Excessive Dynamic Airway Collapse

Excessive dynamic airway collapse is abnormal expiratory narrowing of the trachea and main bronchi caused by bulging of a floppy posterior membrane into the lumen, with the cartilage rings intact. Narrowing of over 70% of the cross-section during forced expiration is the diagnostic threshold.

It is different from tracheomalacia, where cartilage is soft. It is common in patients with COPD, asthma, obesity and chronic cough, and it is often missed because the symptoms resemble those of the underlying disease.

Pathophysiology. Loss of tone in the posterior membranous wall, from chronic inflammation, mechanical stress during cough, obesity and age, allows the membrane to invaginate during expiration when intrathoracic pressure is raised. Airflow limitation, ineffective cough and secretion retention follow.

Dyspnoea out of proportion to spirometry, a barking or honking cough, wheeze that does not respond to treatment, difficulty clearing secretions, recurrent bronchitis and cough induced syncope. Symptoms often worsen when lying down and on forced expiration.

Cough syncope, stridor or respiratory failure needs urgent assessment.

• Dynamic expiratory CT of the chest, at end inspiration and during forced expiratory manoeuvre: collapse of more than 70% of the lumen, with crescent shaped narrowing.
• Flexible bronchoscopy during quiet breathing and cough or forced expiration, with the patient awake or lightly sedated, to measure collapse and exclude other causes.
• Spirometry: a flow volume loop with a notch or plateau on expiration, and reduced peak expiratory flow out of proportion to FEV1.
• Assess the underlying disease: COPD, asthma, obesity, reflux, and sleep disordered breathing.

Differential diagnosis.

• Tracheomalacia (cartilaginous)
• Asthma and COPD without collapse
• Vocal cord dysfunction
• Heart failure
• Bronchiectasis

• Treat the underlying cause: optimise COPD and asthma treatment, reduce weight, treat reflux with omeprazole 20 to 40 mg daily, and stop smoking.
• Continuous positive airway pressure, 5 to 15 cm H2O, acts as a pneumatic stent and is tested during a bronchoscopy or at home with symptom review at 4 to 8 weeks. Non-invasive ventilation is used for respiratory failure.
• Airway clearance with huffing, positive expiratory pressure, and carbocisteine 750 mg three times daily or hypertonic saline when secretions are thick.
• To predict benefit from surgery: a temporary silicone stent, placed for 1 to 2 weeks, with symptom, walking test and quality of life response. Stents cause granulation, migration and mucus plugging, so they are removed after the test.
• Membranous tracheobronchoplasty (a mesh splint on the posterior wall) for severe symptoms, with improvement in dyspnoea and quality of life in selected patients, and a risk of complications including infection and airway necrosis.
• Pulmonary rehabilitation and vaccination.

Monitoring. Review at 6 to 8 weeks after treatment change, with symptom scores and walk test, and yearly thereafter, with repeat dynamic imaging only if symptoms change.

Referral and escalation. Refer to interventional pulmonology or thoracic surgery for severe symptoms, confirmation of collapse, stent test or surgery.