

Distinguishing Features:

• Variable extrathoracic lesions: inspiratory stridor, symptoms during exercise that resolve within 5 minutes of stopping, more common in young female athletes 2
• Variable intrathoracic lesions: expiratory wheeze, progressive dyspnea, may have saw-tooth pattern on flow-volume loop representing mechanical airway wall instability 1